Thyristor behaving as TRIAC?

Discussion in 'General Electronics Chat' started by egonx, Jul 15, 2013.

  1. egonx

    Thread Starter New Member

    Jul 15, 2013
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    Hello Everyone,

    This is my first post. I'm trying to build a thyristor bridge, and being my first time, before i jumped to the real thing, i started playing around just to make sure, everything works, and I'm not making any wrong assumptions. I am going nuts with the circuit i attached.
    I press the pulse switch and i get a waveform (on the load) like the one on the figure. I cannot figure out why the hell is happening...

    NOTE1: The resistor values are not standard cause i had to use what i had.

    NOTE2: If i take away the 190ohm resistor i get the same waveform.

    thanx in advanced,

    egonx.
     
  2. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    They're fairly rare; but some SCRs are "non-blocking" - that is they have an integral diode much the same way as a MOSFET has a body diode.

    BTW: you only have to replace 2 diodes in the bridge with SCRs for most F/W power control applications.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    Try it with the load on the anode side instead of the cathode side.
     
  4. egonx

    Thread Starter New Member

    Jul 15, 2013
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    Thanx for the answer! but i don't think this is the case. It says on the datasheet (http://www.nxp.com/documents/data_sheet/BT151_SERIES_C.pdf):
    "...intended for use in
    applications requiring high bidirectional blocking voltage capability..."

    Regarding your second point, if you mean a half-bridge, that is exactly what i'm doing.

    What do you mean with F/W?

    Cheers.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    656
    Why are you using a 12A SCR on an 18mA load? I don't know how SCRs react to high impedance loads, but I suspect it has to do with the minimum holding current. Try it with a 10, or 20, or 100 ohm load and see if it works.
     
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  6. egonx

    Thread Starter New Member

    Jul 15, 2013
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    I tried, but no luck.
     
  7. egonx

    Thread Starter New Member

    Jul 15, 2013
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    Ok, i changed the load for a 47ohm load and it worked. I am new to power electronics what can i say? :p thanx! Correct me if i'm wrong, but shouldn't i design to get over the latching current?

    It's weird though because i was below both, latching and holding current, and yet, the thing fired when i pulsed the switch and stopped after releasing it (on the following zero-cross). That's a triac right?
    I mean, it shouldn't have fired at all.

    thanx again!
     
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