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Thread: Testing diode bridge with a multimeter

  1. #1
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    Testing diode bridge with a multimeter

    I want to test a diode bridge with a multimeter or megger. I know if you test a single diode with a multimeter (ohm settings) then with your leads in one arrangement you will get continuity however if you reverse the leads the diode will block current and your meter will read open.

    If I am testing a full diode bridge then if I connect my meter leads to the two input of the bridge then I will get current flowing through the bridge in one direction on half of the bridge and will read continuity. When I reverse my leads I should still see contunitly (the same as first arrangement) on my meter as current has reversed direction and is flowing through the bridge on the other half of the bridge. If I do not get continuity from one of these tests and get a high ohm reading then its safe to say that one of the diodes are bad.

    Is it a good idea to disconnect the secondary circuit and just jumper the bridge's output terminals so that you are strictly testing only the brige and not the external circuit?

  2. #2
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    Yes, it would be best to disconnect all four terminals to test the bridge. This way you will not get any interferrence from other devices & you will not damage any other devices. There is no need to short out any terminals. Use the diode setting on your meter and work your way around the bridge.

    On each set of terminals take a reading in both directions, for a total of 8 readings ( 2 reading on each diode ). The diodes will read OPEN in one direction & the breakdown voltage will be shown on the meter in the other direction ( typically .4-.6volts ).

    Another test would be to apply an AC voltage to the input terminals, the DC output will be 10% less than the input. ( no capacitor on output )

    Another test would be to apply DC voltage to the input terminals, it should pass through the bridge.
    Advise is a dangerous gift, even from the wise to the wise.

  3. #3
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    Just for my own sake, how or what good do you think testing a full bridge with a megger do:-?
    Old and in the Way.......

  4. #4
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    Smile Bridge Rectifier Elementary

    Quote Originally Posted by Pitt123 View Post
    I want to test a diode bridge with a multimeter or megger. I know if you test a single diode with a multimeter (ohm settings) then with your leads in one arrangement you will get continuity however if you reverse the leads the diode will block current and your meter will read open.

    If I am testing a full diode bridge then if I connect my meter leads to the two input of the bridge then I will get current flowing through the bridge in one direction on half of the bridge and will read continuity. When I reverse my leads I should still see contunitly (the same as first arrangement) on my meter as current has reversed direction and is flowing through the bridge on the other half of the bridge. If I do not get continuity from one of these tests and get a high ohm reading then its safe to say that one of the diodes are bad.

    Is it a good idea to disconnect the secondary circuit and just jumper the bridge's output terminals so that you are strictly testing only the brige and not the external circuit?
    SG stated it well. See attached sketch. The attachment limits of this site caused me to squeeze it down quite a bit. If you cannot read it send me your email and I will send the full size image.

    Tony
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    Am I in shape?? I get plenty of exercise pushing my luck!!

  5. #5
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    Arrow

    You can easily test a bridge with a scope with XY axis as well.
    Old and in the Way.......

  6. #6
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    I would like to add that it does not make any difference to which AC input terminals the DC is applied. The + DC output will remain positive no matter which way the positive & negative are connected to the AC input.
    Advise is a dangerous gift, even from the wise to the wise.

  7. #7
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    Quote Originally Posted by 76nemo View Post
    Just for my own sake, how or what good do you think testing a full bridge with a megger do:-?
    If you suppected the rectifier was breaking down to the chassis then you could short all four terminals together and apply voltage between them & ground ( the chassis ). The bridge may not be breaking down until 50 volts or more is applied. An ohm meter would probably show an open in this case.
    Advise is a dangerous gift, even from the wise to the wise.

  8. #8
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    Quote Originally Posted by SG-1 View Post
    I would like to add that it does not make any difference to which AC input terminals the DC is applied. The + DC output will remain positive no matter which way the positive & negative are connected to the AC input.
    It was a great way to stop truckers from reversing the polarity to their CB radios, I use to install them in every CB that came into my shop years ago, stopped a lot of headaches.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

  9. #9
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    Quote Originally Posted by 76nemo View Post
    You can easily test a bridge with a scope with XY axis as well.
    That picture would quicky tell you if it was healthy or not.
    Advise is a dangerous gift, even from the wise to the wise.

  10. #10
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    Quote Originally Posted by 76nemo View Post
    Just for my own sake, how or what good do you think testing a full bridge with a megger do:-?
    I think you will destroy it if you exceed its voltage rating. Putting 1000v to a 24v rectifier might not be wise.
    Tom
    TBLO

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