Pentax Spotmatics: The Bridge Circuit
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- Опубликовано: 21 окт 2021
- We've got a great tip for photographers getting into M42 mount cameras and wanting to use internal light meters - Get yourself a Spotmatic!
Spotmatics contain a bridge circuit that allows the light meter to operate independent of battery voltage. This will allow you to use cheap and readily available silver-oxide batteries at 1.55V and still acquire accurate light meter readings!
Disclaimer: I am not an electrician and I may very well be wrong in my interpretation, I know this because I was wrong before! and I’d like to shoutout and say thank you to @stevebrot for pointing me in what I now think is the right direction!
Also, my utmost gratitude and appreciation for Luis lending me his excellent Pentax collection: @PentaxLou // Insta: Spaceman_Lou // flickr.com/photos/picsfromthebirdcage/
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All want to know is what battery works with the pentax spotmatic
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@@ht8764 thankyou
A 12v car battery !!!!!. Why are you so lazy look it up online.
@@mikepxg6406 abit late informing me now
Thank you for the video! It's very informative.
Shame its the wrong circuit.
Useful video, I didn't know those light meters worked like that. Applies to the KM and K1000 (derived from Spotmatics) too I guess.
Re: terminology, I would say that a galvanometer is the central mechanism : the moving coil, magnets and needle. A voltmeter is a galvanometer plus series resistors, dial, casing etc and an ammeter is a galvanometer plus parallel resistors, dial, casing etc. The thing inside the Spotmatic is a galvanometer plus Wheatstone bridge, photocells etc to make a light meter. There are ammeters and voltmeters with central zeros that can read positive and negative, I've seen plenty in industry.
Thank you for sharing that! I’m definitely no electrician, I’m trying to learn all this second hand really :)
No the SPF and K1000 KM KX use a different circuit with a 3 wire dual coil galvanometer its more complex and slightly more accurate across the scale. You can download service manuals for free then take a look yourself. :)
I'm no engineer, just an electronics tech but looking at the design of the Spotmatic F metering circuit, I question the notion that it isn't also able to tolerate a silver oxide cell.
The dual-coil meter is essentially forming two upper legs of the wheatstone bridge, and a variation in circuit voltage would only (as far as I can judge from the diagram) result in reduced/increased deflection to the side (just like the original), but the centre position would remain accurate - when the battery is higher voltage, the "correct" exposure is still when the current through each coil is equal.
The dual-coil system is essentially the same in principle as the earlier design but simplifies the parts count. Both bridge designs are reliant on balancing the current through each branch, the only difference is that the earlier system is measuring the consequent voltage difference induced by the differential current flow, and the newer design just measures the differential current flow directly.
I just got a Spotmatic F which appears to work ok with alkaline 625GH battery at 1.5V...
@@koby1960 Yeah, it should work on anything within a reasonable margin of 1.3-1.6V
The early spotmatics have a single coil 2 wire Ammeter design, later ones like SPF have dual coil 3 wire galvenometer design. Best to do your homework before writing.
@@mikepxg6406 I appreciate you looking to correct me here, but the two-wire and three-wire meters were precisely what I was talking about.
The early two-wire design uses a typical wheatstone bridge design of four resistances and and a galvanometer to measure the difference in voltage between two nodes to determine the difference in current flow between each branch of the bridge.
The three-wire design effectively substitutes two legs of the wheatstone bridge with each of the two coils, directly measuring the differential current between each leg of the bridge.
The video asserts ( 7:30 ) the latter design isn't a bridge-type (and is rather the less robust 'calibrated for an expected voltage' style circuit) and would be less tolerant of different voltages than the earlier design, but I don't see any justification for that argument - they're both bridge-type differential circuits.
Rotating the SP:F schematic clockwise helps make it more visually obvious; it's two parallel current paths, with the reference resistances on one side and the metered brightness on the other.
If I've misunderstood you here though (or you can tell me how I'm off base with this assessment), please let me know.
Massive error. you have drawn the circuit completely incorrectly. the wiper of the shutter/ASA potentiometer is connected to A, there is a ground connection on this potentiometer connected via a screw to ground the third connection on this potentiometer goes to the meter switch and back to the battery. RX on your drawing should be a pair of CDS cells and R3 is the calibration resistor set at factory. Also the spotmatic battery has a positive ground and the negative is switched the current flows in the opposite direction. Why don't you download a spotmatic service manual and check out a real schematic.