Driving a mosfet

Discussion in 'General Electronics Chat' started by rjjenkins, Dec 29, 2012.

  1. rjjenkins

    Thread Starter Member

    Apr 16, 2011
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    Hello

    I am trying to get a mosfet to drive a DC heating pad. The pad is essentially just a resistor of about 10 ohms and I want to switch 9V to it. I am using a Stp36nf06l Mosfet and the circuit is attached - it couldn't be much simpler. I want to turn on the mosfet with a remote control unit, the R06A, which is also simple: when the rx unit is on, the output goes high to 5V. That bit works fine, so long as the output is not connected to the mosfet.

    For some reason, the output won't turn the mosfet on. The current through the heating pad, which should be about 1A, fluctuates round about 0.15A. The voltage on the gate fluctuates round about 0.6V. I suspect that actually the rx output is going on and off rapidly - too fast for my meter to register. The voltage from the battery doesn't drop noticeably.

    The version of the R06A I am using has the "momentary" chip, so the output remains high so long as the button is pressed. Oddly, if I use the toggle version of the R06A it all works fine, which is pretty baffling (to me)

    I can get the momentary R06A to switch on a reed relay (with a transistor) and use the relay to turn on the mosfet but that seems a pretty silly way of doing things.

    Any ideas, please?
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Your Mosfet conducts very well when its gate is +5VDC.
    But if your remote is turning it on and off many times quickly then its average on time is low. The gate-source of a Mosfet is a high value capacitor that needs a high AC current to charge and discharge quickly.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Try putting a small value (22-47) ohm resistor in series with the mosfet gate.
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
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    90
    Bite the bullet and get a mosfet driver chip. It will supply the gate current needed to turn on the mosfet quickly and also shut it off. I don't think a remote control receiver has the cajones to properly drive mosfets.
     
  5. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Thanks -

    Audioguru - I expect you are right but the question is why the remote is turning itself on and off quickly. If it is not connected to the mosfet, then it doesn't do this, it just stays on. It's as if drawing any current from the remote causes it to switch off. But I didn't think that a mosfet would draw current at the gate.

    tubeguy - is your suggestion to reduce the current drawn from the remote? I will try it; but


    Jaguarjoe - this is supposed to be a logic mosfet so I thought it would not need a lot of current to drive it, but I am willing to try a driver chip if necessary: what do you recommend?
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Are you sure that the remote transmitter isn't sending pulses, which are simply being reproduced by the receiver? And perhaps the pulses are rapid enough that the reed relay doesn't have time to release between pulses, but the MOSFET does?
     
  7. gootee

    Senior Member

    Apr 24, 2007
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    You seem to have found the solution in the first paragraph above. And then you found another potential solution in the second paragraph above. Yet at times you appear to be thinking that the mosfet should simply turn on and stay on, when you send a momentary signal, which is probably a pulse train.

    It looks like you will need to temporarily latch (or maybe just low-pass filter) the receiver output and use an edge of the result to toggle the state of a relay or a flip-flop that controls the mosfet gate voltage. You might need a buffer/amplifier/driver or two.
     
  8. Jaguarjoe

    Active Member

    Apr 7, 2010
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    A LL MOSFET, as previously posted, works well at 5v. The problem is overcomming the gate capacitance. It takes a lot of current to quickly charge that capacitance so that the MOSFET turns on quickly. If it does not turn on quickly, it will glide through a region where it is neither on nor off creating heat, sometimes lots of it.
     
  9. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Yes that is possible. However, when I measure the raw output from the rx with a meter (i.e. when not connected to the mosfet) there is no pulse visible - it appears continuous (so long as the tx button is held down). Would you expect this if it's a pulse train?
     
  10. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    I meant that the mosfet should stay on so long as the tx button is pressed

    So I could maybe use a 555 to go high for (say) a second once it gets a leading edge... although IIRC every 555 circuit I remember seeing is triggered by the leading edge going low. But no doubt that is solvable

    Frustratingly, the problem is v erratic - sometimes the circuit works fine for a bit then it gives up. Also, even a reed relay doesn't always hold but sometimes clicks on and off.
     
