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Diode Bridge Rectifier Verification

Discussion in 'Repairing Electronics' started by givknow dge, Apr 4, 2015.

  1. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    • when working on TV's with 15~30kV old TV repair guys used to say .. keep one hand in your pocket... meaning avoid thru heart shocks
    • Alligator adaptor on ground lead frees up one hand.
    • I would say try not to short out the mains with the tip to chassis., it can vaporize probe tips as copper or steel spray paint on your safety glasses in a millisecond with a loud bang.

    • But with experience as I do, I can install AC sockets and switches live.
    • Although it gets interesting at 100kVdc testing a large distribution transformer in a factory and insulated pipes on the wall start arcing 100 ft away beyond me from stray E field partial discharge.. going tick tick tick.... like the faint corona tick noises on HV transmission line bushings.
     
    Last edited: Apr 25, 2015
  2. givknow dge

    givknow dge New Member

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    Thanks guys.
    Is it possible to know which part of the following circuit should be taken as ground(to attach the ground terminal of voltmeter via alligator clip):
    [​IMG]


    Regards
     
    Last edited: Apr 26, 2015
  3. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Here again, if you could clean up the board, i.e., remove the dust, carefully wipe the top surface of the ICs and, if possible, improve the clarity of the photo (extremely hazy) we could better help with your request. Clean ICs will, hopefully, expose the identity, from which we can determine which pin is ground.

    Often (but not always) the PCB mounting holes are connected to circuit ground. Remove the board (POWER OFF) from its mounting bolts and see if an obvious wide, continuous band of copper is evident around the hole(s), which connects to other portion of the PCB's traces. Then you might be able to, using a continuity tester, see if the mounting hole trace is continuous with the ground pin of one of the ICs.

    Another approach is to very carefully solder a short wire to the ground pin of one of the ICs and use it to attach the ground lead of your voltmeter.

    Of the two approaches, I would would use the second. It is most likely to be the correct ground since, even with multiple DC sources (as evidenced by multiple bridges) powering the circuits, it is logical that the grounds would be common.
     
  4. dave

    Dave New Member

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  5. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Some cap lids will be ground for +V, others V-
    Ohm connections to pin 7 of 14 or 8 of 16 to logic chips to ground to locate a connection << 1 Ohm with power off.. Check for Voltages 1st on meter.

    Use Tulip icon on Camera for macro focus, with a flood light if using a smart phone.
     
    Last edited: Apr 26, 2015
  6. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    The LM339 (as an example) is arguably a digital (A to D logic) 14 pin device and does not follow the pin arrangement you mention, rather pin3 is Vcc and pin12 is gnd. Thus the request for IC identifications.

    As a result, we can then determine all possible grounding points without a power up which will facilitate soldering a short lead to that pin. I am, of course, assuming the OP has adequate soldering skills (since he was considering a cap swap).
     
  7. givknow dge

    givknow dge New Member

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    upload_2015-4-30_0-32-37.jpeg
    Pasted above is a clearer picture of the circuit, though it misses out on one of the relays. (is it possible to know how to shrink the size of the diagram to fit more appropriately within the page)

    The two relays each have their activation coils being traced back to the ATMEL ATF16V8B PLD.

    Soldering is definitely possible. I'll follow the ground instructions accordingly. Thanks guys!
     
  8. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Fascinating . If you cant locate ground. We are in trouble ;)

    Salam.
     
  9. givknow dge

    givknow dge New Member

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    I've set up the system as suggested and taken the initial readings. I am working on putting them on the above diagrams next to their respective chips. Will post the results in a day.
     
  10. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Measure starting with;
    Vac input to board,
    Vdc Bridge out,
    Vcc on DIP chips using datasheet. Usually diagonal corner pins like 16,8 on DIP16 with dot showing pin 1.

    Then go from there to locate fault.
     
  11. givknow dge

    givknow dge New Member

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    The voltages on three chips are mentioned. These chips drive the two black relays on the top right of the board. Since the relays are not getting activated, the voltages on these chips has been mentioned only. Two of the voltages are bigger in font size and of a different color as well. This is only an editing problem. IMG_3513.jpg
     
    Last edited: May 20, 2015
  12. givknow dge

    givknow dge New Member

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    Also, the voltages across the two 4007 diodes attached across each relay read 0v.
     
  13. givknow dge

    givknow dge New Member

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    Though this image had been shared and discussed earlier in this thread, I had some confusion over the voltage reading across the 4007 diode at the bottom-left of the board. With the polarity as indicated on the board, it reads -2.40 V.

    What does this imply?Should this diode give a reading between 0.5-0.7V? IMG_3254.jpg
     
  14. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    The ATF16V8B-15PU is an EE PLD (Electrically Erasable Programmable Logic Device). That said, it has the correct Vcc (5Vdc) and ground. Beyond that, the other voltages you've listed have little meaning since we don't know how the device was programmed or its current state (it does, however, seem to be providing reasonable logic levels).

    Except for pin 11 (Active Low Enable). With 4.8Vdc applied, the chip is disabled. I do not know why that is the case nor do we know where the 4.8Vdc is coming from.

    I am, unfortunately, going to have to admit defeat on this one, givknow.dge.

    As a lark, and because it is a likely device to fail from an overvoltage, I'd swap out the Opto isolator. If that doesn't fix it, I'd part out the board(s).

    Perhaps some other forum member might have some ideas.
     
  15. givknow dge

    givknow dge New Member

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    I see. Thanks for your feedback throughout this thread. It has been helpful in getting a clearer picture of the system.
     
  16. tronitech

    tronitech Member

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    Isn't 230V the official UK specification for mains voltage? ;)
     

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