I am wanting to control a voltage range of 0-10v across a 10k load with PWM from a micro. Now I was told to use this circuit with an n channel mosfet:
This is what I understand so far: R1 is to reduce current flowing into the base R2 is used to pull mosfet closed when its not conducting
I was wondering if I could have help understanding what R3 is doing?
Cheers
R3 is a pullup resistor. When the transistor is not conducting, R3 provides 10V to the load. When the transistor is conducting, R3 prevents the transistor from shorting 10V to ground. It can't be too large, because current to the load has to flow through it when the transistor is off, but it can't be too small, because current to ground will flow through it when the transistor is on (which will waste power and create heat.)
I am surprised the circuit is wired this way, instead of putting the load directly between the transistor and the 10v supply. That approach seems more normal to me. However, it reduces the voltage to the load by the voltage drop of the transistor. But it also provides a lot more current to the load than you would normally want to allow through R3.
Transistor switched on: current goes thru R3 to GND -> your load is switched to GND
Transistor switched off: current goes thru R3 and your load to GND -> you got a voltage divider. e.g. if R3 = Rload your load is switched to 5V.
After all as @Glenn mentioned this is not a really good solution. Better put your load at the position of R3.
