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mosfet help

Discussion in 'Electronic Basics' started by Tristan, Jan 6, 2004.

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  1. Tristan

    Tristan Guest

    I would like to make a mosfet switch using a 555 timer connected to a
    4017 counter and then use the signal given to me by the counter to
    open and close a MOSFET switch. I would like to switch a reasonably
    large load (about 5amps at 10 volts). Can anyone give advice?

    In particular I am having problems controlling the MOSFET devices
    (philips BUK456-60a/b). They don't respond to the counter signal and
    even when I disconnect the counter from the MOSFET - so that no signal
    current is supplied – the MOSFET still allows the load current to
  2. John Fields

    John Fields Guest

    If load current flows flows with the driving source (the counter)
    disconnected from the MOSFET's gate, then that means that the gate is
    floating in a charged state, allowing the MOSFET to stay turned on.

    Connect the gate to ground when you disconnect the counter and, unless
    the MOSFET has been damaged, the load current will drop to zero.

    Assuming that your circuit looks like this:


    you must drive the gate to +10V to fully enhance the channel (turn on
    the MOSFET).

    If you're using a pullup resistor from the gate of the MOSFET to your
    positive supply it could be that the 4017 can't sink enough current to
    pull the gate down to ground to turn off the FET, but you haven't said
    anything about what your circuit looks like, so it won't be possible to
    zero in on what your real problem is unless you supply some more
    details, among which the logic supply voltage would be nice to know.

  3. For those tuning in the MOSFET datasheet can be found here:

    Have you used MOSFETs before? There are a number of possible causes of the
    symptoms you describe.

    What does your schematic look like?

    Are you sure you don't have the source and drain hooked up backwards? The
    naming on those pins seems most counter intuitive (at least until you
    understand the semiconductor device physics for the device's behavior) for
    N-channel devices. In a normal N-channel MOSFET application current is
    supposed to flow into the device through the "drain" pin and out through the
    "source" pin. For normal use the source pin is connected to ground as shown
    in John Field's little schematic.

    What is the voltage drop accross the MOSFET when the device appears to be on
    all the time?
  4. Tristan

    Tristan Guest

    Fritz / John,

    Thanks for your help – I have corrected the circuit - each peice of
    advice was very useful!

    I did have the source and drain connected incorrectly, that was the
    significant part of the problem. I have also now use a couple of
    diodes to protect the mosfet from voltage spikes caused by my load and
    also placed a ULN 2003A chip between my circuitry and the mosfet

    Again many thanks and sorry for the delay in getting back to you both.

    -- Tristan.
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