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Topic: inverting or non-inverting transistor Fet driver? (Read 1 time) previous topic - next topic

lemming

Most circuits for driving a fet show the inverting setup as show below.







I would like to keep the logic intuitive by having the fet output go high when the micros pin goes high. Are there any pitfalls using the following circuit (I would use 10k for R1)?


larryd

#1
Apr 17, 2018, 04:16 am Last Edit: Apr 17, 2018, 04:18 am by larryd
If you are driving R2 with an Arduino O/P, the transistor will not turn on fully.


Don't worry about "intuitive".  ;)



No technical PMs.
The last thing you did is where you should start looking.

larryd

#2
Apr 17, 2018, 04:27 am Last Edit: Apr 17, 2018, 04:27 am by larryd
You can define a variable.

#define heaterOn HIGH
or
#define heaterOn LOW



digitalWrite(heaterPin, heaterOn);



No technical PMs.
The last thing you did is where you should start looking.

Wawa

I would like to keep the logic intuitive by having the fet output go high when the micros pin goes high.

Are there any pitfalls using the following circuit (I would use 10k for R1)?
Something you just have to get used to.

1) IRF520 (typo?) is not logic level, so will struggle with 5volt logic (circuit#1 would be better for that one).
2) Emitter follower drive circuit 'steals' another ~0.65 volt (BE junction) from the gate drive voltage.
Circuit#2 would be better without the drive transistor (and with a logic level mosfet).
Leo..

lemming

#4
Apr 17, 2018, 08:12 am Last Edit: Apr 17, 2018, 08:13 am by lemming
Thanks guys. Yes, its meant to be IRF520. and the second circuit would have 12 volts at the collector of the the transistor (and the source of the FET) so the voltage drop would not matter. I would like to open up my options for more suitable fets without being limited to logic level ones.

septillion

and the second circuit would have 12 volts at the collector of the the transistor
You, but the emitter will always be lower than the base (around 0,7V even). So the emitter can never reach even 5V.
Use fricking code tags!!!!
I want x => I would like x, I need help => I would like help, Need fast => Go and pay someone to do the job...

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TomGeorge

Most circuits for driving a fet show the inverting setup as show below.







I would like to keep the logic intuitive by having the fet output go high when the micros pin goes high. 

That circuit works as follows.
Arduino OUT = LOW, Q2 OFF, Gate of T1 pulled HIGH to 12V wrt Source, Drain of T1 = LOW (0V)
Arduino OUT = HIGH, Q2 ON, Gate of T1 pulled LOW to 0V wrt Source, Drain of T1 = HIGH (12V).


Where is the inversion?


Unless you talk about LOAD current with Low Side Switching
Arduino OUT = LOW, Q2 OFF, Gate of T1 pulled HIGH to 12V wrt Source, Drain of T1 = LOW (0V), LOAD Current flows.
Arduino OUT = HIGH, Q2 ON, Gate of T1 pulled LOW to 0V wrt Source, Drain of T1 = HIGH (12V). No LOAD Current flows.


Which is the OP talking about.


Tom... :)
Everything runs on smoke, let the smoke out, it stops running....

MarkT

Use a MOSFET driver chip, not a transistor, they are available inverting and non-inverting and do a better
job.  They are also as fast as a MOSFET, not an order of magnitude slower, which is a real advantage when
using PWM at any significant power levels where switching losses can be a real issue.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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