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Silicon Controlled rectifier

Discussion in 'Electronic Basics' started by joe, Mar 27, 2007.

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  1. joe

    joe Guest

    A curcuit I would like to build calls for a 6 amp. 200 volt SCR. I
    can't seem to find that particular component and was wondering if I
    could use a SCR that is rated for 200 volts but has a amperage rating
    of over 6. After reading a little bit about SCRs I think the voltage
    rating is a trigger voltage so it shouldn't be changed. I'm not quite
    sure what 6 amp. means. Is it the maximum current a SCR can take?
     
  2. As long as you meet the minimum and it fits it should be fine, so a 400 volt
    or 10 Amp or both would work OK.

    There are some sensitive gate models but I doubt yours is one of those.
     
  3. DJ Delorie

    DJ Delorie Guest

    The voltage and current ratings on SCRs are maximums. I.e. in your
    case, if you put more than 200 volts across the SCR, it will damage
    the device. Also, if you put more than 6 amps through the SCR, it
    will damage the device.

    So a 200V 6A SCR can handle *up to* 200V and *up to* 6A. An SCR with
    larger voltage and/or current rating should be a suitable substitute.

    Note that SCRs *do* have a trigger voltage (and current), but it's not
    the 200V you see listed. It's usually a lot less, like a volt or two.
     
  4. Phil Allison

    Phil Allison Guest

    "joe" <>

    **Groper alert !

    ** For god's sake, post the post the damn part number !!

    No-one can advise you unless you do.




    ........ Phil
     
  5. Mark Zenier

    Mark Zenier Guest

    Won't it just self trigger? So any damage will depend on
    whether the rest of the circuit limited the current.

    Mark Zenier
    Googleproofaddress(account:mzenier provider:eskimo domain:com)
     
  6. ehsjr

    ehsjr Guest

    No, an SCR needs a trigger current on a separate lead.
    It has an anode, cathode and gate. The trigger is applied
    to the gate:

    + --o
    \S1
    o
    |
    +----------+
    | |
    | [Load]
    [R] | a
    | [SCR]
    | _ g/ | c
    +--o o--- |
    PB |
    |
    Gnd ------------+

    When the momentary pushbutton (PB) is pressed, current is
    supplied to the gate until the pushbutton is released. The
    SCR turns on. It will remain on after the pushbutton is
    released, until S1 is opened. Closing S1 again will not
    trigger the SCR. It won't trigger until the pushbutton is
    pressed again.

    Ed
     
  7. Mark Zenier

    Mark Zenier Guest

    That's the ideal case. What happens to an SCR if used outside of its
    ratings?

    As I understand it, if you apply too much voltage to an SCR anode, it
    leaks current, and that current acts the same as the trigger current into
    the gate, causing the device to flip into conduction. But since that's
    behavior outside of the device specs, it's not guaranteed at what voltage,
    just that it's higher than the rating.

    Mark Zenier
    Googleproofaddress(account:mzenier provider:eskimo domain:com)
     
  8. ehsjr

    ehsjr Guest

    There is a forward voltage at which the SCR will conduct in the
    absence of gate current - and you might get away with it, without
    damaging the SCR. But you might not.

    It is better to treat the ratings as they are intended:
    they are maximums which are not to be exceeded.

    That does not invalidate your observation. In fact, they used
    to provide a breakover voltage level at which conduction would
    occur in the absence of gate current, and a peak forward voltage
    rating not to be exceeded. As long as your voltage was between
    the two levels, you could get the effect you mentioned, without
    damage.

    Ed
     
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