Variacs DO short their windings
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- Опубликовано: 13 фев 2021
- A variac is basically a variable auto-transformer that allows you to adjust the voltage between zero and the local mains supply voltage, or more if tapped mid-winding.
They find use in specialist applications where it's useful to ramp the mains voltage up slowly for regulation or testing. Some applications would be inverter, motor drive repair, soft start of vintage electronics and some specialist lighting applications where lights need to be dimmed without phase angle noise.
These units do not provide isolation from the mains supply, as they are just a single winding with an adjustable tap.
I've sometimes wondered how they can seamlessly jump from one winding to another without bridging loops and crating shorted transformer turns. Since I had my unit open for fitting a new digital meter, I thought I'd test that with a thermal imaging camera.
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This is my variac. There are many others like it, but this one is mine.
Full Insulated Metal Jacket.
I picture electroboom with a crazy look in his eye and a big capacitor ready to blow.
This is me bottle o' scrumpy. There are many like it, but this one is... MINE! :D
Where do you get one?
I have never seen a variac like that. What’s the name and model? I’m interested in a low current one with dc possibility my variac will run up to 20 amps and I have a extension cord I made for testing ac powered stuff with an incandescent bulb wired in series on one leg. That way if there’s a short the bulb gets bright instead of seeing the magic smoke coming out of the equipment that you’re fixing. One last question do they still have that motorcycle race on The Isle of Man? I remember that when I was younger and thought it was awesome going around an island as fast as possible. Thanks for your help and time.
I just had a look at the brush on my 10 amp variac.
It's a 10x10x10mm cube of brass, with a small T-profile length of graphite set into it, about 10x5x5, the edge that contacts the winding being about 1.5x the width of the winding pitch. The graphite measures about 1.5 ohms in the winding-to-winding axis, and 0.3 ohms from the brass to the contact face - the whole length of the graphite insert is effectively all in parallel to keep the resistance to the brass low, while having enough resistance in the winding-to-winding distabnce to minimise heat from shorting turns.
I know someone who tried replacing a broken variac brush with brass. it didn't end wel...
Hey Mike, have you ever popped into the synth-diy mailing list? I think you might find it interesting A lot of analog electronics design happening.
Likewise on my 2A one. Mine came with a spare brush, which I have never needed and occasionally find lying about the workshop (it's supposed to live in the bottom of the Variac cabinet). I notice that the windings are actually set slightly apart and embedded in epoxy so that the contact surface is quite flat and the 'dwell' is very short. I imagine if and when the leg of the T wears down it will be another story, but it will probably outlive me.
Someone else mentioned a tiny brass roller that rode across the windings. I tried measuring mine, but didn't see significant results with my ordinary meter. Maybe I should have tried my installation tester for a higher resistance test current.
@@bigclivedotcom I have a heavy one, that has five ! ~1 cm carbon sliders side by side so with spacing the entire takeoff area is ~6 cm , using the height of the wingdings on the outer circumference. (for normal use I have a smaller 2Amp one though)
i have second hand ones with copper cored graphite wheels, but they got stuck and have been rubbed flat on one side
I remember as a kid having one of these for the lights on the Christmas tree, you could dial down the brightness when the room was dark. Of course being a kid I had to plug all sorts of things into it to see how they were affected.
That was likely a rheostat, not a variac. resistor as opposed to transformer. Rheostats were very common for lighting dimmers in the day.
I really like the direct relation between turning the dial and the analog meter moving. I've got too used to watching digital meters dance around as their electronics fuss over what the actual value should be.
My Variac is Digital, but the display and the dial are not at all accurate. !!! It's important to use a Volt Meter unless you want to blow up something !!!!
It never occurred to me that connecting adjacent windings would do that, but it makes sense
I've wondered about the shorted turns in the past on variacs in the past. Thanks for actually measuring one.
Years of armchair theorizing destroyed in under 10 minutes with a thermal camera and hopi.
Found my variac in the e waste bin at my university. Not much was wrong with it. Fixed it up. Works like a charm.
Hmmm... I imagine that they can get away with shorting the windings because the voltage difference between the adjacent windings is very small. I'm commenting this before your measurement on the HOPI, but if there is a 80mm diameter variac and a 3mm wide contact, the maximum voltage across the contact would be 240 * 3/(3.14*80) = 2.8V. Obviously if it's shorted, the voltage is ~0V, but this shows the scale of things relative to 240V.
/\ this /\
Small voltage drop = small current dissipation.
Good 'nuff engineering.
@Flat Sign Big Clive mentioned he checked for that, and it seemed to be normal carbon with the same resistance in all directions.
@Flat Sign He tested this in this video. Some fancier high-powered variacs might use this, but this one doesn't. Also, if there are multiple vertical contacts, which one is connected to the actual wiper?
@@LazerLord10 Depending on the physical preciseness of the contact - I would say the 1st wire that makes contact is the effective one.
the problem is not voltage but magnetic flux
Yep, the brush is designed to bridge at least two turns. But it is carbon so it has some resistance, and thus it isn't a dead short, it is just a low resistance across a small voltage.
The brush is designed to bridge turns so that it will work effectively as a rheostat and the output of the variac will not step in voltage, but will be a more or less smooth increase or decrease as the arm is moved. The rheostat effect comes about as the area of contact changes from one winding turn to the next.
