I want to switch 24V with a P-channel MOSFET through the digital pin of an Arduino.
I have the following questions:
Is the circuit basically correct?
The data sheet of the MOSFET states that max. Vgs = +-20V. Am I correct in saying that if I were to connect the gate to ground, the Vgs would be -24V and I would destroy the MOSFET?
Is it correct that Vgs = -12V due to my voltage divider and the MOSFET is therefore not destroyed?
Is there an easier or better way to achieve the same goal? If so, which one?
Do you have any other tips for the circuit?
Just for the sake of understanding, what would happen if I applied Vgs = +20V to the gate of the transistor?
Of course, this means that these values should not be exceeded. If they are, the component may be destroyed.
So Vgs should not be above +20Vgs and below -20Vgs.
As I understand it, Vgs is the voltage at the gate minus the voltage at the source.
This would mean that Vgs would be -24V when ground is applied to the gate. This would destroy the MOSFET, as -24V < -20V.
I think my uncertainty comes from the fact that in most examples the gate is simply connected to ground. However, the examples I found work with a source voltage of +12V. As I understand it, this would result in a Vgs of -12V, which is within the operating range of the MOSFET.
My voltage divider consisting of R1 and R2 applies a voltage of -12V to the gate. This is within the tolerance range. But is this correct? Or am I making a mistake somewhere?
There is an other, maybe more clever way of driving the P-channel fet.
Connect base of Q1 directly to the Arduino pin.
Use a resistor in the emitter of Q1
Now Q1 acts as a constant current sink.
Collector can be connected directly to the gate, for a higher constant drive current.
The resistor between source and gate of Q2 limits drive voltage, because of the constant current sink.
See diagram #4 on this page.
Nick added a zener for extra safety, but this was not really needed.
Leo..
When the NPN is Off, the Gate is basically at Source potential; and the FET is Off.
When the NPN is On, the Gate is at +12V (from GND), or 12V less than Source potential; and the FET is On.
When working with MOSFETs the reference is the source and the threshold voltage is defined that way Vgs (Voltage Gate Source). The circuit as shown is relatively common should work fine, I would suggest adding a zener from gate to source to protect from transients. It doesn't matter what is connected to what as long as the Vgs is not exceeded the MOSFET should survive assuming it is not overloaded with current.
