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Clues on controller circuit problems

Discussion in 'Electronic Basics' started by Pete, Aug 21, 2004.

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  1. Pete

    Pete Guest

    Hi, I'm not sure whether anyone can help with this problem but I though
    I would post on the of chance because I haven't a clue. I have made a 12
    V speed controller/light dimmer circuit that came in kit form. It uses a
    LMC555/7555 CMOS timer and BUZ71A power mosfet transistor. A 10K pot
    controls the motor speed. I was testing the circuit tonight and it
    worked flawlessly then suddenly the pot stopped controlling the speed.
    It now operates at full current irrespective of where I set the pot. The
    pot seems to work alright tested with a multimeter, so I'm anticipating
    the problem is elsewhere. The transistor gets very hot (it has a heat
    sink connected) as does a big 3A diode. There are no obvious signs that
    anything has blown at all. Any clues?

  2. John Fields

    John Fields Guest

    Could be you've toasted the MOSFET or the diode. To find out,
    disconnect the diode and measure its resistance in one direction and
    then the other using your meter. If you don't have a DIODE TEST
    function, then get 1.5V cell (battery) and a 100 ohm resistor and hook
    everything up like this:

    +--[-BAT+]---[100R]---[DUT]---[DMM (20mA)]--+
    | |

    If the DUT (device under test) is OK, when you have it in the circuit
    in one direction the meter will read about 8mA and when you reverse it
    the meter should read zero milliamps. It might show a few microamps;
    that's OK.

    If you've popped the MOSFET, you can find out by connecting the gate
    to the source and measuring the resistance between the source and the
    drain. It should be very high with the drain connected to the +
    output of the ohmmeter, and it should look like a diode with the
    ohmmeter connected the other way around. If it reads like a low
    resistance both ways, it's gone.

    You can also test the 555 by disconnecting everything from its output,
    setting your DMM to DC volts and measuring the output of the 555 as
    you turn the pot. It should be low at one end of the pot's rotation
    and high at the other.

    Depending on your meter, though, you might have to do something like

    555OUT>----[1N4148>]--+------+ <--------+
    | | |
    [0.1µF] [1K] [DC VOLTS]
    | | |
    GND>------------------+------+ <--------+
  3. Pete

    Pete Guest

    Thanks very much John. I will do just that. I received a post from Kim
    off list also suggesting that i fried the transistor. I gave her some
    additional information regarding the extra demands I was placing on the
    circuit by trying to control the heat generated by a a number of
    resistors in parallel. I've reposted seeking a more robust design under
    "Need heater strip circuit" above because Kim reckons this design isn;t
    going to work for that purpose.

    Clear and dark skies!

    Home Page:
    Lat: 42° 53' S; Lon: 147° 19' E

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