Question on using power MOSFET & PICAXE

Discussion in 'General Electronics Chat' started by izon, Jun 3, 2015.

  1. izon

    Thread Starter Member

    Mar 17, 2013
    163
    5
    Hi,
    I'm trying to connect a power MOSFET (BS270) to one output pin on a PICAXE
    controller chip. The device the MOSFET is to turn on is an LED array that is constructed to look like a beacon light. It uses 24vdc and the PICAXE uses 5vdc.

    I've connected things as shown on my attached drawing. When I power on my
    project, the LED array immediately operates for about 10-15 seconds and then goes off.
    It should not come on at all. It must be running at about half the normal voltage during
    this time since I see only about half the LEDs lighting.

    What am I doing wrong here.... how can I get it to stay off even when just applying
    power to the project via a wal-wart? I can tolerate this but I'm concerned maybe Ive
    done something wrong that will cause a failure later. Since the light is a warning, I
    don't want it to ever be on falsely.

    Thanks!
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,972
    3,219
    What is the gate connected to when you ran your test? A MOSFET gate cannot ever be open (floating).
    The gate must be connected to ground, either through a resistor, or the PICAXE output has to be in the logic low state, for the MOSFET to be fully off.
     
  3. izon

    Thread Starter Member

    Mar 17, 2013
    163
    5
    Sorry my details and drawing were not the best. The gate is connected directly to one of the
    PICAXE output pins. I guess I would have thought the output upon initially applying power
    would be logic low and fully off. Maybe there is some stored up voltage until everything
    gets into normal state.
    Thanks for trying to help...the PICAXE is a 20M2.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,246
    Connect a resistor from the gate to GND, something between 10 K and 100 K. This will prevent the gate from floating if the picaxe output is in tri-state.

    ak
     
  5. izon

    Thread Starter Member

    Mar 17, 2013
    163
    5
    Hey thanks fellas ! Put in a 10k and it solved the problem and all else seems to work ok too..thanks again!
     
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