BTW, in place of that simple digital volt meter, you might want to look on eBay for a Peacefair model PZEM-021 or model PZEM-022 meter. This will do everything the Hopi will, and fits in about the same hole shape as one of those digital LED voltmeters. They claim to have a minimum input voltage of 80V because they are powered from the same line they are measuring. However, a little looking at the PC board will show you where you can cut a trace and separate the power input from the measuring voltage input. At that point it is fairly easy to adapt either one for use with a variac. They use the same chips as utility-supplied electric power meters, so they are pretty accurate. Search terms are "digital power watt meter monitor voltage voltmeter ammeter" and the like.
I have an old GE variable reluctance transformer. It has a rotatable core in a cylindrical stator. It creates variable voltage without the steps associated with brushed windings. It's much heavier than an equal powered Variac.
As a young lad, many years ago, I had a model train speed controller. Imagine my delight when, after taking it to bits (as one does) I found it was in fact a variable transformer from about 6 volts to 25 ish. The variable part was an isolated secondary. Amazing for a toy. Needless to say it was never connected to the train again.
I've seen a system much like that, including a fixed tap for 12v AC lighting! Very cool stuff.
@@jercos Yes, I remember that now, made by the German Märklin model railways. Much better than the Hornby stuff that used DC and a tacky resistance to control the speed.
I can never see or hear about a Variac without thinking of Andy from photonicinduction cranking them silly in his loft. I hope the guys doing well and can make an appearance again. I miss his "I ain't havin' it attitude"
Dave from EEVblog posted a community post here on youtube a while back saying he was in good health and working on some videos to be posted soon
@@michaellynch1018 yeah,. I tried to follow it best I could a while back and saw his wife was in the UK finally. Last I saw was about wanting to make videos "soon". Guess he needed more time than he thought
Finally we got to see your Variac. Cute small one, my 5kva one is a pain to move!
I've got a 2KVA (120V 17A), and it turns easily.
@@michaelmoorrees3585 It turns super easy, but moving it from one place to another is a pain. That beast is heavy. It also loves to pop breakers sometimes if I don't hook it up to the soft-start. :D
@@DrakkarCalethiel You could attach some caster feet
to it. Did the same with my 10kVA 60kg monstrosity : D works well
@@aulus4163 60kg 10kVA, what a beast! My 5kVA one is just around 16kg with all the control stuff.
When I'm going to pair it with a couple beefy transformers on the output, everything will get mounted on a rollable platform. Still need to find some tranies that don't cost an arm and a leg new...
My first acquaintance with this was when I took apart a model railway transformer. I had read about transformers in the Great World Encyclopaedia of Science, and it was immediately obvious how the wiper selected a different number of coils in the output stage. It was linear rather than circular, and there was a long streak on the copper windings where the wiper had been wiping!
My brother used to take apart his Christmas presents of electric train set transformers so much so that he was given this Dr Frankenstein Chemistry set / welding kit instead next years because they were less costly.
I was wondering how they constructed that linear variac you seen. Was the magnetic circuit completed like a square toroid of transformer steel plates or was there a long airgap with coils wound on group of rectangular transformer steal plates, but not completing the magnetic circuit?
@@kreynolds1123 that 'linear' thing may have been a high power variable resistor with sliding contact, not a 'variac' or transformer of any kind
@@andygozzo72 Thank you. It is a little difficult finding many examples of linear autotransformers.
I had a Märklin one and it was my first "lab equipment". I was around 9 and my dad had to get a resettable porcelain fuse in the fuse box in our apartment. The fuse blew so often when I set fire to steel wool and tried arc welding with sewing needles. It sure had some punch 😁
Love how you can tell how new the video is by how scuffed looking the desk and BigCliveDotCom sticker is.
When I have a print adhere too well to the build plate, I unclamp the build plate and print from the printer and put them in the freezer for about half an hour, so far that's always let me get the print off without damaging the glass.
You tell it to chill out 🥶👍
I find my glass bed sometimes barely sticks so I use glue stick and that makes things way too hard to pry off. Hard to find a balance really, I used to have the flexible magnetic sheet that came with the printer but I was an idiot and scraped it with the scraping tool too much and it wore it down. I guess painters tape is something to try out next
@@oswith971 Why not just buy another magnetic sheet?
@@oswith971 Painter's tape worked well for me with PLA, but with ABS, the tape would melt into the print and was near-impossible to remove. The other problem was that I had to reapply it regularly. Kapton tape worked better with abs, but it had to be reapplied often as well, and it was very hard to get it on without wrinkles. A sheet of glass and a glue stick has been the best so far, but sometimes the prints stick too well, then I put the whole sheet in the freezer, and the print pops right off.
this is also what you are told to do with troublesome prints that come off of FDM printers that cost 5-6 figures
Dearest Clive! I never regret when you digress in your videos, and I think many of us like the content you make. ^^
Interesting. I've got an old RS Components (I think) metal cased variac from the early~mid 1980s at work. I now have the desire to find it and open it up for a look-see :)
maybe would be interesting to slide a thin piece of plastic under the brush and see how much of the loss is due to the turns being shorted by it, and what remains
It's worth reminding viewers that this does NOT provide insulation from the mains, it is not an isolation transformer, so needs to be used with care.
I've added that in the description.
Clive, you are amazing. I love your videos and you make complicated things very accessible and even friendly with your kind delivery. We use variacs in the music industry to get the “brown sound” by slightly lowering the voltage for guitars amps. I have wondered for years how these things worked. I knew it was “like a transformer” but couldn’t figure out how you would build something like this. You, as always, really connected the dots! Thank you.
