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Can I use 4 scr's as a ordinary rectifier bridge ?

Discussion in 'Electronic Basics' started by Ray, Feb 16, 2008.

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  1. Ray

    Ray Guest

    I have a bunch of old 80 amp scr's (GE C46h) out of an old lighting dimmer

    I would like to use 4 of them to make an ordinary bridge rectifier for a
    heavy duty 12-15 vdc power supply...

    What do i do with the gate, and the other wire sticking out of the scr ?

    When these are connected, do I gave an 80 amp bridge, or a 160 amp bridge ?

    I have read arguments about this, havent heard a definite conclusion...(Is
    each diode only in use half the time ?

  2. Phil Allison

    Phil Allison Guest


    ** The C46H SCR has two terminals on top of the case - the gate is the
    smaller and the larger is the cathode. Connect a 1 amp diode from the case
    ( ie anode) to the gate to convert the SCR to simple diode operation.

    ** Neither.

    ** For a bridge feeding a capacitor input filter - each diode conducts only
    about 10 % of the time.

    Down load the pdf on this page - it has all the gory details.

    Your idea will maybe create a 30 amp ( average DC current) bridge.

    It will need a massive heatsink too.

    Buy an ordinary 40 amp bridge.

    ....... Phil
  3. You have to figure out a way to trigger the gates each time
    the SCR is forward biased, but not over voltage the gate in
    reverse, when the SCR is reverse biased. And you will have
    to put up with about twice the forward voltage drop compared
    to normal rectifier diodes, since each SCR has two junctions
    in series when they are conducting.

    Are actual rectifier diodes so expensive?

    If I were building a 12 to 15 volt supply, I would use
    Schottky diodes and cut the forward drop about in half,
    compared to silicon junction diodes, and cut the waste heat.
  4. Ray

    Ray Guest

    I am looking to make an 80 amp or greater power supply, and havent found any
    bridges in that category, and 100 amp diodes seem expensive and hard to

  5. Phil Allison

    Phil Allison Guest


    ** You CAN use more than one 40 amp bridge - you know.

    Either wired in parallel and mounted on a common heatsink - OR and much
    more technically elegant, have the AC transformer wound with 2 or three
    *identical* secondaries.

    Then you only parallel the + and - terminals.

    ....... Phil
  6. Do you want a two diode (half bridge) to use with a center
    tapped winding, or a 4 diode (full bridge) to use with a
    single winding?

    70 amp stud diodes from Digikey run $8 each, and can be had
    in reverse polarity, so two of the cathode to stud polarity
    can be mounted on one isolated heat sink for the positive
    output and two of the anode to stud polarity can be mounted
    on another isolated heat sink for the negative output.

    Those 4 in a bridge can deliver more than 80 amps out.

    The ones to watch for on eBay are two diodes in series (the
    common terminal connects to the anode of one and the cathode
    of the other) in an isolated module. A pair of them make an
    easy to cool bridge. Here is an example:
  7. BobG

    BobG Guest

    Get a couple of 80 amp mosfets and make a synchronous rectifier. If
    the diodes are wasting .7V * 80A (56W!) the mosfets should be able to
    do better than that.
  8. BobG

    BobG Guest

    John... I saw a dozen of those at Skycraft Saturday
  9. Ray

    Ray Guest

    I want a full bridge, as I have a couple of 80 pound transformers with a
    single output winding,

    I guess I'l just buy the appropriate diodes.... I was just trying to
    utilize some of the stuff I've been collecting, and the scr's seemed like a
    good idea. I want to build a heavy duty general purpose power supply for
    battery charging large batteries at my cabin, driving a winch, maybe light
    welding. I have built a pwm controller for the output so I can use it with
    both small and large generators, I see some 250 amp diodes on Ebay i'll
    probably bid on

    I learned something, I didnt know they made dides in a series
    Thanks for the input


    " tive output.
  10. I can't find any through their web page.
    Maybe you had to be there.
  11. Ray

    Ray Guest

    The synchronous mosfet seems like a good idea, I'l l be reading up on that,
    as the 75 amp controller was the only thing I've ever done with mosfets.
    probably could be able to integrate it all into one circuit.

  12. Phil Allison

    Phil Allison Guest


    ** Be aware that mosfets are diodes when non conducting.

    ........ Phil

  13. Their web page has always been useless, and about as informative as
    the people who work there. The last time I looked at it, it was nothing
    but pictures of them at the Orlando Hamfest.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  14. BobG

    BobG Guest

    They do a lot of mail order and ebay sales.... but I bet if you called
    and described the gizmo and tell them its sitting on the top shelf
    right by the bridge rectifiers they'd put it in a bag and mail it to
    you. I think they were marked about $2. I was looking in the box next
    to them for IGBTs... they get those once in awhile. Hamfest was last
    weekend. Place was shoulder to shoulder with folks from all over the
  15. default

    default Guest

    Don't count them out entirely. If you want welding, battery charging
    etc. you can use them in a partial bridge - two regular diodes and
    two SCR's for a pretty good adjustable power supply. Much more
    efficient than a linear supply of the same amperage.

    You just need a phase control for them - the basic unijunction and
    phase shifting network to trigger them. I did something like that
    awhile ago and was happy with the results. I used a pair of SCR's and
    a center tapped transformer, but if I were to do it again I'd use a
    pair of diodes and scr's.

    The only downside was I needed a source to trigger the SCR's that was
    more positive than the cathodes (positive output). If you used pulse
    transformers that would be unnecessary. In my case I had a toroidal
    transformer so just wound a 10 volt winding on top of the windings
    already there. It just supplies a low current so doesn't need a
    massive second transformer.

    The upside is it is relatively bullet proof. Mine had no feedback so
    didn't "regulate" just controlled the output. Mosfets in a welding
    environment wouldn't be my first choice. SCR's TRIAC's or IGBT's
  16. Jamie

    Jamie Guest

    Learn a lot more electronics before even thinking about this?
    No, you get only what the max of a single item can deliver and that is
    with proper heat sinking and voltage ranges. (80 amps)
    only 2 diodes out of a bridge are active at once. the pairs alternate
    in a time sequence.

    that's about as simple as I can make it.

    Unless you're planning on making a phase controller for these SCR's
    I would suggest you stick to diodes rated for the load and voltage.
  17. Jamie

    Jamie Guest

    Come one Philip, you know better than paralleling bridges with out
    load balance compensations. And I really don't think you can achieve
    that with bridges since there is no guarantee all diodes used in the
    bridge are matched. In the end, one bridge is going to be doing more
    work than the other. Using resistors is just going to deteriorate the
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