SgtWookie's Simple PWM Circuit for a P-Channel driver

Discussion in 'The Projects Forum' started by zebbo, Sep 11, 2011.

  1. zebbo

    Thread Starter New Member

    Mar 28, 2010
    First, thanks to SgtWookie for his advice regarding posting my problem/project. Much appreciated as I'm a newbie around here.

    I'm trying to construct a circuit to drive a 12 volt automotive cooling fan. Hence I plan to use a P-Channel mosfet as a high side switch, driven by a suitable PWM circuit (I need to do this because the fan motor has a chassis earth to negative)

    I have tried a 555 based PWM circuit (as attached but driving a p-channel mosfet, high side, a schottky for D2 and a 47ohm gate resistor), but the mosfet overheats - due, I suspect, to slow switching. I also tried a totem pole drive circuit, connected directly to the 555 output, but the high current from the 555 fried the totem transistors.

    I came across SgtWookies "Simple PWM Circuit" and was wondering if I could use this to drive the mosfet - perhaps even via a dedicated driver chip (I can easily source a TC4428A for this purpose)

    (original circuit at

    Any help would be appreciated

  2. wayneh


    Sep 9, 2010
    The circuit you attached, are you saying it overheats the IRF540? Is your Vcc really 12v, or 6v, and what is the voltage of the output, ie. the gate voltage when "on"? Anything less than 10V or so will not turn it fully on.
  3. shortbus

    AAC Fanatic!

    Sep 30, 2009
    wayneh, he's using a P channel mosfet, doesn't the gate need to got to zero volts to turn on, when the source is at 12V?

    Just looked at the data sheet, duh, OP is calling it a P channel and it is a N channel?
  4. SgtWookie


    Jul 17, 2007
    Sorry Zebbo, but this places your circuit outside of what we can discuss on the forums. Automotive modification discussions are prohibited according to paragraph 6 of the boards' Terms of Service.

    Even experienced, professional automotive engineers make mistakes that cause injuries and property damage. I don't know of anyone on the forums who is an automotive engineer. Trying to make such modifications yourself will most likely result in early failure, and overheating of your engine.

    Really, the only thing that we can suggest is to maintain your vehicle as it was delivered by the manufacturer.

    Besides, the diode and MOSFET that you are trying to use are woefully inadequate. Cooling fans can draw in the neighborhood of 30 Amperes, and your chosen devices are not even close to those specifications.

    Also, the nature of automotive electrical systems is such that you will have "spikes" up to 60v and higher that will destroy typical hobbyist circuits in the blink of an eye.

    Combine the noisy electrical environment with the very broad temperature ranges involved, high shock/vibration, exposure to the elements, caustic fluids, etc. and you have a very tough challenge to create a reliable system. This is partly why we don't support such discussions.
  5. zebbo

    Thread Starter New Member

    Mar 28, 2010
    Ok - first, although the fan I am driving is from an automobile, I'm not using it as such (I'm actually building a ventilation system into a caravan). The reason I want to vary the speed is so I can regulate the ventilation.

    I'm mounting the fan at the front of the van, directly to the steel chassis. It will only be used when the van is parked.

    Also, I'm driving the circuit with 12 volts (my van uses 12v electrics).

    I did mention I'm using a p-channel mosfet - an IRF9540N (to repeat, I'm only using the DRIVE part of the circuit attached to the OP - not the mosfet).

    I'd still be interested in any positive suggestions related to the mosfet driver circuit.
  6. Wendy


    Mar 24, 2008
    They make much better MOSFETs, and they are common and cheap. Look to your local vendors, that or post an part you think might work and ask.
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
    @zebbo - your circuit won't work for a P channel mosfet. The P mosfet has to have a negative gate voltage to turn it on.

    Your schematic through me off, it shows a IRF540 not a IRF9540