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Bridge rectifier question

Discussion in 'Electronic Basics' started by Dan Beck, Feb 9, 2004.

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  1. Dan Beck

    Dan Beck Guest

    Hello all,

    I have a 35A, 400V bridge rectifier that may be the source of a problem.
    When I use the diode test on my DMM, with the power to the bridge off, the
    forward voltages measure 0.470 V, which I thought was normal. The AC input
    to this bridge is 29 VAC; I am getting 36 VDC output at the DC leads. Is
    this normal, does this represent a problem with the bridge, or with the DC
    circuitry downstream?

    Thank you for reading.
  2. An AC voltage that measures 29 volts on a volt meter produces the same
    heating effect as a DC voltage of 29 volts if applied across a
    resistor. But AC sine wave has two moments per cycle of zero volts
    and two other moments per cycle that are the square root of 2 ( or
    1.414) times this effective voltage. For 29 volts AC, the peak
    voltage is about 41 volts. When you take a DC reading of the output
    of the bridge, you measure the average voltage, not the effective
    voltage the AC meter measures. The average of a rectified sine wave
    that has a peak voltage of 41 volts is 63.7% of the peak or about 26
    volts. If you are measuring something much higher than that, there
    must be a capacitor after the bridge that is charging up to almost the
    peak and discharging very little between peaks.

  3. Bill Vajk

    Bill Vajk Guest

    Or to put this another way.....

    Take the area under the sine wave and redraw it into a square
    wave and you get .707 times the peak voltage. That's the
    voltage read by your ac voltmeter, the 29 volts.
  4. No. That would be the average value. The effective voltage square
    root of the area under the squared sine wave. That is what RMS
    means. Root of the mean of the square.
  5. Bill Vajk

    Bill Vajk Guest

    You're generally right, John, but alas I think you're missing
    something this time out.

    The problem starts with an ac voltmeter reading of 29 volts.

    Unless there's something special about the meter it registers
    rms voltage for oerdinary sine wave alternating current. I
    don't see where you're getting average voltage out of
    anything I wrote but I'm certainly willing to listen.
  6. Perhaps I misunderstood what you meant by "redraw it into a square
  7. Fred Stevens

    Fred Stevens Guest


    Full wave rectification will give an output near the peak voltage of
    the AC wave which is 1.4142 times the RMS voltage (29V in your case),
    minus the forward drops of the diodes may explain why you get 36V.

  8. John G

    John G Guest

    Fred, If only John P and some others could be as succinct as
    All the averages and RMSs etc only serve to confuse someone
    asking such a basic question.

    Yes a simple rectifier with a capacitor across the output
    will give about 1.4 times the transformer AC voltage with no
    load and something less as the load increases, depending on
    both the capacitor value and the current ratings of the
    rectifier and transformer.
  9. John Fields

    John Fields Guest

    Unfortunately, ignoring the details doesn't make him right.
    You may want to go back and read the original post, where no mention was
    made about a "capacitor across the output", rendering both your and
    Fred's comments useless.

    You may also want to go back and read Popelish's first post, where the
    possibility of the existence of a filter cap was pointed out, further
    nullifying the need for your and Fred's posts.
  10. Monk

    Monk Guest

    I think those other guys may have already answered your question but
    this is good practice for me as im studying power supplies right now
    in my devices class. anyways i would say no, there is no problem with
    your rectifier.

    29 VAC / 0.707 = ~41 Vpeak

    41 Vp subtract the voltages from the diode drops and the voltage
    variation of the ripple and you should get a voltage DC of about 36.
    This really depends of if there is a capacitor there to filter out the
    ripple or not but overall it seems like your rectifier is working

  11. Dan Beck

    Dan Beck Guest


    Thank you for the prompt, informative responses. I appreciate the
    scientific rigor of the answers, even though I had to dust off the calculus
    based college physics text from 20 years ago! Sometimes I wonder if I
    should have pursued electronics further than that...

    The bridge rectifier in question is for a pinball I am working on (my own,
    as a hobby). There are a number of feeds off of the DC output; many of them
    have filter caps, as Mr. Popelish first speculated. The game has had an
    intermittent sound problem I thought related to the output of this bridge; I
    believe I need to search a different avenue!

    Again, thank you.