I've always wondered this since photonicinduction started posting videos of his variacs and now I know. Although as I've always been warned not to accidentally short a normal toroidal transformer with the mounting lug in the middle I imagined there to be a fair bit more heating.
I also have some of those ZnSe (Zinc Selenide) lenses for my FLIR and they do slightly affect the readings, it can go both ways due to temperature of the lens but it's usually reporting lower than reality by 1 or 2 degrees. The pure germanium ones don't suffer from this problem but are 3x the price.
I've used variacs for over 40 years and always wondered about this. Thanks for answering some questions for me!
The windings on these variacs aren't always with fine wire. I've got about a dozen old Statco 20 Amp, 0 to 120 VAC variacs, and the windings look like 14 GA. The carbon brush looks like it contacts more than one winding as well, but I guess it doesn't hurt it. Does dim the lights in the room for an instant when the power switch is first turned on, even with no load connected.
That sounds like the classic behaviour of a toroidal transformer.
Yeah, I got my hands on an old Grundig RT5A combined Variac/Isolation transformer that can output up to 3,5A, and it actually has a "soft start" mechanism so it does not blow the breaker when you switch it on. It actually needs it, when I first got it I forgot to reset the power switch and turned it on at the outlet - and suddenly it was dark ^^ )
Built like a brick outhouse, really heavy and the wires on the secondary are really thick don't know what gauge, though. But must be able to take afair bit of heat.
(www.radiomuseum.org/r/grundig_regel_trenn_transformato_2.html)
Now I am kind of intrigued to pull the cover off again and look at it with my cheapo thermal imager.
Magnetizing inrush current. It happens to all transformers, but it goes larger as the transformer size.
Power transformers at substations makes crazy sounds when started because of this.
My General Radio W20 Variac is probably very similar in construction, and yeah, its windings are about that size. It tends to pop the circuit breakers in my apartment if I energize it without a load. But it helps if I place a light resistive load on it, say a 40-W incandescent lamp, and the knob around 20-30%.
FWIW, the large aluminum disk that rotates and holds the brushes is referred to in GenRad/IET documentation as the "radiator". The whole disk is electrically 'hot' during operation -- found that one out the hard way! 🤣
That's how my 14 amp Staco Energy one is. Wound with coarse windings that are just ground flat on top, and the brush is the width of the core thickness. These things are ridiculously expensive, too. Got mine in new condition for free, but I think it was like 400-500 USD new. The sudden inrush is due to the core saturating since there is no magnetic field present at turn-on to resist the incoming AC. Turning the variac on near the zero crossing causes the windings to pretty much only present their DC resistance once saturation is reached, and draw LOTS of current until the field builds up. I have a large Denon receiver that does the same thing.
A neighbor gave me one of those from probably the 50's. Big heavy thing. the wire has cracks but it still works. I've always wondered how it works, so thanks for showing me.
All these years using these things and I never considered that this was happening. I have had many experiences with variacs, not all of them pleasant.
Working within electrical and mechanical engineering for 35+ years, looking at the back of that variac in the video reminded me of the 3 phase 415V ganged variac that I used to use in the Electrical Test Department of a local firm I used to work for. It was in a test rig for high power DC motor testing with the outputs of the variacs going to a 3 phase diode bridge rectifier.
It was all open live connections mounted on a piece of thick paxolin board. The only protection from certain electrocution was a flap of cardboard over the front of the connections secured with gaffer tape. The whole rig was bolted to the side of the test bench and the floor, on top was a large wheel very reminiscent of a water gate valve. I did manage to get zapped occasionally on the output side of the motor test rig, think it was due to leakage current from the big diode bridge.
The other one I remember was "The Puppy" as it was referred to. This was a large wooden box on wheels that contained a variac and high voltage step up isolation transformer and DC bridge rectifier. On the front was a large red "jewel" indicator (remember those anyone?) lamp that had a 15W pygmy lamp in it . It's purpose was a high voltage leakage current detector, I think it was 1200 volts maximum output. There was a long high voltage test lead came out the front with a pointed bakelite probe holder and brass probe tip (not insulated or retractable!).
Of course it had gaffer tape ears, tongue and mouth, the variac pointer was the nose, and the indicator light was one of the eyes! Dangerous kit by today's standards and probably would send an HSE representative into palpitations!
Also good to see an MK 655 D8 red mains plug still in use. Does it have "Hospital Property" stamped on it? I can remember my father (he was a hospital electrician) having one or two of these in his works toolbox. Used to see these on everything in the local hospitals, from ECG machines to electric floor polishers, vacuum cleaners etc.
We used to carry a 20A Variac when I toured with a sound system (1980's). This would allow us to adjust the voltage at venues that were out of spec (for the console and other sensitive gear. Nowadays with better power supplies this is not as much of an issue.) For the connections to the windings there were actually 4 connections, 0%, about 10%, about 90% and 100%. We would connect the hot at 90% and the neutral at 0%. This would allow us to move the output wiper past the hot connection at 90% and boost the output voltage above the input. Very handy when we were at a venue with low voltage. Being from the US, we would also bring this variac along when touring in 240 volt lands, etc., allowing easy adjustment. (We would find some places where 240 was actually 260+, being able to tweak as needed was handy.
Oh that's just brilliant, there was me thinking about having a lazy week but instead, I've now got to take apart my Variac and examine it with my Seek 😁 camera.
I had the same incident with my 3D printer a few times. I discovered that adding a very thin layer of PVA glue to the bed helps with adhesion on those zones where bed level is not perfect and, at the same time, stops the plastic from fusing with the glass bed.
