14 VDC power supply putting out 24 VDC

Discussion in 'General Electronics Chat' started by Johnnyb60, Mar 15, 2015.

  1. Johnnyb60

    Thread Starter Member

    Sep 6, 2013
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    I’ve been having some problems with some circuits and decided to check the supply voltage to find 24VDC. I’ve had this power supply for over 25 years and did have some circuit cards connected to this power supply fail about 6 or 7 years ago, but I can’t remember if I checked the power supply voltage at the time. I know the voltage will be a little high until a load is applied, but I just don’t remember it being this high.

    Anyway I pulled off the cover to see if there might be an adjustment someplace and there isn’t anything. There are a couple of large resistors on the output of the transformer and wondering if they might be going bad. I haven’t actually measured the resistance and I don’t have a schematic, but I just wanted to run it by someone who knows this stuff before I start tearing it apart.

    [​IMG]
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Is it a linear supply or a switching supply?
    What is the AC output of the transformer?
     
  3. Brevor

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    Apr 9, 2011
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    can you post a picture of the nameplate that shows what the output voltage should be?
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Its common for an unregulated supply to output voltage higher than its rating when unloaded..
     
  5. BReeves

    Member

    Nov 24, 2012
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    The photo doesn't really show enough to be able to tell a whole lot. Need the other side, maybe the top of that circuit board, name plate and rear.
     
  6. Johnnyb60

    Thread Starter Member

    Sep 6, 2013
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    The transformer is AC output and uses a fullwave bridge inside the enclosure for DC. I don't know about a switching supply. It is a Motorola power supply for 12 volt radios.
     
  7. Johnnyb60

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    Sep 6, 2013
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    [​IMG]
    and
    [​IMG]
     
  8. Johnnyb60

    Thread Starter Member

    Sep 6, 2013
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    [​IMG]
    [​IMG]
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    Well I wanted you to tell me what the AC output voltage of the transformer was. If the supply is not a switcher which seems likely given it's age and application then 24V on the output would be a bad bad thing. There is a failed component that is allowing the input to the regulator to pass through to the output. If the AC output is about 18VAC, then multiplying by 1.414 is 25.5VDC from which we subtract the bridge drop of 1.5 V to get 24VDC out of the bridge rectifier.
     
    Last edited: Mar 15, 2015
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  10. MrChips

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    Oct 2, 2009
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    Firstly, check your voltmeter against a know voltage source such as a 9V or 12V battery.
    If the meter checks out ok then it looks like the pass transistor is shorting.
     
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  11. Johnnyb60

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    Sep 6, 2013
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    I don't know which one is the pass transistor. Is it one or both of the big ones in the huge heat sink shown in the last photo?
     
  12. MikeML

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    Oct 2, 2009
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  13. MrChips

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    You have two in that unit, the big ones on the heat sinks marked M9639.

    Remove both transistors and measure the resistance from collector to emitter.
     
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  14. Johnnyb60

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    Sep 6, 2013
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    WOW that's fantastic! I can't thank you enough MikeML :) The schematic is a big help and I think I can now make some checks.
     
  15. Johnnyb60

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    Sep 6, 2013
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    Thanks I think I'll be able do that much :)
     
  16. MikeML

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  17. Johnnyb60

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    Sep 6, 2013
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