Car battery charger-no output

Discussion in 'General Electronics Chat' started by drop2_c, Jan 19, 2009.

  1. drop2_c

    Thread Starter New Member

    Jan 19, 2009
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    Ok here is where I am. The unit is a cheap chinese made Chicago Electric/Harbor Freight special 12v 2or6amp charger. I have 25vAC on the secondary of the tx so I believe the tx is ok. From there I have a very small pc board with lots of transistors and resistors which all of them check out fine. Then I have the two (not sure-power fets) bolted to the heatsinks which the tx secondary taps attach to. Every thread I have read seems to target the two chips on the heatsinks. I have one out of the circuit but there are no numbers at all to source out.
    Any help in getting this equipment fixed and back to the owner would be greatly appreciated.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    If there are two of them, they may be (or have been) rectifier diodes. They would be suspect, as well as the SCR. Diodes should check as any semiconductor. The SCR won't show a gate to cathode PN junction, but you should see a moderate resistance G to K.
     
  3. drop2_c

    Thread Starter New Member

    Jan 19, 2009
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    That's funny I didnt see the SCR silk screened on the PCB til I took the pic. This is much different then the tube amps I have been building for the past 3 years. Forget the 2 years of EET training I had 15 years ago. The brain forgets quickly.
    Beenthere thanks for taking the time. All my transistors are fine, the diodes and resistors also good. The SCR's do have a very high resistance. Both from gate to cathode or anode are reading about 25Mohms. What do you think?
     
  4. drop2_c

    Thread Starter New Member

    Jan 19, 2009
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    I think I figured out that both SCR's are bad. Because I should be able to get continuity from the gate to cathode and I dont. Let me know if I should look elsewhere.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    One other test is to place an ohmmeter on the SCR with the leads placed so positive is to anode and negative to cathode. If you can manage to jump the positive to the gate, the SCR wil turn on and you will see conduction. It helps to be part octopus.
     
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  6. drop2_c

    Thread Starter New Member

    Jan 19, 2009
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  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    If it fits, it should work. Plenty of overhead with those specs.
     
  8. Acrizona

    New Member

    Aug 2, 2011
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    I replaced both regulators with RadioShack #7812. Works perfectly !
    * The 7812 cost less than $2

    Low-cost chargers have no built-in protections for crossed wires (reversed polarity) or any other simple misteks we make. I have smoked several chargers by forgetting to disconnect them BEFORE starting the engine.
     
  9. woodman

    New Member

    Nov 24, 2012
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    Nice job! I have two chargers exactly like yours - Harbor Freight model #45005. If your SCRs blew from reverse polarity to battery you could modify design so that could never happen again. Also fuses if not already in place, could be added to prevent "smoking chargers" when we forget to disconnect chargers while cranking engine.

    I'm thinking about improving the design for better charging ability. Steve at Electronics Point forum is helping because the circuit is way too complex for me. Here's link for forum:
    http://www.electronicspoint.com/battery-chargers-doa-model-hf-45005-a-t254890.html
     
    Last edited: Dec 28, 2012
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