  11. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Sorry to be dense but are you saying that even a logic mosfet might need a driver? If so, can you suggest one?
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    No.
    Most Mosfets need a gate-source voltage of 10V to completely turn on. Some of them are barely turned on (a current of only 0.25mA) when their gate-source voltage is 4V (the maximum threshold voltage for most Mosfets).
    For a Mosfet that turns on well with a gate-source voltage of 5V then you need a "logic-level" Mosfet.
     
  13. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Try putting a small cap (.1μF) in parallel with the pulldown resistor. And as previouslly suggested, a small resistance in series with the gate might help. As to a MOSFET driver, I have built lots of circuits where the MOSFET is driven directly from and op amp or a microcontroller with no need for a specialized driver.
     
  14. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I think JaguarJoe's post means that a logic level (A LL) MOSFET will work with 5V at the gate.
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    LL means Logic Level in Text Speek?:eek:
     
  16. gootee

    Senior Member

    Apr 24, 2007
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    Your meter is probably low-pass filtering what is probably a pulse train. Either way, you said you only get 0.6 Volt at the gate (according to your meter), with the button held down. But would you really want to hold the button down, to keep the heater running? I assume not. If the receiver contains a toggle function that the other type of remote chip would have used, then you got the wrong remote chip. But you can implement your own toggle function, as has been mentioned. On the other hand, if the remote's output is what would have been toggled, with the other chip you mentioned, then you probably would not want to use that, if the heater is to be left running for any extended period of time, since that would tend to drain the remote's battery, and the remote would have to stay in proximity to the heater, etc.

    If you still want to find out what's happening when the button is held down, use your 9V supply to make an op amp (or discrete transistor) buffer amplifier (with a high-impedance input) with its input connected to the receiver's output, and see if you can set the amplifier's gain high-enough to boost the signal until your meter reads 5 Volts, for the amplifier's output.

    If you can't get to 5 Volts, even with very high amplifier gain, then it probably means that the receiver output is a pulse train and the duty cycle is too low to make your meter register 5 Volts when the amplitude of the pulse train is <= 9 Volts.

    If you CAN get the amplifier output to register 5 Volts on your meter, you could try connecting it to the MOSFET's gate, assuming 9 Volts or somewhat less won't hurt the MOSFET. If you wanted or needed to, you should then be able to insert a low-pass RC filter, to get rid of the up-to-9 V peaks and the pulses and make the output a more-or-less constant DC value, around 5 Volts. (You could try putting the filter on the input side of the amp, or the output side, or you could make the amplifier be an active filter with gain.)

    At any rate, if you don't want the heater to work only when the remote button is held down, you will need to create some sort of toggle circuit that applies 5 V to the gate.

    As jaguarjoe mentioned, you don't want the gate voltage to linger anywhere between 0 and 5 Volts, since the mosfet's Rdson would then be relatively high, compared to the Rdson with a 5-Volt gate voltage, and a lot of heating of the mosfet could be the result.

    Edit: I didn't check the datasheet of your particular MOSFET. I thought someone said it was a logic-level mosfet, which needed 5V to turn it fully on, which is what you need to do. If some other gate voltage is required, to turn your mosfet fully on, then you need to use that voltage instead of 5 Volts.
     
    Last edited: Dec 31, 2012
  17. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Yes that is what I want - not the toggling.

    Thanks for the other ideas. I feel that the best solution (at least the one I will try next) is to put a 555 monostable between the rx output and the mosfet.


    point taken


    Yes it is a LL
     
  18. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    I'll give it a go - the series resistor on its own didn't seem to have any effect
     
  19. gootee

    Senior Member

    Apr 24, 2007
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    I think that you should consider buying at least a cheap, used oscilloscope. It seems like you could put one to good use. If nothing else, at least download one of the free ones that runs on your PC and uses the sound card input, for low-frequency stuff, and playing around.
     
  20. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    I think this might be the decoder chip on the R06. Found a clear enough photo of the R06 to get the chip #

    http://www.adafruit.com/datasheets/PT2272.pdf

    Per pg 6 you can drive an led thru a 560 ohm res for testing.

    It would be interesting to measure the voltage on the output pin while driving the LED,
    Both AC and DC.
     
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