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Topic: [sorted] Tesla member but never used a MOSFET: clarity please? (Read 4676 times) previous topic - next topic

nickgammon

Not "wrong" and generally workable, just not best design.

Best design should be the goal.  (Ignoring the other argument re series resistor.)

According to my calculations that voltage divider would reduce 5V output from the Arduino to 4.926V at the MOSFET. That's the objection, I gather?
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

nickgammon

There's a bit of discussion here.

There seems to be a bit of a feeling that unless conditions are marginal (ie. you need 5V rather than 4.9V) it wouldn't make much difference.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

Paul__B

According to my calculations that voltage divider would reduce 5V output from the Arduino to 4.926V at the MOSFET. That's the objection, I gather?
Indeed.  Why design a circuit with any performance restriction which can be avoided by simply doing it right (given that there is absolutely no performance, cost or risk penalty whatsoever of doing it right)?

Now the point here is also about the mind-set.  What is the problem?  Well, the problem is that in the reset phase, the Arduino outputs are open circuit.  It is incidentally, not a problem when it is unpowered as the protective diodes pull the lines down to the unpowered VCC rail.  But this is a problem because a floating gate on the FET may turn it at least partially on.

Here's the trick:  Is this a fault of the FET?  No!  The problem belongs to the Arduino (ATmega), so the correct approach is to pull down the Arduino pin, not the FET gate.  If you think by this stepwise reasoning, it is pretty obvious that the pull-down should be on the Arduino side.

Now here is an interesting muse on the matter of whether the series resistor is desirable.  What could be wrong with inserting a resistor in series with the gate?  Well, it is suggested that this will slow the transition of the gate voltage through the linear conduction region and having the FET only partially conducting will increase its power dissipation during the transient.  Which is indeed correct.  So then this argument, given the description of how brief this charging transient is, is inherently suggesting that it is more important to minimise the transient dissipation in the power FET, than it is to minimise the transient dissipation in the ATmega output driver.  So - which is it now?  :smiley-eek:

JimboZA

By the way, the [sorted] in the title meant it was sorted as far as I was concerned; the rest of you feel free to carry on. The threads paid for....  :P
Johannesburg hams call me: ZS6JMB on Highveld rep 145.7875 (-600 & 88.5 tone)
Dr Perry Cox: "Help me to help you, help me to help you...."
Your answer may already be here: https://forum.arduino.cc/index.php?topic=384198.0

nickgammon

Quote
Here's the trick:  Is this a fault of the FET?  No! 
The gratifying thing about these threads is that we progress from a good solution to a very good solution. I think I see your point here. The problem is that the Arduino pins are floating, not that the FET pins are floating.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

nickgammon

I've changed my schematic, which unfortunately means this thread makes less sense, but I have posted the image in a few places now, and would rather that the (more) correct one appears in all those places.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

pito

This forum shall have some educational impact on the people who start with electronics (as the most people who are asking here do not posses such background, and, some who give an advice do not have too), so a good engineering practice has to be evangelized.

Wiring a power mosfet directly to a logic output is not a good practice, even it may work somehow.

The same as with an LED - most people think they can simply hang an LED to an atmega output because the output is "40mA" (it is not) and an LED is "20mA" (it is not).







Paul__B

I've changed my schematic, which unfortunately means this thread makes less sense, but I have posted the image in a few places now, and would rather that the (more) correct one appears in all those places.
Did you really drop R1 to 10 Ohms?

nickgammon

That's odd. I somehow reverted to an older schematic in ExpressSCH. I've updated the image now to be more consistent with the former version.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

Paul__B


nickgammon

I uploaded a fixed version. You might need to refresh the page.
Please post technical questions on the forum, not by personal message. Thanks!

More info: http://www.gammon.com.au/electronics

Paul__B

I uploaded a fixed version. You might need to refresh the page.
Interesting - caching problem!  I had refreshed the page (in order to see the most recent replies, and the page was still in the tab in contrast to navigating from the index), but it was on a different machine which presumably was holding the original graphic.

Trick is - I browse here on no less than five machines at different times and days.  :smiley-eek:

Frankly, I am getting somewhat cheesed off with Firefox, considering moving to Chrome.  Very slow and faltering performance, even worse on Windoze.  The only reason I stay with it is the familiar and essential add-ons - NoScript, Adblock Plus and Tab Mix Plus, but I understand these are all available on Chrome so ...

And Firefox on Mint baulks at IView for some reason related to the rotten DRM.

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