To apply the PVA I use a generic Dealz (Poundland) spray bottle filled with Dealz liquid PVA lightened with distilled water (filtered is good enough) and a bit IPA, to help the water to evaporate faster.
Everybody has a slightly different solution for this issues, so you'll get lots of suggestions, my recommendation would be to test and find the one that works better for you.
I uses Kapton/polyimide tape for my 3D printer or those flexible reusable plastic beds. No more scraping.
you don’t need to add anything to a glass bed to induce adhesion. I print fine on mine without anything.
Prrit stick
@@reme2493 It depends a lot on the printer and the material you are using. Ideally, you don't need to add anything to the bed, you are right, but reality is a bit more complicated. I personally was a huge fan of the "nothing better than a perfectly clean bed", until I started doing more and more prints and started having more and more different issues.
Some printers, specially the ones with bigger beds, almost always have issues with the bed not being perfectly flat, which affects levelling and adhesion, even when using ABL (that I strongly recommend). Please, keep in mind that I'm talking about machines on the price range of 300 to 500 €.
On the other hand, some materials really love glass, specially PETG, but also some blends of PLA. In that case, the problem is the opposite, your piece will literally solder to the bed, that is what happened to Clive here and that has happen to me too. In my own experience, adding a layer of "something" between the glass and the piece that you are printing helps a lot.
Of course, I'm talking about my own accumulated experience, that is only about 4 years, I'm no expert here, just another amateur. Every individual experience may and most likely will vary, that is why I'm suggesting to keep trying different techniques until you find the one that works the best for you, that is what really matters at the end of the day.
I have heard of people adding a waft of aerosol hairspray. Adds an easily breakable bond.
Ahh, what a cute little Variac. I have one I call Big Bertha, over 50 years old with a rated current of 45A and still going strong. She is a beast!
Used a variac in our shop. Rated for 15A and the windings were much more 'course' as it was about 14 AWG wire. As you turned the knob, you could 'feel' the bumping as the brush moved from one turn to the next. They are a sort of auto-transformer (uses the same winding for primary and secondary) with 'taps' on every turn of winding.
Nice video. And some VERY nice FLIR photos.
When I heard back in the 80's that Eddie Van Halen used one on his amp, I got a variac and put it on an old Yorkville Sound amp. Since it wasn't really meant for guitar specifically, it didn't have a distortion circuit. But the variac, when turned down, allowed me to crank the volume "up to 11" which overdrove the tubes something spectacular, turning that amp into an insta-Marshall. Seriously dangerous project, but it worked amazingly well.
And this is why I love these videos, every once in a while you have things I'm just about to buy, and you dispel the mystery I wonder about. This time it was nothing to do with the video, you just happened to explain why the contacts work (they're bare, I know I'm ignorant, it's ok, I learn quick). Thanks for another great one, I'll keep watching long as you're making em.
Thank you BC. I'd been aware of this for a very long time but it's good to have it highlighted by you in your usual professional way. My 8 amp variac dissipates 30 watts on standby and the brush has an heatsink which gets quite warm on standby. When you think of it, there are only a few volts between each turn of the winding so the current flowing as the brush shorts them out is minimal on a miniature variac but it gets significant on a large variac with a correspondingly large brush hence the brush's heatsink on my 8 amp one.
The brush seems to create a virtual single turn secondary winding with the turns it shorts out. Much like the shading rings do in a shaded pole motor. That can muchly explain why they heat up just in a small spot. It's a secondary short circuit. The shading rings in a shaded pole motor also get rather hot.
I've always thought that was likely. But, so many say it's not a shorted turn. Glad someone actually proved it either way.
I can think of ways to avoid shorting turns, but it'll always come down to either a different transformer style, major cost/size increases, or opens replacing shorts.
It depends on your definition of "shorted". The wiper is carbon, and carbon makes a resistor. So it isn't a dead short, it is a very low resistance resistor connecting a few turns, and the turns have a relatively low voltage difference between them, so it is a fairly small power loss.
@@lwilton You're really just splitting hairs. I could claim everything has resistance so there's no such thing as a short, but we both know that's absurd.
I patiently waited for “Let’s take it to bits.” 😉
I think you need shirts. That bearded outliney-sunglassey icon.... w your dotcom, and a couple choices of sayings. Don’t print them yourself, stock, and ship. Do some kind of third party host thing. Don’t want you tied to regular scrambles to the post.
That glass delaminating thing is very common with PETG, but that was PLA. I had it happen at a small scale once. Maybe it was cheap glass?
I've had pla delaminate on PEI sheet. Temp wasn't high enough to get good bonding. Increased first layer temp and problem solved.
I use cheap soda glass, and delaminating is never a problem because of the high coefficient of expansion. Once it cools the prints quite literally pop themselves off.
Borosilicate glass is much more expensive and is normally recommended because it has a very small coefficient of expansion. I got 3mm glass from a local place for about $2. I got two for a spare, but I've not broken any of them in over a year.
So, I'm thinking that the better the glass (eg borosilicate) and if the surface is textured (ie sandblasted) so the adhesion is better, that's when you are more likely to have problems if the print sticks really well and there's no contraction of the surface to help get it off.
If I leave the bed to cool down completely I literally hear when the print releases. I've also had prints complete while I'm out, and come back to find they've popped off so forcefully they are on the other side of the bed.
@@Chrisamic you should set up some targets and see how many you can pop.
Yep, I had it happen on PLA as well. Tore off a good chunk of the glass bed.