  12. Louis Bybee

    Louis Bybee Guest

    I understand, and accept the fact that DC voltage from a rectifier with
    capacitance across the unloaded/lightly loaded output can be modeled
    mathematically. Is it also possible to model mathematically the DC voltage
    from the same output where the load, and capacitance levels are varied?

    Thank you.

    Remove the two fish in address to respond
  13. In principle it is certainly possible to model the rectification and
    filtering mathematically. In practice it is pretty messy. Some
    complicating factors:
    * resistance of transformer windings
    * leakage inductance between primary and secondary
    * distortion in the line waveform
    * non ideal diode drop (diode series resistance)
    * non ideal capacitance (ESR and inductance)

    So most models are simplified to the point of being only good guesses
    at the exact DC and AC ripple components out of any given system. At
    least the ideal assumptions put an upper limit on the DC component.

    There are quite a few pages on the web on his topic:
  14. Louis Bybee

    Louis Bybee Guest

    Hmmmm..... How high is up? :-]

    As I expected.

    Thank you.

  15. Guest

    if you still come back to this thread
    Ok you said sound problem so your machine is solid state for the most
    part. But you said it had a transformer so it most be about 20 years
    old. You could just buy a new bridge it will only cost you a $1.40-6.
    If that dosn't fix it try the caps. You said sound was a problem hmm
    might it be just a cap. But .4 volts across the bridge means you have
    problems with the bridge so just replace it. It should be with a load
    ..7-.88 per diode.

    what brand is your pinball
    is it Stern, Illinois, Capcom, Gottlieb
    Williams/Bally, Sega. Or some other brand
  16. Dan Beck

    Dan Beck Guest


    You may regret opening this can of worms, but I would be happy to oblige
    your curiosity!

    The game is a 1989 Gottlieb System 80B pinball called Big House. I have
    performed the following:

    Replaced all the electrolytic caps on the sound board power supply board, no
    electrolytics on sound board itself.

    Repinned the edge connectors carrying the sound lines from CPU board to
    solenoid driver board to sound board.

    Upgraded the circuit grounds; approximately 40 wires. Gottliebs of this
    vintage are notorious for having crummy grounds.

    Replaced various TTL chips that Gottlieb gurus suggested I change--no

    The sound board has a dual CPU system using 2 Rockwell 6502 processors.
    Each processor has its own sound rom, amplifier chips, and various TTL
    chips; both circuits share a DAC. When certain playfield switches close the
    CPU circuit that provides speech cuts out. These playfield switches are
    associated with transistors that control 24 (actually 36) volts DC either
    through light bulbs or a solenoid. The diode on the solenoid tests ok. I
    was thinking the 24 VDC bridge was the troublemaker. This is the bridge
    that actually outputs 36 VDC, discussed in my first post. This 36 VDC is
    pared down to 12 VDC on the sound board power supply board using a 12 volt
    Zener; this 12 VDC then goes to the sound board to power the amplifier
    chips. My oscilloscope shows all the address and data lines cut out when
    the speech crashes; the other CPU circuit seems immune to the crashes. The
    crystal still ticks, and the 5 VDC is good. I have replaced the speech CPU
    with a new one--no change.

    There are probably other things I have done and not mentioned, but those are
    the major ones. Good thing this is a hobby, and not my day job--my family
    and I would be starving. Most of the Gottlieb gurus I have spoken with
    think it is a flaky sound board; perhaps, but what bothers me is that it
    seems an insult external to the sound board is causing the sound board to
    crash. Ah well. I have spent way too much of your time; I apologize for
    the length. Thank you for humoring me!


    P.S. I have a fresh batch of bridge rectifiers. When I test them their
    forward voltages (juction drop) are 0.490 volts as well; does this mean they
    are bad?
  17. Guest

    The game is a 1989 Gottlieb System 80B pinball called Big House. I have

    Hmm model 713
    With all the things you have done I guessing you have the manual and
    TTL are rock steady no ESD issues but and easy fix.

    What about the PROMS/roms can go bad in 10 years you may have just
    lost some data when a prom goes bad only sections go bad.


    hm maybe its the eprom
    the little guy in my heads says it might be this! If their is one bit
    wrong it will crash.
    No its good there is nothing wrong with you bridge and with a scope
    you would now.

    I thought that the power supply was supposed to give 30 volts DC to

    Sorry for not being able to help.
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