Fuzzlight fades out when running from a transistor

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  1. #1
    Join Date
    Jun 2007
    Mansfield, UK

    Default Fuzzlight fades out when running from a transistor


    I have a 12V fuzzlight that uses about 0.4A current. I'm trying to run it from a 1A 30V transistor (with flyback diode) on a 12V supply. After about 2 minutes the motor suddenly slows down and the bulb fades out until it's completely off.

    At first I thought I was overloading the transistor but I've measured the current draw on 2 different bench PSUs and I'm always getting around 0.4A which I know these transistors can handle easily.

    I attached a scope to the fuzzlight. The motor appears to be causing some spikes about 10V above the supply voltage so I've added a 13V zenner diode which seems to have solved the spikes but not the overall issue.

    If I turn the transistor off and back on then the fuzzlight comes back on full speed/brightness and lasts from a few seconds to a few minutes depending how long it's been off for.

    Has anyone seen anything like this before? I'd like to solve it without using a relay if possible.

  2. #2
    Join Date
    Mar 2003
    Commerce Michigan USA

    Default Re: Fuzzlight fades out when running from a transistor

    An actual schematic would be nice.
    Dave Purola,

  3. #3
    Join Date
    Jun 2007
    Mansfield, UK

    Default Re: Fuzzlight fades out when running from a transistor

    Hi, Sorry for the delay. I've managed to solve it and it turns out it was my own stupidity.

    I was testing with 2 power sources (additional to the bench supplies).
    1 of them has 12V regulators in and was actually designed for turning small solenoid valves on for a few seconds.
    It looks like the fuzz light was drawing enough current to cause the regulator's thermal protection to kick in (hence the slowing and dimming).

    The second source is unregulated and uses 20V 1.2A transistors running from 12V toroids.
    Originally the spikes caused by the motor were blowing the transistors as they were hitting 25V so I added the 13V zeners.
    With the zeners in place the transistors were blowing immediately after turning on.
    Eventually I remembered that the house gets 250V+ mains so my 230 to 12V toroid is actually outputting over 13V.

    I'm already in the process of converting everything to use 12V switch mode supplies and have had a fuzzlight running for over 12 hours without a problem.

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