@@Chrisamic haha, that's amazing. I use cut mirrors (I found some really thick 1/4 inch ones), and a smearing of sugar water to keep things attached. It works unreasonably well.
I've been wondering about this for decades, thanks for the answer. I guess the windings have a good thermal bond to the transformer iron, so the shorted turns don't overheat. As 99% of the coils are unshorted, the overall losses are low.
Used those for many years on stage to be able to vary the output on singular lights! Quite handy bit of kit!👌🏻
Much kinder to transmission lines than modern triac dimmers too... phase-fired control inherently has terrible power factor.
Interesting that you want to replace the analog meter with a digital one. I used to have digital ones but have the odd feeling, that it is much faster and more intuitive to adjust the voltage/current to a desired spot. That will be due to our instincts, we have a hardwired closed-loop in our brains for moving things to where we want it. There is no such instinctive support for 3 digits of numbers.
I added digital volt and amp meters to a variac that never had a readout. Except for the last few volts I never attempt to set a voltage from the meters, they are too slow. The variac has a 0-100 scale, and I've marked the actual output voltage at a couple of interesting points like 115V and 125V, so I can go directly there, or estimate some other voltage as a fraction of those voltages. Once I'm close I can be precise with the meter, if it matters.
It looks like a cheap eBay meter with a slow update rate too.
@@Agent24Electronics It might be relatively fast. I used eBay meters when I did my variac probably 10 years ago, and they update once or twice a second. Not wonderful, but not horrible either.
I just like analogue meters. Another advantage is if the load is varying, a meter shows it better than wandering digits...
@@lwilton twice a second is slow. 3Hz is typical
My stepfather had several of these for the lights in his animation studio. He found that the lights lasted much longer if you wound them up gently and didn't run them at full power, which was important as the special bulbs were expensive. Of course he'd got them second hand and much used, so big blue sparks would shoot out the sides as you wound them up. Also they had to make sure that the lamps were always running at the same voltage throughout the shoot, since mains voltage varies from day to day, which would show up as a noticeable flicker when sped up by the animation.
Your almost at 1 million! But don’t change. Keep going around the same way your going on.
Very good variac video. I have been looking for one for a while. Thanks
I recently had a massive 15 amp / 120v one burn out just sitting there. I'd been using it to control a Harbor Freight spot welder, for which it was very useful. I forgot and left it energized for half an hour (spot welder inactive) and then there was that smell in the room - and about 3 turns at the position of the brush were black... The variac was collected by my Dad in the 50's, no idea of the actual vintage. Things like those stay in the family for generations, as many know, they don't grow on trees and the big ones cost real money. Luckily, we've been avid collectors...
Yet another great myth-busting video Clive. Thank you! There are many assumed theories about the graphite brushes on variacs, but it's obvious that they simply short out a few turns of the coil which after all amounts to only a few volts precisely as you say. I have an 8 amp variac and it dissipates 13 watts quiescent AND it has a finned heatsink on the graphite brush. That's where the quiescent power goes.
Thanks Clive, very interesting. I've always wondered about shorted turns on a variac.
I’ve never noticed before that the Hopi has Frequency misspelt as ‘Frequence’.
I think that’s French.
@@5roundsrapid263 The other labels are all in English, even “Annual Power Consumption”, so I don’t see why that one would be in French.
Another 5 star video! Thanks! Love this
You’re a very wise and naughty man..I truly wish I understood what exactly you’re speaking of but I’m hoping to absorb things subconsciously in the end..
I've had mine over 40 years. Still use it! It's a DURATRAK type V5HMTF, rated at 240v 50Hz in, 0-270v 2A out
I’d wondered about this myself, thanks for the video!
I believe it is a actually an autoformer . A transformer would have at least two independent magnetically coupled windings and this has only one winding
@@lwilton "I know you are but what am I ?"
@@robstorms "Transformer" just doesnt tell you anything about how many windings something has. Autotransformer, however, does.
@@Basement-Science Just funnin with you ! Thanks for the clarification !
I've always wondered about this. Why have I always wondered about this?... Thanks.
Very neat video! I've wondered about that same thing, seeing how a variac doesn't have any "dead" areas between windings as the control is swept across the entire scale.
Makes me think of a power transformer with an "on load tap changer" which momentarily bridges out a segment of the winding during tap changes. Those have a more complex system than a variac, but the idea is similar. There is no off state between taps. A low-value resistor is used in series with each winding for a very brief moment during tap changes.
I've got two Chinese variacs which have developed some dead spots and audible arcing. Should tear them down and inspect to see what went wrong. Both of them are less than 2 years old and followed the same path to failure. New out of box, there were many dead spots, cleared up by repeatedly sweeping the knob with power off. Then after several hours of use, dead spots reappeared at the more commonly used ranges such as 100V and the very low range used to check for signs of life in very old equipment before ramping up.
Both of these now take very little effort to rotate the knob and I expect that winding varnish was on the end turns where the brush rides, causing the initial problem; and that now the brush has become stuck due to more gumming up from varnish.
Thanks, Clive. I had wondered about that myself.
I used one to vary the speed of a fixed speed wall fan without having a buzzing noise that you get if you use a normal resistor in the circuit. Work fine and never had a problem.
I think it's more to do with the resistance of the brush in general.
If you assume the brush to have no resistance and do the calculations, a 10A variac would dissipate hundreds of watts in a single shorted turn. (Assuming 1.2 ohms end-to-end resistance, i.e. 5% impedance, and about 240 turns. This gives 5 milliohms per turn, put 1V across that and you get 200W!)
