Understanding MOSFETs: MOSFET as a Switch (Supplemental Video)
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- Опубликовано: 5 фев 2025
- This is a supplemental video for the blog post at motbots.com titled "Understanding MOSFETs: Key Concepts and Practical Examples".
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UPDATE: As of 01/29/2025, it’s been brought to my attention by @RexxSchneider that a mistake was made on this example circuit on the website’s page titled “Understanding MOSFETs: Key Concepts and Practical Examples" (link below). THIS MISTAKE HAS BEEN CORRECTED ON THE SITE.
The mistake does not affect the making or working of the circuit in this video, but some of the resistors used here now have different values than represented on the website, so any mention of calculations here won't be the same as on the site. Also, the mention of using the multimeter has been disregarded in the updated example on the website and other information has been given. Sorry for any confusion or inconvenience. Thanks for your understanding. Enjoy 😊
P.S. I will link the updated video for the website example in the description of this video when it becomes available. Thank you.
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In this video we go over each component that has been set up on the breadboard. The website explains a problem in detail on a circuit similar to this one. If you're interested in learning more, please visit the website using the link for that example and for the subject matter on MOSFETs below:
PARTS LIST:
motbots.com/un...
SCHEMATIC:
motbots.com/un...
Video for Our Website Page: "Understanding MOSFETs: Key Concepts and Practical Examples"
Page URL: motbots.com/un...
Description: This is a supplemental video for a post made on our website.
Website: motbots.com
Also check out our page where we ask and answer 12 essential questions about MOSFETs.
Understanding MOSFETs: 12 Essential Questions Answered:
➡️ motbots.com/un...
#motbots #motivatedrobotics #learnelectronics #electronicsproject #mosfet #switch
Thanks to @RexxSchneider for pointing out a mistake I made for this example circuit on the website. As mentioned in the description of this video, the mistake has been corrected on the site. Please read the UPDATE portion of the description of this video for more information. I'm very appreciative for people like Rexx who help in pointing out mistakes and take the time to help in correcting them. Time and knowledge are valuable assets, and sharing both are much appreciated. Mistakes are opportunities to learn, and boy do I make a lot of mistakes at times! Hopefully you all are learning everyday, as I try to.
I just took a look at your blog post and unfortunately, you have a misconception about about how a mosfet behaves in the saturation region. The voltage from gate to source is _not_ equal to the the voltage from drain to source when passing current. You can see this by checking the voltage you think is at the gate when the switch is closed. You incorrectly suggest that Vgs = 2.9V because Vds = 2.9V, and you correctly calculate the voltage at the drain (i.e. the voltage across R3) to be 4V. That would mean the voltage at the gate would be 4V+2.9V = 6.9V. But you also calculate the voltage at the junction between R1 and R2 to be 3V, and that is also the voltage at the gate. It cannot be both 3V and 6.9V.
When the mosfet is in the saturation region, the drain current is essentially independent of the voltage across the mosfet. If you raised the supply voltage V2 by 3V to 12V, the current would be unchanged, so the voltage across R3 would remain at 4V and the voltage across the LED would remain at 2.1V, so Vds would increase by 3V to 5.9V. But clearly Vgs doesn't change because Id doesn't change. Your assumption that Vgs = Vds is unfounded.
You will find that the threshold voltage is fixed for a given sample of 2N7000 at somewhere between 0.8V and 3.0V, and at 20mA will require an additional 150mV or so. Depending on the sample, you will find the source is at somewhere between 0.95V and 3.15V _below_ the voltage at the gate, and that determines the current flowing through R3. The voltage at the gate is indeed 3V if you use R1=1K and R2=500R, so the voltage across R3 can be anywhere from 0V to 2.05V, resulting in a drain current of anywhere between 0mA and 10mA, depending on the sample.
I would recommend that you check the voltages across each of the resistors with a multimeter, as well as checking Vgs and Vds across the mosfet, and then calculate the drain current. It should give you a clearer idea of how the mosfet behaves. Hope that helps.
You are absolutely correct my friend! Thank you so much for pointing out my mistake, and what an awful mistake it was! I've promptly corrected the mistake on the website and will be needing to update this video in the near future. I'll have to be sure to tag you and mention you for observing my mistake and helping correct it in the next video. It's people like you I'm truly grateful for who watch out for mistakes like this and from whom we can all learn from. Again, thank you so much. You are greatly appreciated!
@@motbots You're most welcome. Thank you for all the effort you've put in to your website and this video. It is appreciated.