Fuzzlight fades out when running from a transistor


Closed Thread
Results 1 to 3 of 3
  1. #1
    Join Date
    Jun 2007
    Location
    Mansfield, UK
    Posts
    697

    Default Fuzzlight fades out when running from a transistor

    Hi,

    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
    Location
    Commerce Michigan USA
    Posts
    1,166


    Did you find this post helpful? Yes | No

    Default Re: Fuzzlight fades out when running from a transistor

    An actual schematic would be nice.
    Dave Purola,
    N8NTA
    EN82fn

  3. #3
    Join Date
    Jun 2007
    Location
    Mansfield, UK
    Posts
    697


    Did you find this post helpful? Yes | No

    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.

Similar Threads

  1. Replies: 8
    Last Post: - 28th January 2014, 15:21
  2. NPN Transistor
    By rondo2 in forum General
    Replies: 1
    Last Post: - 10th September 2009, 03:44
  3. Choosing a transistor
    By The Master in forum Off Topic
    Replies: 51
    Last Post: - 1st October 2008, 23:16
  4. Generating Timed Fades in PBP - ideas ?
    By bcd in forum mel PIC BASIC Pro
    Replies: 2
    Last Post: - 10th August 2008, 01:18
  5. LED with Transistor
    By mimi_deviL in forum Schematics
    Replies: 2
    Last Post: - 9th February 2008, 09:03

Members who have read this thread : 1

You do not have permission to view the list of names.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts