Revers voltage to PSU

Discussion in 'General Electronics Chat' started by Skiivari, May 17, 2013.

  1. Skiivari

    Thread Starter New Member

    May 17, 2013
    2
    0
    Hey guys.
    My friend lended his GPS tracking device to another friend and this guy manages to connect it with reversed voltage. It's supplied by 12V off a car.
    So I opened it up and there's some obvious damage on a zener (looks blown). A rather large resistor has obviously gotten quite hot, as the PCB has gone a little brown close to it. I plugged it in for some quick testing and what I found out was that the large resistor gets hot, too hot. The voltage regulator IC doesn't get 12V, it gets about 4 volts and outputs 1.5 volts. The part number of the voltage regulator is 78m05 so it seems that it should maybe be outputting 5 volts (not quite sure on this). I measured the blown zener, another zener close to it and identical looking ones further away on the board. The results were that the ones that were probably ok measured 1900 and 639 on the diode-testing-mode of my multimeter (I guess these are ohms?), and the obviously blown one 730-ish one way and no reading the other way around. The polarity is so that this 730 was the "same" reading as the 639 on the others. The one that might be blown measured 639 and no reading. I don't really know what values I need for a replacement zener, can you guys help me by telling how to measure these? If you can give some insight on what else is probably fried on the board that would be of great help :)
    (If this is an inappropriate board for this message I'm sorry :( first post here)
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  2. Kvarner

    New Member

    Nov 16, 2012
    2
    1
    Helo
    I think 7805 is ok beceuse you need 8 or more voltage on pin where is diode.
    That diode in conection with 7805 must be cathode to prevent oposite voltage.
    So replace 2 diode with new. Check connection from input + pin to 7805.
    Somewhere must be burned thing.
    Dismantle resistor and check
    brgd Igor
     
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  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    Ok unsolder the 7805 regulator and input 5 volts on the output side and see if it powers up,

    it could be there is a protection diode that is blown taking the supply down causing the resistor to get hot.
     
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  4. Skiivari

    Thread Starter New Member

    May 17, 2013
    2
    0
    First I replaced the blown diode (the only one actually, the others are ok). Still heated up the resistor. Desoldered the 7805 and it works. Then I started checking basic stuff, and surprise surprise the big cap had failed, shorted. Didn't quite make it to the parts shop today but I'll look if I can salvage a suitable cap in its place. I don't know if something else is blown but I'm delighted to make progress!
    I still have to work on being careful with pcb tracks, I demolished the one next to the blown diode and used jumperwires on the rear of the PCB.
    I'll update this if I continue to have problems after I get a new cap but thanks for the help :)
     
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