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: enter image description here

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?


  • 1
    \$\begingroup\$ R1 and R2 would be appropriate if that were a BJT, but not a FET. Does your 10k resistor need to be grounded? \$\endgroup\$
    – TimWescott
    May 17 '19 at 3:50
  • \$\begingroup\$ It's pretty typical to see gate resistance like R1 to prevent ringing and connections like R2 to ensure it switches off if PWM is undriven or disconnected. \$\endgroup\$ May 17 '19 at 4:04

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.


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