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Bridge Rectifiers to repair battery charger

Discussion in 'Power Electronics' started by w4audi, Feb 23, 2013.

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  1. w4audi

    w4audi

    6
    1
    Feb 23, 2013
    Hi guys I am new to the forum. I have Exide battery charger model # (70-100). I need to replace the 4 button diodes that is rated at 75amps 400 volts with a bridge rectifier. Can someone tell me which bridge rectifier I can use to replace these diodes with and how to connect it in the circuit of the charger? The button diode is press into a plate two in each row. The center pin of diodes is connected to a copper wire coming from the transformer one on each side.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    If there are 2 diodes, and three connections from the transformer then the supply is already obtained by full wave rectification. Employing a bridge rectifier would not be advisable.

    If there are 4 diodes (almost certainly in at least 2 heatsinks) and only 2 wires from the transformer are used, then yes, you may be able to replace these diodes with a bridge rectifier (which is how the individual diodes will be arranged).

    In the second case you will need a 75A bridge rectifier rated at about double the transformer voltage (or higher), and it will need to be connected to a heat sink.

    The big question is 1) why do you want to use a bridge rectifier? If there is a fault in these diodes, why not replace the faulty one(s)?
     
  3. w4audi

    w4audi

    6
    1
    Feb 23, 2013
    Only 2 wires from the transformer is use and 2 diodes are connected to each of the wires. I am not getting any output dc voltage from the charger when I put a meter across the leads so I am thinking that the diodes are dead. I am having a hard time taking out the button diodes that is the reason I am exploring a bridge rectifier . see pictures below I cut 1 of the connection to remove diode but having a hard time getting it out.

    IMG_0985.jpg

    IMG_0987.jpg
     
  4. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    You could fire up the circuit and measure the output at the diodes.

    You could disconnect the wires and test each diode for integrity - conduct one way, non conduct the other.

    One pair of diodes may be of different polarity to the other - possibly not.
    Draw the circuit, it looks as if there might be two diodes connected in parallel.
    Is there another wire lurking somewhere which comes from the transformer and goes to the output?

    With 75A, there will be about 75 to 150W of heat dissipated. I doubt if you can find a bridge rectifier that will do this. There is a reason for using pressed in or bolted diodes.
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    I'd say the circuit is connected like this:
    [​IMG]
    A center-tapped transformer with both ends connected to the diodes (note: in your picture all diodes are connected in parallel, housings connected via the metal plate). I may have the diodes backwards, in that case + and - change places.
    You should not replace that circuit by a bridge rectifier, if will give you twice the output voltage. Better replace the diodes.

    Before that, however, measure the AC output of the transformer to ensure the fault is not elsewhere.
     

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  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yes, I agree with Harald. That is what I expect to see. There are THREE wires coming from the secondary of that transformer, not 2 W4Audi.

    If one or more diodes has failed short circuit, you would have no (or very little) output and the transformer would get very upset (if you're lucky, just blowing a fuse).

    In addition tot he tests mentioned above, check for continuity of the primary and secondary.
     
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