I'm trying to simulate crowbar circuits, which is mentioned on page 8 of ON SEMI TL431 datasheet (https://www.onsemi.com/pub/Collateral/TL431-D.PDF). I'm using TINA TI for this. There are two circuits, one with TRIAC, another with SCR. Trip voltage formula for both is the same, Vtrip = Vref*(1+R1/R2). However, simulation of TRIAC circuit shows that it trips at about 10.8 V instead of planned 15.25V:

enter image description here

If I change internal resistance of VS1 from 0 to 50 mOhm, simulation renders out expected result. Also, adding reverse polarity protection P-MOSFET makes crowbar to trigger at any input voltage: enter image description here

Circuit with SCR demonstrates expected simulation results both with and without P-MOSFET: enter image description here enter image description here enter image description here

20 Ohm resistor in each case is imitating a load. In real application load will be some complicated device with about 500 mA consumption.

Questions are:

  1. Why TRIAC circuit simulation shows such results?
  2. In SCR circuit did I chose R5 and R6 values (1k both) correctly?

1 Answer 1


Both circuits using triac connect the gate to the positive supply via 330 ohms resistor. This will trigger the triac when the gate receives enough current through the resistor.

To confirm this, remove TL431 from simulation. It should still trigger. Also trigger voltage should depend on the value of 330R resistor. Lower resistor value means higher gate current, so it should trigger at a lower voltage.

SCR circuit, on the other hand, uses a BJT to push current in the SCR gate when it needs to be triggered, so it works. So, you should either use the BJT in both setups, or add a diode in series (cathode on TL431 side) with the triac gate to make sure it can only be triggered by negative gate current (drawn from the TL431) not positive gate current (from the 330R resistor).

Notes about the SCR circuit:

R4 should be low enough to make sure the transistor is not biased into conduction unless threshold voltage is reached.

The BJT needs a base resistor. Value uncritical, like 4k7.

R6 is unnecessary.

R5 should set the SCR gate current ; check how much gate current it needs from SCR datasheet, and calculate R5 accordingly. When TL431 triggers it will have about 2-2.5V between A and K, so with 15V supply there should be about 12.5V-13V across R5, which gives gate current.

  • \$\begingroup\$ Isn't R6 required to keep the gate of SCR tied to GND (not floating)? SCR I've chosen has trigger current of 45 mA, which renders R5 at 220R (13/0.045 = 289, so choosing next value). Not so sure about R4, how do I calculate it? I thought it is required in order to keep TL431 in it's working current limits (1-100 mA). I think in real application I'll go with TL431, so TRIAC scheme isn't so important for me. However, removing TL431 doesn't help. Anyway, thanks a lot! \$\endgroup\$
    – Zhenek
    Nov 24, 2019 at 17:26
  • \$\begingroup\$ R6 required? Hmm, if the transistor is not conducting, no current flows into the gate so the SCR should not trigger at all. I don't think R6 is necessary. I'd set R4 at 330R so even if TL431 draws 1mA the transistor is sure to be off. \$\endgroup\$
    – bobflux
    Nov 24, 2019 at 18:06

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