If you expect around the same impedance again in the overall brush (from winding to the output connection), and pull a figure out of the air to say that the turn-shorting resistance would be 25% of that, then you burn 3.33W in the brush and 55mW in the shorted turn.
Which is bupkis.
It doesn't make any sense that the brush doesn't look hot in your thermal camera images, so I may be totally wrong.
Edit: When I say transformer impedance, I mean the one that ruins your voltage regulation... not the one to do with transmission line characteristic impedances etc.
Edit 2: Ah yes, you tested your wee lil bugger with HOPI. I expect that figure is dominated by iron loss, just like any transformer.
Thank you for doing this expose'. I have always wondered about this exact phenomena. I guess the carbon brush is the "trick". A little lossy on the contact prevents high currents to flow in the shorted winding.
I read about putting your build plate in the freezer to get stubborn, stuck prints unstuck for years before I tried it. Works every time. You even get an audible "pop" when it comes off. Build surfaces are consumables IMO.
Clive, you will probably find that your digital meter does not measure below ~70V. You can modify it by adding a third wire, disconnect the supply from measured voltage. In my case it was removing one 0ohm resistor
I'll check that out to see where it taps its monitoring supply from.
I now know more about Varics than I thought I did, 8 and a half minutes ago 👍
@bigclivedotcom I don't understand some of the stuff you talk about or show, but I still enjoy watching your videos. Thank you for not having music, I find it quite distracting when trying to watch/learn from some channels' videos. I know you mentions things on many videos during the process, but I'd love to see a general tips video for faults, issues, traps, etc, when making or fixing things. I think it would help the less experienced and knowledgeable people like me.
The brushes on large DC motors normally bridge several commutator segments as well, I believe that this improves the commutation and reduces the sparking by ramping the current up & down more slowly than if a brush just contacted a single segment. Brush manufacture is a black art ( pun intended) with a secret mixture of anthracite, other carbon clay & copper to get the correct resistance. One UK supplier had big problems when the Welsh anthrasite mine(s) closed, the replacement seemed to had sufficiently different characteristics to result in an increase in brush & commutator wear.
I have one that I put into a box with several switches: A dual-pole on-off switch that interrupts both sides of the line, just in case. A switch that selects whether the high side goes to the end of the variac or to a tap, allowing me to "boost" the voltage to somewhere around 130V, for some tests. And a DPDT center-off switch that in one position puts a light bolb in series with the output, in the other position connects directly, and in the "off" position allows for the connection of a meter to some binding posts so I can measure the current. There are also neon indicators to tell me when the power is on and when I've blown the fuse.
In a relatively recent acquisition of a pile of "stuff" I got another one. The rather distinctive knob that came with it is one that I've seen installed in premise wiring, as light dimmers. I had no idea that they were using variacs for that purpose.
Variacs were used as orchestra pit music stand lighting dimmers to avoid phase angle control noise near microphone cables.
Cool info Prof!
That Digital panelmeter normally begins to work and show voltage from 60a65V, but you can hack it to show voltage from 0 to 240V.
Lift one side of the brown capacitor and use that for measuring the voltage, use a separate 12V DC and connect this parallel to the black electrolytic capacitor.
It's the same hack that is used on the Peacefair panelmeters.
Morning Clive👍
"I'm not sure why I did that ... but i did." This is exactly me when I'm working on a small Arduino project at the moment 😂
The volt per winding is very very small so there's a very very small eddy current in two adjacent shorted windings.
Another thing to consider: A carbon brush has more resistance than copper has. Two wires being shorted through a brush results in less "shorted" current than if the brush were made of copper. But, as pointed out, the brush may sometimes span more than two windings.
@belly tripper Kindly, while "Were" is used in the second person singular and plural and first and third person plural, it is also used in the subjunctive mood to indicate unreal or hypothetical statements.
www.grammarly.com/blog/was-vs-were/
@belly tripper Past tense "Jack was early to work", vs subjunctive unreal or hypothetical "if the goose were to lay a golden egg."
@belly tripper ok, roger that.👌
I'd like to propose that you brush up some. Chech out the section on 'The Subjunctive Mood with "Be" and "Were" ' www.merriam-webster.com/words-at-play/getting-in-the-subjunctive-mood.
If you are to be a grammar nazi, you might want to be an informed grammar nazi.
Please have a nice day.
@belly tripper With all due respect, given your two statements ' your grammar is horrible, you mean ""if the brush **was** copper"" ', please understand that when one wants to criticize another person's grammar, one should know that an informed grammar nazi would not start either sentance with a lower case letter. Nor would they end either statement with either a comma or nothing. Nor would a grammar nazi use "" where a single " is required.
Lighten up on people please. Putting them down on something does not lift you up. And when you are the one that falls short on the same measuring stick you used to measure another person, the heaping dish of crow pie doesn't taste very good.
In the case that the last idiom is not common wherever you are, I'll share this from wikipedia "eating crow is a colloquial idiom, used in some English-speaking countries, that means humiliation by admitting having been proven wrong after taking a strong position.The crow is a carrion-eater that is presumably repulsive to eat in the same way that being proven wrong might be emotionally hard to swallow."
In the 1970's I built a variable power supply using a variac rated at 10 Amps. It was used to run an electromagnet.
Because I was a laboratory technician I was able to get items from RS Components then. Since I retired they are no longer in a position to fulfill any orders from me.
