Bridge Rectifier issues.

Discussion in 'General Electronics Chat' started by Joshmann, Dec 3, 2008.

  1. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    Greetings,

    Im a bit of a Noob here but here it goes:

    I have 2 sources, both 24VDC. Actually one is 27VDC off the charger and the other puts out 24VDC. One is a battery and the other is a converter. I have installed a bridge rectifier but hooked it up as to be an isolator for the sources. (See photo, battery charger not shown)

    The problem I have is that I get 38V at the end of the circuit BEFORE I close the breaker. AFTER I close the breaker, I get 27VDC (same as the battery with the charger on it). Any ideas of what the problem may be??

    This looks like it should be an easy one looking at the other posts.

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    Thanks,

    Josh
     
    Last edited: Dec 3, 2008
  2. mik3

    Senior Member

    Feb 4, 2008
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    When the breakers are open there is no current through the circuit. The voltage out of the converter is higher than the battery's voltage and thus it appears at the output. When the breakers close, the output of the converter drops below the battery's voltage due to discharging of the capacitor filter it contains, thus the battery's voltage appears at the output.
     
  3. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    Using a fluke multimeter, the DC output of the converter reads 24VDC, not higher, when the breaker is open. There IS a possibility that there is a miss-wire but I can't figure out what kind of miss-wiring would cause a 38VDC output before the breaker is closed.
     
    Last edited: Dec 3, 2008
  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    When you say that you "...get 38V at the end of the circuit BEFORE I close the breaker.", does "end of the circuit" mean the two terminals at the far right of your schematic? If so, it looks to me like you would be measuring an open circuit when the breaker isn't closed.
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Assuming you want a positive voltage, then typically you would connect the (-) terminals together without any diodes (call this point GND, it will be common for the source outputs and load). Then use a forward biased diode between each positive terminal and the output ...whichever voltage is higher wins.

    Note that the diode amp rating must be larger (typically 2x) than the current drawn by the load.
     
  6. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    But WHY do I get 38V? It wouldn't make a difference in the output voltage to not have the common negative, correct?
     
    Last edited: Dec 3, 2008
  7. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    Thats right. The open circuit voltage is 38V. There is no load on the end of the circuit but when the breakers close, the voltage is 27V (same as the battery)
     
  8. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Try it the way I suggested, and see if it works (be careful to know the supply polarites). If you're using an integrated diode bridge instead of discrete diodes, you can still use it by connecting the inputs to the AC terminals, and the + (joining cathodes) is the output.

    I think The Electrician was pointing out that if you are measuring on the far side (load side) of the breakers when they're open, then you should have nothing or 0V. Otherwise, something is generating 38V on the load side.
     
  9. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    Ohhh I see. The Electrician makes more sense to me now :)

    I will give 'er a shot and see if it makes a difference.
     
  10. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Another thing to try: while you're making the measurement that gives a reading of 38 volts, unplug the two items that are plugged into the power line, one at a time, and see if you get a change in the reading.

    Fluke meters have a high input impedance, and hum picked up from the line can be rectified by diodes to give erroneous readings sometimes, especially if you're measuring what is essentially an open circuit. There may be leakage at the megohm level through the breakers contributing to this effect.
     
  11. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    I appreciate your input but I have tried to unplug one at a time. Regardless as to which source was unplugged, I would get 24-27 Volts, not 38VDC.

    Now, this is DC and unless you are speaking of the avalanche breakdown phenomena, I'm afraid I don't understand where this hum you speak of would come from.

    Again, Im new at this, but I have done a lot of troubleshooting in my days. This would be the first time I've seen a my Fluke multimeter give me any errors do to its impedance, especially on such a simple circuit. I will try the test again tomorrow with an analog voltmeter and see if that gives the same result.

    Thank you again for your input.
     
  12. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    So, are you saying that both sources have to be plugged in to get the 38 volts?

    There's 60 Hz hum all over any building that is supplied with power from the grid.

    Try the following experiment to see what I'm talking about. Take any old silicon rectifier, such as a 1N4004, and connect the voltmeter probes to it, one on each side. Have the meter on a low DC volts range. Spread the leads apart and walk around the room, watching the meter reading. Touch one of the legs of the rectifier and see how much voltage you get. You probably won't get 38 volts, but you will get something, and it's DC. A lower impedance meter won't get so much.
     
  13. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    Yes.


    This is on a boat with no AC on it at all.

    Hmmm! That would be great to see. I will try that.

    Thanks again
     
  14. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Now I'm really confused. You just said they both had to be plugged in to get 38 volts. Where's the AC coming from? A generator or inverter?
     
  15. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    When the batteries need charging, they are charged from an alternator (I suppose that constitues AC). The converter is run by a shore line The shore power is isolated from the boat. What I mean is the boat does not have any AC running through it (no plugs, etc)
     
  16. Joshmann

    Thread Starter Member

    Dec 3, 2008
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    Here's an update: The circuit is feeding a ground fault meter which has a voltage divider in it. Somehow when both sources were hooked up, the voltage divider fed back to just before the diodes that are attached to the sources. It was a fault in the meter but only when it was hooked up.

    Confusing I know. Replaced the meter, everything works fine.

    Thanks for all of your help
     
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