Strange (to me) circuit shorting?

Discussion in 'The Projects Forum' started by bluesguitar03, Mar 18, 2009.

  1. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
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    0
    Hello,

    I have been reading and following this site and forum for quite some time and finally have a question to post! I have a circuit I've been working on for quite some time now (just reconditioning and making for use outside of its original console).

    I know that it worked a few months ago (it's been dragged out over quite some time....) because I powered it up with the same power supply I'm using now. The power supply is a 48V linear supply (Power-One HB48-0.5-A). I recently put the whole project in an enclosure and redid some of the wiring and now it is has a short I think but it doesn't make sense how it is happening. When everything is connected and I power it up the voltage coming out of the power supply will go to 0.3V, then drops to 0.2V then the fuse blows only after a few seconds.

    This to me says it's a short because the output isn't getting to 47.2V where it is normally. So after this I disconnected everything from the supply to see if the PS was still good. Powered up just fine and stayed at 47.2V for a minute. I then connected the LED power indicator and it still worked just fine. After that I hooked the circuit back up to the supply and it had the exact same symptoms as stated above. So then being curious I connected just the (+) from the PS to the (+) of the circuit and the voltage jumped up to ~66V! I didn't take that to be a huge problem and attributed it to the circuit not being connected to ground. Doesn't make sense how exactly that would happen either though.

    So next I connected just the ground of the PS to the ground of the circuit and the voltage was only 0.3V again so I shut off the power before it blew another fuse.

    Now logically thinking, if there were a short when the grounds of the circuit and the power supply were connected, shouldn't there be a short when just the (+)s were connected?

    I am very confused at this point and was wondering if anyone had any ideas or suggestions to offer?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    When the voltage rose to 66 volts, how did you measure it (actually, where)?

    It would seem that the short circuit is in the external circuit, as the power supply voltage doesn't dive until both leads are connected. The wiring you redid may be the cause of the problem if no load is connected.
     
  3. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    20
    0
    I measured it from the output terminals of the supply.


    The supply voltage did dive when I connected just the ground too.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Does the PS use a three prong line cord?
     
  5. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    20
    0
    The power supply only uses the live wire and the neutral. I have it hooked up with a 3 prong cord that has the safety ground secured to the chassis. I did test with the safety ground lifted (used a two prong adapter plugged into the wall socket) and nothing seemed to change (still had the same issue).

    I have another power supply (exact same unit but never been used) and did some testing. The power supply that is in the unit now seems to have a capacitance between the (+) out (47.2V) and the power supply chassis and the Ground out of the supply seems to have capacitance as well. I did the same tests on the unused supply and it did not appear to have that capacitance. I'm wondering if something is wrong with the supply?

    But if there were something wrong with the supply, my thought is that it wouldn't operate normally even with nothing attached.
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Are you putting it in a metal enclosure?

    Using anything conductive to mount it (i.e. screws)?

    Heat sink tabs of transistors and some regulators (TO-220, TO-3, etc) are often hot, make sure two of them aren't touching each other, or the same bit of metal, heatsink, or screw when in the enclosure.
     
  7. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    20
    0
    Well the transistor (TO-3) is attached to the metal chassis of the power supply and the power supply is bolted to the enclosure (aluminum). I didn't figure that should be an issue. This weekend I'm going to take everything 100% out of the enclosure and see if that fixes anything.

    Heat shouldn't be an issue as it's only been on for a matter of a few seconds before it blew the fuse.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    If that TO-3 is a pass transistor, the case is hot (meaning it is electrically active) and probably the reason for your short.
     
  9. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    20
    0
    I will look into that. Before I came to work this morning I actually wrote down what transistor it was but forgot to bring that paper today. :(

    When I get home tonight I will check. Thanks!
     
  10. bluesguitar03

    Thread Starter Member

    Jul 19, 2008
    20
    0
    Well I checked the datasheet for the transistor and the case is indeed the collector. I have yet to try out the circuit with that separated from the chassis, but my guess is that was the issue.

    Thanks for your help! It is much appreciated!
     
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