Confused about a bridge rectifier

Discussion in 'General Electronics Chat' started by drbenne, Apr 10, 2014.

  1. drbenne

    Thread Starter Member

    Jul 30, 2013
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    Hey guys,

    So, I'm troubleshooting this linear power supply (12Vdc) by doing the basics first. Tested primary side of transformer, tested fine (120Vac). Tested secondary output, seems fine (33.6 Vac). This is then connected directly to a what I'm pretty sure is a bridge rectifier (part no. 70CT01P, couldn't find a datasheet). Tested the DC output of the rectifier and get 0V

    Unplugged all connections to the rectifier and used the diode test function of my meter. I'm getting a forward voltage drop across each diode and no voltage drop in reverse bias across each. Using these conditions I then drew out a schematic according to my tests and I'm getting confused as to how this particular rectifier is connected.

    I attached a schematic I drew out according to my diode tests which doesn't seem right. Also, the labeling on the rectifier doesn't specify positive or negative for the DC output, it only has 2 positives. One can be seen on the top of the picture I took. The bottom side looks the same except the sine wave symbol says AC.


    Thanks,

    Dave
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think you probably have a center tap transformer then everything would make sense. The center tap would be ground, the 2 +'s tied together for the positive output.
     
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  3. drbenne

    Thread Starter Member

    Jul 30, 2013
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    Thanks :)

    I looked at the bottom of the transformer and sure enough there was the center tap. Measured from the center tap and got a reading. Did a little more probing and found two blown pass transistors (2N3771). Replaced both with two 2N3055s temporarily to see if that fixed the problem, doesn't seem to have. Gonna order some 2N3771s today.

    Dave
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Your schematic of the bridge is incorrect. It matches the pin locations in the photo IF you reverse the two diodes going to the lower left pin and mark this pin -. In your photo the upper right tab (the one that is oriented differently from the other three) is the +DC output. The upper left and lower right tabs are the AC inputs, and the lower left tab is the -DC output. In other words, AC input on diagonally opposed corners, DC output on other two diagonally opposed corners.

    The bridge is the same for transformers with and without a cernter tap.

    ak
     
  5. drbenne

    Thread Starter Member

    Jul 30, 2013
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    That's what I thought too but, again, using the diode test function on my meter yields the schematic I drew, unless both diode directions spontaneously changed direction which I think is highly unlikely . I don't have the supply near me at the moment but on the underside of the rectifier it has a positive sign where you said should be the negative DC output in the lower left corner.
    With the black probe on the center screw (ground) and the red probe on either DC output corner I get ~+22Vdc I believe. This confuses me also because shouldn't a rectified wave yield around the same as the AC input.
     
  6. praondevou

    AAC Fanatic!

    Jul 9, 2011
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  7. profbuxton

    Member

    Feb 21, 2014
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    I also believe you have a center tapped tx. That would make sense of the diode layout.
    Your measurement of 22v dc sounds about right for a measurement to ground from one side of the output. Were the filter caps in circuit? As I understand your measurement you are only measuring half of the output. Is your 33vac across the total sec winding or from the center tap to one side?
     
  8. drbenne

    Thread Starter Member

    Jul 30, 2013
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    Yes, the filter caps are in the circuit. The transformer is actually a dual secondary output. One output is 10.6 Vac and the other is 33.6 Vac. They are both center tapped with the two centers tied to each other. I had to desolder one center tap from the board to confirm this. The 33 Vac is across the sec winding. From center to one side is 16.7 Vac.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Seems all as it should be then. It sounds like your transformer is probably 32 volt center taped that is reading a little high so 16.7 volts to the center tap. The meter measures RMS on ac, but when the voltage is applied to the caps it charges them to the peak of the AC waveform so 1.41 X as high. 23.57 volts, but the diodes drop it by .6 X 2 or 1.2 volts less at 22.4 or so.
    The 3055's probably should have worked so there might be something else going on. Is there an adjustable voltage regulator as well (Maybe like an LM317)? What ever drives the big transistors may be bad. If you have a schematic it could be a life saver.
     
  10. drbenne

    Thread Starter Member

    Jul 30, 2013
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    unfortunately I don't have a schematic. I tried looking but couldn't find one anywhere. It's a Tripp-lite PR-20 if someone happens to know where one is :confused:
     
  11. inwo

    Well-Known Member

    Nov 7, 2013
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    Someone suggested they don't release schematics, but it might be similar design to Astron.

    I have an RS-20

    Does this look similar? (fat chance:))
     
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  12. drbenne

    Thread Starter Member

    Jul 30, 2013
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    No, this is one is pretty different. I couldn't identify a regulator on this one. I did see a LM614CWM and LM393N though.
     
  13. inwo

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    Nov 7, 2013
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    They may send a schematic.
    You need a serial number to request tech support though.:(
     
  14. drbenne

    Thread Starter Member

    Jul 30, 2013
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    I guess it wouldn't hurt to ask. In the mean time ill probe around to see if something jumps out at me. :(
     
  15. ronv

    AAC Fanatic!

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    Maybe a picture would help. Or trace back from the base of the power transistors to the next transistor.
     
  16. inwo

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  17. drbenne

    Thread Starter Member

    Jul 30, 2013
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    Here is somewhat of a crude schematic I drew up to help me diagnose. I checked as best as I could to ensure accuracy. I'll probably have to go back and add resistor values, perhaps at a later time as needed. The ICs are as follows:

    2n3771
    LM614CWM
    2n6388
    LM393N
    TL431
    MCR692G
     
  18. drbenne

    Thread Starter Member

    Jul 30, 2013
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    Wow guys.. :eek:

    I put it back together to start probing in different areas and went to check the output and lo and behold, there is a nice 13.8Vdc . Put a load on it and it stayed the same. Not sure what the problem was. I know for sure those 3771s were no good. :confused:
     
  19. LDC3

    Active Member

    Apr 27, 2013
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    The way this bridge is used is the outer taps from a center tap transformer are connected to the 2 AC inputs and the DC ground is connected to the center tap. That way you get a positive output from each 1/2 for a half cycle. If you look at circuit that was drawn by drbenne, you can see that is how it is wired.
     
  20. inwo

    Well-Known Member

    Nov 7, 2013
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    I think we all know how a full wave center tap supply works.:cool:
    The question seems to be about a rectifier package that has two common anode and two common cathode.

    That couldn't rightly be called a bridge.:confused:

    I can't think of a reason for designing that package, nor have I found another.
    If the only use is parallel diodes. Then why not a half bridge of sufficient rating?

    I guess maybe a dual supply from the same ac source. But what would be the advantage?
     
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