1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Diode Bridge Rectifier Verification

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

  1. givknow dge

    givknow dge New Member

    Joined:
    Apr 4, 2015
    Messages:
    29
    Likes:
    0
    upload_2015-4-4_19-20-8.jpeg

    Hello
    I am trying to verify the diode circuit labelled "Diode Bridge Rectifier" above. Diagrammatically, it can also be represented as:
    [​IMG]
    On placing the positive terminal of my voltmeter (switched to diode mode) on one of the AC terminals and the negative on the other I get a reading of 0.130 V. When I swap the terminal positions, the same reading repeats. Is it safe to conclude the bridge is broken? My google search say the reading should be OL. But verification from this forum would be helpful.
     
    Last edited: Apr 4, 2015
  2. JimB

    JimB Super Moderator Most Helpful Member

    Joined:
    Sep 11, 2004
    Messages:
    6,393
    Likes:
    593
    Location:
    Peterhead, Scotland
    ONLINE
    Your schematic of a diode bridge is correct.

    When testing the diodes, you will almost certainly get some odd results while they are still connected in circuit.
    I suggest that you disconnect one end of each diode and repeat the test.

    I would expect that you get a reading of 0.6 to 0.7v with the meter probes one way round on the diode, and OL with to probes the other way round.

    JimB
     
  3. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,139
    Likes:
    566
    Location:
    AZ 86334
    Are you testing an isolated, unconnected bridge with an Ohmmeter? Or is it still connected to the power supply?
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. cowboybob

    cowboybob Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2011
    Messages:
    3,053
    Likes:
    480
    Location:
    James Island, SC

    Certainly odd readings. Do as JimB suggests and try again.

    What were the operation symptoms of the board you posted?
     
  6. givknow dge

    givknow dge New Member

    Joined:
    Apr 4, 2015
    Messages:
    29
    Likes:
    0
    I've tried JimB's suggestion and the diodes seem fine as well as the bridge when disconnected from the rest of the circuit.

    But the DC output of the bridge comes at 0V when the circuit is active.

    The operation symptoms of the board are that it is basically the battery charger module of an un-interruptable power supply (UPS). When

    power is available at the A.C mains, the circuit starts charging the battery. When power fails, it stops.



    However, there was a sudden voltage surge at the AC mains and the UPS has not been functioning properly ever since. It is not able to

    detect power at the AC mains and keeps supplying power from its batteries.



    Thanks for the feedback. Any suggestions on further debugging the circuit would be quite helpful. Also, clicking on the small image at the top enlarges it thus giving a detailed view of the actual circuit.
     
  7. JimB

    JimB Super Moderator Most Helpful Member

    Joined:
    Sep 11, 2004
    Messages:
    6,393
    Likes:
    593
    Location:
    Peterhead, Scotland
    ONLINE
    OK, it is difficulty to tell from the picture, but the whole of the equipment looks very dusty and dirty.
    Cleaner is always better.
    Brush off the loose dust with a 1" paint brush, use a vacuum cleaner to suck up the dust as you brush.

    What about the other diode bridge, near the transformer? Hve you tested that?

    What about the capacitor, the big blue one with the cross on top?
    It looks slightly "bellied out". Worth trying a replacement.

    JimB

    Editing with further thoughts:

    Turn the board over and examine the solder joints.
    Pay particular attention to:
    The two white wires from the big transformer.
    The pins on the little transformer.
    The pins on the two large transistors.

    All of these are prime candidates for the wire breaking free from the solder.

    JimB
     
    Last edited: Apr 4, 2015
  8. givknow dge

    givknow dge New Member

    Joined:
    Apr 4, 2015
    Messages:
    29
    Likes:
    0
    I've verified the following:
    white wires from the big transformer
    Pins on the little transformer
    Pins on the two large transistors

    Replacing the capacitor without being sure its faulty is not an option at the moment.
    The other diode bridge is working fine giving the appropriate DC output.
     
  9. givknow dge

    givknow dge New Member

    Joined:
    Apr 4, 2015
    Messages:
    29
    Likes:
    0
    This bridge verification is actually part of the process of determining which section of the UPS is responsible for power sensing
    at the mains as I am assuming that section has broken down thus preventing the UPS from detecting AC mains. However, I could
    be wrong and it could be that the bridge would work once it gets a control signal from some other circuit. To cater to that possibility,
    I've pasted below the three main circuits of the UPS: Main board, Battery charger and Over-load protector.
    upload_2015-4-6_0-25-40.jpeg

    upload_2015-4-6_0-26-32.jpeg

    upload_2015-4-6_0-26-32.jpeg



    Someone with familiarity of UPS circuits would know how the control flow and that which circuits together determine if the UPS should switch
    to backup or the mains supply. It would be of immense help if that circuit along with the possible issues due to a voltage surge could be pointed out.
     

    Attached Files:

  10. cowboybob

    cowboybob Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2011
    Messages:
    3,053
    Likes:
    480
    Location:
    James Island, SC
    Sort of flying blind here. Maybe a member recognizes the boards, but until that happens, please give us your UPS Brand/Model. Not all UPS units use the same circuit(s).

    Many UPS units, especially older ones (which yours appears to be) have schematics available on the net, which would make troubleshooting a much easier task.
     
    • Thanks Thanks x 1
  11. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

    Joined:
    Aug 31, 2012
    Messages:
    3,152
    Likes:
    282
    Location:
    Richmond Hill , ON Canada near Toronto
    One cannot expect DC behaviour of a bridge diode set while in-circuit with inductive transformers with low DCR and with some load impedance.

