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Drive MOSFET with 555

  1. Feb 29, 2012 #1
    I've currently using a signal generator into the gate of a MOSFET to switch an inductive load.

    I've built a 555 circuit to replicate the signal generator output (TTL 5V 192kHz).

    When I connect this to the gate, the power supply driving the MOSFET limits the current and shuts itself down. I've tried adding a FWD to the output of the 555 but no success.

    Any idea how I can successfully replicate the signal generator output?
  2. jcsd
  3. Feb 29, 2012 #2


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    You would be using something like this:

    There is usually a diode across C3, cathode pointing upwards, but otherwise yours would probably look like that.

    Do you know the inductance of your inductive load? Do you have access to an oscilloscope?
  4. Feb 29, 2012 #3
    Thanks for your quick reply, the circuit is like that. It's 9.53uH, and yes I do have access to a scope.
    Last edited: Feb 29, 2012
  5. Feb 29, 2012 #4
    I haven't got C1 in my circuit, sorry. I hope thats not the reason for my error?
  6. Feb 29, 2012 #5


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    It would help, but it probably isn't why it is stopping.

    Have you had a look at the waveform coming out of the 555 when it is driving the Mosfet? There is a lot of capacitance at that input and it may not be a square wave any more.

    Are you sure about the frequency? 192 kHz has a period of about 5.2 μS.

    Are you running the Mosfet off 5 volts or 12 volts?

    Would you have two different supplies you could run the 555 and the Mosfet from separately, just as a test?

    Do you have a high powered 22 ohm resistor you could substitute for the inductor? About 10 watts?
  7. Feb 29, 2012 #6
    Ay the input to the gate the signal has a small oscillation at the rising edge but settles fairly quickly. I've measured the period and it appears to be correct.
    The MOSFET is running off 15V and the 555 is going through a 5V regulator, I tried two separate supplies but it made no difference.
    I have a resistor like that but haven't tried it, what would it tell us?
  8. Feb 29, 2012 #7


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    OK on the tests, you seem to have most of it OK.

    The resistor would give you a chance to measure the drain voltage waveform without the power supply shutting down.

    If the resistor fixed the shutting down, you could introduce the inductor in series with it and measure the voltage distribution between the two.

    9 μH isn't much and has only 11 ohms reactance at this frequency. So, it would be worth trying to limit the current until you find out what is going on.
    Last edited: Feb 29, 2012
  9. Feb 29, 2012 #8
    Thanks for your help. I'll try that when I get back to the lab tomorrow to see what I find out.
  10. Mar 1, 2012 #9
    I tried with the resistor and it behaved the same. Any further ideas?
  11. Mar 1, 2012 #10


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    With that resistor, the circuit should only be drawing half an amp.

    If the power supply has current limiting, maybe it is set too low? Try putting that 22 ohm resistor across the power supply output and see if that is enough to limit the current?

    Otherwise, it sounds like a fault or a wiring error in the circuit.
  12. Mar 2, 2012 #11
    You might try a reverse-biased diode across the inductive load (pointing up in the schematic shown) to clamp back-EMF kick when the FET shuts off.

    Also, to my I'm shaky FET knowledge, only an _Enhanced_ N-channel FET will switch correctly in that circuit -- they work just like switching bipolar NPNs -- other FETs need different biasing. So your circuit may be switched ON all the time...
  13. Mar 2, 2012 #12

    i've tried changing the limit on the power supply, and tried a different power supply, it doesn't seem to make a difference.

    I'll try putting the resistor straight across the supply.


    My first thought was to try the protection diode, but it stopped the transfer between my mutual inductors from working. So that's a no no.

    As for the FET switching, it's an IRF630, it seems to be working fine with the signal generator?
  14. Mar 2, 2012 #13


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    You mentioned earlier that you didn't have a bypass capacitor across the 12 volt input.

    If you haven't fixed this, you need to.

    At 192 KHz, you will need at least 1 uF, preferably not an electrolytic, as well as a large electrolytic of at least 100 uF.

    These capacitors would be in parallel and rated for at least double the supply voltage.

    Without bypassing, it is possible this 192 KHz could get into the power supply and disrupt the operation of its regulators.

    Also, 5 volt regulators are not as simple or innocent as they may appear. They are quite capable of oscillation if they are not wired or bypassed carefully, and when they do, they can cause chaos with volts of square wave being injected into a power line.

    So, you always need to check the operation of any regulators, under load.
  15. Mar 7, 2012 #14

    Could a pull down resistor help with the gate on the MOSFET? It may not be fully discharging (thus always on).
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