To buy from RS components you have to present yourself as a company. It's purely a box ticking exercise as they are simply not allowed to sell directly to the public at their trade counters for red tape reasons. If you tell them you are a company they will happily sell to you without needing proof, purely because you have ticked the box for them.
You could call yourself Shieltronics.
That was very fascinating
I really hope you show the installation of the new meter.
Always informative. Please consider making a video on the crimp connectors you used. Tips and tricks for making them? What kind of tools do you use? etc... Thanks
Unsmoothed rectified is useful for measuring very large inductances in the presence of DC... maybe you wanted to test chokes?
I used to use variacs a lot where I used to work, we tested motors and used them for fault finding when mains equipment was faulty and we could ramp up the voltage gradually. Useful things. I figured out how they worked years ago, and always thought they shorted windings out because it was just designed that way, or 2 or 3 windings would deliver a better surface area for the carbon brush with less resistance/heating in that particular spot. Prob just a rough and ready design though. Who knows lol. We read too much into design sometimes.
The most likely reason for the shorts is breakdown of lacquer on the windings of the Variac. Over time, and especially on tight windings the lacquer starts to crack, which causes shorts. Heat accelerates this. Buffing the top off the lacquer for a contact point doesn't help, but it is the most direct way of providing the varying voltage. Cracking of the lacquer over time caused the OEM car manufacturers to stop using ammeters in their cars and they switched to voltmeters (because the resistance was very big or would fail open circuit). The cracks in the turns of the D"Arsonval movement caused I(squared)R problems where the resistance got very small which, would cause the power consumption to go way up. This would melt down vehicle wiring harnesses (yes, even with bypass style ammeters). Tons of vehicles from the 40's up into the 70s were sent to the scrapyard because the wiring harness would melt down when the ammeters shorted out.
When I was in college I had the fortune of attending one of their liquidation sales. Typically, they had one of these every so often, when they wanted to replace lab equipment with new, & had to spend allocated dollars lest the get less the following year (politics). At any rate, I managed to procure a Variac & a Multi-tape Isolation Transformer. Combining these two made for some interesting experiments!
That's one of life's mysteries solved. Reminds me of a large Tesla coil built by someone I met. He put two windings on the secondary so he could later remove one of them and leave a fine gap along the entire winding. This made the coil less capacitive, less inductive and less prone to internal arcing hence boosting the output. He made the mistake of testing it indoors which caused the arc to find a path to earth through the ceiling plasterboard to his domestic lighting and ring main cables.
His landlord was not amused.
My ZnSe 20mm (aliexpress, ~$5) lens attenuates the reading with about 2°C on the Seek Compact.
Made my Seek *very* useful with SMD work.
Oh FFS, now I have to rewatch all of photonicinduction's videos. Again.
The carbon brush on a variac touch / shorts always 2 or 3 windings at the time, this "shorting" forms a small voltage divider, if the brush was only touching 1 winding, there was between each winding a small disconnect and you get arcing on each movement, also the output would be very "jumpy" and not smooth when rotating the variac.
Due the contact of 3 windings at the time, there are no disconnects between each movement, because 1 winding in the middle of the 3 windings still keeps contact, the outer 2 windings are connecting and disconnecting during movement of the axle.
The dog licked my one and it broke. 🤣 As always a big fat 👍 for the COVID free big Clive! 👏🇬🇧
I hope you can fix your dog.
Use a glue stick on the build plate for the printer. It allows the print to stick firmly and releases easier when done. Or a flexible plate would work too
Yes, they short out the windings which is why it is nice when they are overbuilt like the old ones were, so that the thing can actually handle *real* usage...
The new-style Chinese units have a very narrow pointy-cone brush to help reduce the strain on the undersized windings but will never hold up over extended usage, especially as the brush wears down and more windings come into play on the "shorted turn" section.
There's a thread on the EEVblog about this, I'll see if I can find you a link...
I have a couple of very old 6kw autotransformers from an old theatrical dimmer board. The ends do terminate on one wire, but the brush does bridge more than one winding. Not sure what the brushes are made of (lost now) but I think there was some resistance across the brush itself being that it was carbon or some mixture of materials. Could be this micro-resistance is enough to not dead-short a winding. Otherwise, I would think it would heat up like an instant-on soldering gun.
I hope we get a short video of you replacing the display!
These are fantastic bits of kit, i like your idea of having the dc output aswell thats something i havent done to mine yet currently saving up for lots on contactors to modify mine further.
I definately would reccomend earthing if possible as the main spindle on these can have the tendancy to become live in some instances not common but it can happen, i would also reccomend maybe having a little pannel fuse built in fuse after the output of these to protect the variac just encase of short or overload on the load side as in some instances you can quite easiy flash the windings in no time at all leaving you wondering what the hel just happened l i was a bit naughty once with some destructive testing and managed to get mine a tad hotter than desireable in no time at all at the time i didnt have a bigger variac which was surely called for however i am looking into putting in some kind of thermal protection on on the windings however same variac thanks to someone whoever owned my variac before me did flash the windings and deemed it dead i was lucky enough to get it for peanuts and repair it on my smaller 4amp variac onthe principle you have explained perfectly of that some of the windings do indeed short out when the wiper arm passes over them...
Myself i use 20mm glass fuses in an unscrewable pannel fuse cutout i know i should really use ideally ceramic fuses because of the surge currents fuse design of glass ect ect but i use glass as theyre cheap as chips on ebay in packs of 100, on the larger one i use an appropriate sized mcb per phased variac.