    There are several bridges , so I don't know why just one is singled out for this simple discussion.

    upload_2015-4-5_20-47-29.png
     
    • Thanks Thanks x 1
  12. givknow dge

    givknow dge New Member

    Joined:
    Apr 4, 2015
    Messages:
    29
    Likes:
    0
    cowboybob:
    It is indeed old and I can not figure out its brand/model. The casing doesnt seem original and only reads the name of the guy I purchased it from. Its as if the guy copied the design from some brand and customized it into his own.
    Given this situation, would you have any pointers on how to proceed?

    tony stewart:
    That is a good question. Since the UPS has stopped sensing power at the AC mains, I am looking for the circuit that performs this sensing. Usually such circuits make use of an opto-coupler and in this case, only the battery charger circuit has one (the white rectangular IC on the extreme bottom left in the image above). My assumption is that some thing has gone wrong in this circuit and is preventing the opto-coupler from working. Hence it is the bridges in this circuit and specifically the one feeding the opto-coupler that are being singled out. BUT, I have no reason to be sure that my assumption is correct and I am looking for someone to verify/contradict it.


    Thanks guys
     
  13. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

    Joined:
    Aug 31, 2012
    Messages:
    3,152
    Likes:
    282
    Location:
    Richmond Hill , ON Canada near Toronto
    optos would be used for inverter feedback, primary isolation is transfo.

    You will have to measure DC and AC voltage with respect to ground and on big caps to check the circuit... starting from Line input.

    230Vac line will be converted to 550Vdc or so then bucked down to charge battery etc...

    It takes a clean 1 mm gap to arc 500V but 3mm is safe unless it is dirty and moist.
     
    Last edited: Apr 6, 2015
    • Thanks Thanks x 1
  14. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    I realise you have 110V mains, but 240V or 220V (no one has 230V - it's just the specification that equipment has to meet) doesn't give you 550V, it's 'only' about 340V DC when rectified.
     
    • Thanks Thanks x 1
  15. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

    Joined:
    Aug 31, 2012
    Messages:
    3,152
    Likes:
    282
    Location:
    Richmond Hill , ON Canada near Toronto
    Thanks Nigel, haven't had coffee yet this morning.. Here they like to keep revenue up by running at ~125~128V, we have pretty reliable power except when road salt and water freeze in underground old wiring.


    givknow dge... unless you know how to design something, thus reverse engineer it easily, or have test experience like a TV repair guy... its' pretty hard to debug with an ohm meter unless it is basic fuses, dry caps or burnt parts.

    ... so clean off the boards with some compressed air can spray a bit or just poke with (-) permanently clipped to gnd with one hand away to record levels. and other hand to probe.

    AC will read DC at twice the level on cheap meters.
     
    • Thanks Thanks x 1
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    He really needs to draw the circuit out :D
     
    • Thanks Thanks x 1
  17. cowboybob

    cowboybob Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2011
    Messages:
    3,053
    Likes:
    480
    Location:
    James Island, SC
    This is not, obviously your UPS (since it's the whole UPS, not just the charger circuit), , but it's a start:
    [​IMG]
    NOTE! Lethal voltages present. Take all necessary precautions.

    With NO POWER applied to the board(s), I would start by carefully wiping off the tops of the ICs and the transformer (which seems to have a circuit displayed on it) on the board you showed in post #10. Let us know whatever numbers that are displayed, as this will help with the troubleshooting. I'm approaching this from the assumption that one or more of the solid state devices (bridge diodes, opto-coupler or IC chip) was fried by the surge. Could, of course, be something else, but got to start somewhere...

    With the board powered up, confirm absence/presence of AC voltage (and what that value is) at the output of the transformer. Then confirm that AC level is at the appropriate diode junctions of the bridge (presumably those 4 diodes closest to the transformer - check the traces under the board for connections to the transformer). If present, check the other two junctions for a DC level and its value.

    If no DC, replace all diodes. Note that they may blow again at power up due to some downstream problem, but diodes are cheap.

    If DC is present, stop, power down and we'll continue with the troubleshooting.
     
  18. givknow dge

    givknow dge New Member

    Joined:
    Apr 4, 2015
    Messages:
    29
    Likes:
    0
    coyboybob:
    I'll work on your suggestions and post the result

    Tony Stewart and Nigel Goodwin, thanks for your input. Though I don't get
    How is the inverter feedback being routed to the opto here? I was under the impression that the opto-coupler was being used in a situation similar to the one shown in cowboybob's UPS circuit.



    Thanks
    giv know
     
  19. givknow dge

    givknow dge New Member

    Joined:
    Apr 4, 2015
    Messages:
    29
    Likes:
    0
    Last edited: Apr 7, 2015
  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    Interesting that it's got a PIC 16F676 on board :D
     
  21. cowboybob

    cowboybob Well-Known Member Most Helpful Member

    Joined:
    Oct 22, 2011
    Messages:
    3,053
    Likes:
    480
    Location:
    James Island, SC
    A PIC was the last thing I expected to see but, then, I am no UPS expert. I can see how an 8 channel ADC uC could be useful though.

    Anyway, givknow dge, let's check, with the board powered up, the voltage levels (if any) at the two ICs in the snip below: (would expect to see 2.0 to 5.5VDC on the PIC and perhaps ≈3VDC across the two on the opto-coupler. Other pins on the opto-coupler MAY BE LETHAL.
    upload_2015-4-7_17-46-33.png
    Without any idea of the traces layout, we're flying by the seats of our pants now. Just trying to chase down where the DC is going or is lost.

    Just out of curiosity, have you ever tried to "reverse engineer" (match copper traces to components) a PCB before? It is not easy, but it would help considerably with the trouble shooting.
     

Share This Page