I am designing a circuit that controls DC power to a load through the use of SCR thyristors. I have been testing things out using the CLA20EF1200PZ part from Littelfuse. It has a 25 mA holding current which is low enough for my application. Here is a schematic of my testing setup:

SCR test setup

When the SCR is triggered at the gate by shorting the gate to VCC, a 100 mA current starts flowing through the 240 Ω resistor. Then the 16 Ω load is switched ON which adds 1.5 A of current to the total current flowing through the SCR. However when the 16 Ω load is disconnected by disengaging the mechanical switch, the SCR cuts current flow, even though 100 mA should still be flowing through it as the current is above the rated holding current.

From my understanding the only conditions for a thyristor to switch OFF is either the current falling back to or below 0 A or have a negative voltage across the SCR.

Is there a known reason for this behavior to occur?

  • 2
    \$\begingroup\$ Parasitics. You probably have enough stray inductance and/or capacitance in your setup so that the transient generated by opening the switch is enough to "commutate", or switch off the SCR. \$\endgroup\$
    – Dave Tweed
    Commented Jun 9, 2023 at 12:47
  • \$\begingroup\$ The 25 mA holding current is specified with 6 V across the SCR. With If the voltage across the SCR is lower in your case, the required holding current may be higher. \$\endgroup\$
    – Hearth
    Commented Jun 9, 2023 at 16:32
  • \$\begingroup\$ @user263983 The original IXYS datasheet says 25 mA at 25 °C and Vd=6 V. I'm not sure where you're seeing 100 mA. \$\endgroup\$
    – Hearth
    Commented Jun 9, 2023 at 16:33
  • \$\begingroup\$ @user263983 Littelfuse bought IXYS a few years ago. The link on that page goes to exactly the same datasheet that I linked in the first place, just hosted on Littelfuse's site instead of Digikey's. \$\endgroup\$
    – Hearth
    Commented Jun 9, 2023 at 17:14
  • \$\begingroup\$ @user263983 But your link is to a page that covers a lot of parts, are you sure you're looking at the datasheet specifically for the CLA20EF1200PZ and not for something else? Link the datasheet you're talking about directly if you want to be unambiguous. \$\endgroup\$
    – Hearth
    Commented Jun 9, 2023 at 19:33

1 Answer 1


Here is an example of what Dave is talking about in the above comment (my choices may not exactly represent your setup). Unfortunately Circuitlab does not have a simulation SCR model so I've substituted a 1A diode.


simulate this circuit – Schematic created using CircuitLab

enter image description here

As you can see, the diode current briefly drops below zero.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.