inverting or non-inverting transistor Fet driver?

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)?

If you are driving R2 with an Arduino O/P, the transistor will not turn on fully.

Don’t worry about "intuitive". :wink:

You can define a variable.

#define heaterOn HIGH
or
#define heaterOn LOW

digitalWrite(heaterPin, heaterOn);

lemming:
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..

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.

lemming:
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.

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.

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... :slight_smile:

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.