Thyristor behaving as TRIAC?

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

  1. egonx

    Thread Starter New Member

    Jul 15, 2013
    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,

  2. ian field

    AAC Fanatic!

    Oct 27, 2012
    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
    Try it with the load on the anode side instead of the cathode side.
  4. egonx

    Thread Starter New Member

    Jul 15, 2013
    Thanx for the answer! but i don't think this is the case. It says on the datasheet (
    "...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?

  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    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.
    egonx likes this.
  6. egonx

    Thread Starter New Member

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

    Thread Starter New Member

    Jul 15, 2013
    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!