2 SCRs going to a diode bridge

Discussion in 'General Electronics Chat' started by dick56, Nov 20, 2013.

  1. dick56

    Thread Starter New Member

    Apr 27, 2011
    23
    3
    We have a battery charger that used 2 SCRs with a trigger coming from a circuit board that is kaput. I would like to put a diode bridge in place of the 2 SCRs. I have a 20 amp 400 volt bridge for a 24 volt charger that puts out 10 amps. The transformer outputs went to the AC side of the SCRs and the output to the battery on the + side of the SCRs with the trigger wire going to a circuit board.

    Would I be better off to use two large 15 amp 400 volt diodes in place of the bridge? I am confused on how to wire the bridge, but understand how to wire the straight diodes. The bridge, a KBPC 50-10, has one post labeled as + and two labeled as ~ (AC) with the fourth post not labeled.

    Any help would be appreciated.
     
  2. #12

    Expert

    Nov 30, 2010
    16,313
    6,817
    Depends on the transformer, whether it's center tapped and how much voltage it produces. The bridge gets the wiggles connected to two ends of the same output winding, the + mark is positive DC output the the unmarked terminal must therefore be the negative side of the DC.

    If that produces too much voltage, and the transformer is center tapped, two diodes would be the better choice because that would produce half the voltage.

    Do some measurements and I'll go fish up some drawings.
     
  3. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    I just worked on similar today.

    If the scr's are not used for regulation and diodes will work. Turn the scr's on with a resistor. Then you will have what is essentially a diode.

    If you find current or voltage too high for your application, you can phase control them pretty easily.

    In my case an 80 amp fuse was blown. Someone added a second set of leads to it and they were marked + - reversed.:(

    In addition the 4 #4 cu leads were forced thru a 3/4" romex connector.:eek:And shorted.
    I had to saw it in half to remove them.

    Before figuring it out, I thought the scr's were shorted so I got looking for replacements.

    Hows this for overkill? It was a half bridge, so I reversed one puck to make it common anode, like battery chargers use.
     
  4. dick56

    Thread Starter New Member

    Apr 27, 2011
    23
    3
    Yes it is a center tapped transformer. The transformer output is 58 volts AC so two 10 amp diodes should bring the voltage down to the high 20s or low 30s which would be fine.

    Thanks for the quick replies.
     
  5. inwo

    Well-Known Member

    Nov 7, 2013
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    I thought I posted this before, but don't see it now.

    Remember that scr's are diodes basically.

    Trigger them "on" and your good to go.
     
  6. #12

    Expert

    Nov 30, 2010
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    I believe inwo is trying to guide you to the idea that you already have 2 diodes and they are already attached to a heat sink. Very convenient except for the fact that they are disguised as SCR's. Just give them a trigger resistor and you're in.

    In fact, you can use a resistor and a capacitor to delay their start time per pulse and use them as a crude pre-regulator. From the way you talk, I think you can figure this out by looking up the SCR's and trying some resistance and capacitance combinations.
     
  7. inwo

    Well-Known Member

    Nov 7, 2013
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    Yeh, like he say's!:D
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Almost certainly, in the original battery charger, the SCR's where used to throttle back the charging current as the battery reached the fully charged terminal voltage. If you just replace the SCRs with Rectifiers (or fully turned-on SCRs), you will have created a nice battery-destroyer :(
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
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    I agree.
    My suggestion fit with his solution of replacing scr with rectifiers.

    The one I just worked on however, used scr's on or off, sensing voltage knee.
     
  10. #12

    Expert

    Nov 30, 2010
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    That's why regulators and resistors were invented. You can bet there will be some in the final design.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    My point is that the SCRs were part of the the regulator!
     
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