Crowbar protection on power supplies

Discussion in 'General Electronics Chat' started by DexterMccoy, Mar 8, 2014.

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  1. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    What does using a Thyristor or SCR in parallel with the filter caps on the output of a power supply do?

    They use a Thysistor or SCR instead of a zener diode to do the voltage regulation? but why? what is the advantage?

    It's a 2N2325 Thysistor

    Why do you need to limit the gate current?

    What might happen if the current was not limited?


    The Gate has a threshold voltage or threshold current?

    once you exceed above the threshold it triggers the gate ?

    Do most crowbar protection circuit has an RC network or RC time constant?

    The dynamic resistance of the zener
    What do you mean by the dynamics resistance of the zener?

    Do you measure the resistance of the zener diode?

    Here is the schematic
     
    Last edited by a moderator: Mar 8, 2014
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    An SCR is a solid state over current protection mechanism. If too much current flows (set by user), then current to PS pins stops and flows through high current capacity SCRs instead. SCRs are much faster response than fuses and, can be reset by tuning the power supply off then on again.
     
  3. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    Thanks for the info

    How do you know when you have a bad SCR? or signs of one?

    Do SCR through aging start to have leakage?

    The gate is current triggered?

    So you set a resistor value for the gate pin, this resistor value is the "current limit threshold" for the gate of the SCR?
     
  4. GopherT

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    I don't have a lot of experience with SCRs, I just reviewed the whole circuit on my power supply - what brand/model do you have?
     
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    That crowbar is not an over current protection, it is strictly for over voltage. If the output voltage goes above a set threshold, the SCR turns on and clamps the output which is intended to keep the output voltage from going too high. In most designs, there is an input fuse somewhere that blows after the crowbar clamps on.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The SCR only requires a small gate current, usually a Diac is used in the gate.
    If an SCR is shorted you will measure a low resistance in both directions from Anode to Cathode.
    Using a capacitor can affect the turn off, if used in a DC environment some method will have to be used to turn the SCR off if the voltage does not transition to zero.
    What is the schematic of?
    Max.
     
  7. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    Why does the capacitor affect the SCR turn off?

    The capacitor delays the SCR gate trigger i thought?
     
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