I would potentially reccomend using an rcd before the variac preferably rcd plug on smaller more portable ones they work quite well even if you decide to be naughty and feed some equipment too many volts deliberately for some destruction currently looking into going as far as putting in a dedicated socket for the larger variac via rcbo ...
Sometimes throughout the year thanks to industrial upgrades ect you can buy three phase variacs for a fraction of the cost of a new single phase variac for absolute peanuts and perform a reconfiguration to turn a three phase one into single phase to gain an exponentially greater output cureent.
I was lucky enough to get a 3 phase 20amp for 'special uses' i changed the configuration on mine so that essentially as i power each of the three up i can have maximum amperages up to 20 / 40 / 60amp outputs 0-275v as mine has a little bit of over run after the connections i think photonic induction made a superb video going into the ins and outs of modifications and menantioning the over run on these variacs some years ago...
keep making the fab videos sorry about the waffle waffle :)
Would be interesting to hear you and Photonic Induction discuss this.
😠 - had to stop watching Photonic Inductions videos because of him swearing,
I have one of these capable of 4A. has pretty beefy wire to it as a result. I received it loose, and installed it into an enclosure I found in a stream. Doesn't have a meter, but its faceplate voltage indications are accurate enough for government work. I can use my own Hopi on the output if I need higher accuracy. I tapped it to accept American 115v and output up to 280v at full scale, but I have to be careful with the current, as I could hit 4A on the input pretty easily when stepping it up that far.
The idea of unsmoothed DC output is interesting! I might have to build a FULL BRIDGE RECTIFIER for mine. Probably as a module I can attach to the output cable...
I was thinking about this a few hours ago for the first time, wow
I've just got my 2 Kw Digital Variac from Amazon !!! It's a super device, but it's nearly as heavy as my Royal Enfield Motorbike !!!! I shall be using it with a Residual Circuit Device, for fixing power tools, motors and a bit of electronics !!!! Mine arrived in perfect condition. It only has 13 amp Ac output 0 to 240v which is just what I wanted !!!! But if there's a short it should trip the RCD unlike a Microwave Transformer which just kills you instead !!!! If the Brush sparks like a Bumper Car Pole in a Fairground, you might need to fit a new Carbon Brush !!!!!!
Does it really go up to 2000V? If so, be very cautious, as depending on the circuitry it may not trip the RCD.
I have a slightly chunkier one (~4A = 1kW) which has a copper roller on a pin sticking from the slider arm.. that way it makes sure to short out two adjacent windings at best.. and the 0.5mm² solid coil makes a nice ratchety effect while turning.
Speaking of bigger variacs i do have a 20kW variac and it is quite different from what you have here.
For starters it's not a toroidal transformer but a rectangular one, it's very long compared to its width.
Instead of carbon brushes it has multiple carbon rollers and the way is works is a little different from how your variac works, because the rollers make contact on multiple places on the transformer.
I can't remember exactly how it's constructed since i haven't looked at it for a while but i do remember that it does buzz quite loudly, which i guess is part and parcel of a big honkin' transformer.
Well since noone seems to mention it...:
The reason it's not a big deal is simple. P=U^2/R, where U is the voltage of a single turn and R is its resistance.
Both U and R are proportional to the number of turns you short out. But because it's U^2, the power dissipation becomes a lot bigger if you short more turns
Clive kinda did mention it. He showed the power measurement at no load. Just didn't mention why.
I have a few variacs of different power capabilities. On the small one (1 amp) the contact brush shows no lengthwise/widthwise resistance differences, but the ones I have that are over 1 amp have a definite difference in resistance. Maybe the small ones are too small to notice the issue? I'd also like to mention that my taps are all terminated to one wire. Those wires are brought out, given a few twists, and soldered to.
If you spray a thin layer of hairspray on the printer bed it will help with giving prints the right amount of adhesion for staying on put during printing but coming of easily once they are cooled. May require the bed to be heated, not sure. Using hairspray will also give prints a nice matte finish that better matches the finish of the rest of the print.
Uh oh. A Variac ,is he going photonic
It's only a 0-100% variac. It can't overvolt stuff any differently than he already could by plugging a 120v thing into 240v.
@@Ferro_Giconi More importantly, that thing is NOT big enough for any photonic level fun.
I have a couple variacs. One is 240 input with 5-240 output. Another is 115V input with 0-150V output. The third is 110v input and 6. 14. 24. and 0-40v output. Variacs are just basically an auto transformer with one tap that can move. Great bit of kit though.
It's easy for anyone to see this fact just by looking at the brush. The brush is wider than 2 turn surfaces. Note also that they turn / mill the windings flat for the brush; this is obviously necessary for brush movement, and while it may increase the wire's contact area, it drastically reduces the circular cross-section. So the classic name brand units at least (Superior, Staco, and GenRad) always seemed to use quite large wire for a given current rating. I've always assumed it was to compensate for that very problem. And the cheap chinese units do NOT do this, I've noticed they all run hotter. There's a reason used 50A name-brand units (very much in demand among Tesla coil builders, among a handful of other uses) are still breathtakingly expensive; there's a boat-load of copper in them!
The question is: WHY do they short the windings when clearly it is preventable? It's a current density problem, localized heating at the brushes. There isn't sufficient contact surface even after milling the wires to get the rated current into and out of the windings by using only one turn. So designers accepted the heating of one turn because it prevents even higher localized heating at the brushes.
@Railgap Esoterica, while I am a Luddite, this is the explanation that I have been waiting for. I hope that educated corroborators chime in... :-)