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Topic: Understanding Mosfets (Read 192 times) previous topic - next topic

abishur

I've been trying to learn about Mosfets and feel like I'm missing something important.  By all respects they seem like a way to control higher voltages using the Arduino without having to worry about current draw form the arduino like you would when using a transistor.

But whenever I look at enhancement N Mosfets that come in multiple channels I almost only find common drains.  What is the point of this???  Drain is what I connect my load up to, so why would I want to be able to a single load using two different gates on the Arduino?

Am I just misunderstanding how an N type mosfet is wired?  Source goes to ground, Drain goes to load, and Gate goes to my Arduino, right?

I know I'm missing something, but I feel like I'm going crazy as I can't figure out what the benefit is from being able to turn a load on using two different pins on the Arduino.

saildude


abishur

Thanks for the response, but no it doesn't help :) I understand the basics of how to wire it and make use of it, my hold up is understanding why when it comes to multi-channel N mosfets why is that by far and large they tie the drains together?

I can find some multile N channel mosfets that have nothing tied together, it's rare but I can find N channel mosfets that tie source together (which makes sense if I'm using it as a switch), but why is it that most of them tie the drains together?

I'm thinking if I understood that, I would have a better knowledge of what mosfets are really useful for.

JCA34F

#3
Sep 15, 2020, 11:42 pm Last Edit: Sep 15, 2020, 11:42 pm by JCA34F
Which MOSFET? There are hundreds. Post a part number.

hammy

#4
Sep 15, 2020, 11:52 pm Last Edit: Sep 16, 2020, 12:03 am by hammy
Some are used in precision bridge rectifiers and that could be a reason .

( you dont get diode voltage drops)

Try looking for " FET array" it might be more successful ? They are out there , I just found a couple with all pins bought out

Paul__B

Sorry, but until you cite (Web link) the actual datasheet for one of these curious FETs to which you refer, this is all nonsense!  :smiley-roll:

JCA34F

#6
Sep 16, 2020, 12:06 am Last Edit: Sep 16, 2020, 12:09 am by JCA34F
Are you confusing multiple drain connections with the common drain (source follower) amplifier configuration?


abishur

Which MOSFET? There are hundreds. Post a part number.
Well I was asking about any given multichannel N Mosfet where the drains were tied together as that seemed to be the type I was encountering the most (ergo it stood to reason that this was the most commonly used type and there had to be a reason for that)... but to pick one at random, https://www.diodes.com/assets/Datasheets/ds31995.pdf

Now it might just be that as I was looking around I happened to hit only references to common drain type mosfets causing my beliefs to be skewed.  BUT, if common drain types are the most, well, common there's a reason and I'm trying to understand that reason so I can better understanding mosfets.

abishur

Are you confusing multiple drain connections with the common drain (source follower) amplifier configuration?


based off the included picture, I do not believe so.  The ones I saw were all 2 gates, 2 sources, and a common drain between them.  From my basic level searching I had come to believe this was common (as it seemed any time I did a search for (2 channel or dual channel N mosfet) the common drain was the one everyone was talking about.  However, based on the responses here I'm starting to doubt that belief, lol.

CrossRoads

That was the first time I had seen a configuration like that.  I cannot think of a design that would take advantage of that.
I could see connecting the gates and sources together and just using it as a low Rds, Logic Level,  SMD N-channel MOSFET.


Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.


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