# Help me understand thyristor used as a latch

#### robotDR

Joined Mar 17, 2020
76
Hi All,

My first question is:
1) What is the impedance from gate to cathode of an SCR/thyristor?

I ask because I want to use the SCR as a lowside switch/latch for power to a WDT. The gate of the SCR will be connected to the same pulse signal input to the WDT (from my microcontroller).

I want this because my WDT output is latched upon an error. And my microcontroller needs time to boot up and start the pulse. The time can vary and I want something to detect the pulse is live, and to then power up the WDT so it doesn't prematurely latch the output in an asserted state.

I put the led in parallel with the WDT to make sure there is sufficient holding current to keep the SCR on.

Take this SCR for example: https://media.digikey.com/pdf/Data Sheets/Littelfuse PDFs/TCR22-x_Sx02CSx_Series_DS.pdf
It need 5mA holding current. But then it says I_t is 200mA? Is that the latching current I need to go above to maintain anode to cathode current while gate is 0? That is a lot, I just saw that.

Can someone help me design this circuit. The WDT appears to pull like 10 uA. This is why I need something to help pull current to maintain the SCR in closed position. Also an LED would be a good debug indicator in this case i think.

Thank you!

#### dl324

Joined Mar 30, 2015
16,669
What is the impedance from gate to cathode of an SCR/thyristor?
It's a PN junction.

The control signal you show for the SCR won't work. To turn it off, you need to short the anode to the cathode. If you use a sensitive gate type, you still need to provide a signal of the opposite polarity to turn it off.

#### robotDR

Joined Mar 17, 2020
76
It's a PN junction.
Does that mean there is effectively no resistance?

The control signal you show for the SCR won't work. To turn it off, you need to short the anode to the cathode. If you use a sensitive gate type, you still need to provide a signal of the opposite polarity to turn it off.
I only want the initial high to trigger the SCR on. I don't want it to turn off until power down of the system. I want it latched on.

#### dl324

Joined Mar 30, 2015
16,669
Does that mean there is effectively no resistance?
It's a diode, so it has resistance.

I only want the initial high to trigger the SCR on. I don't want it to turn off until power down of the system. I want it latched on.
The signal you show controlling the SCR is confusing. Once triggered, subsequent pulses will have no effect.

#### crutschow

Joined Mar 14, 2008
34,019
Does that mean there is effectively no resistance?
It has the forward voltage drop and resistance of a diode, so the current through the gate must be limited to below its maximum rating (typically with a series resistor of the appropriate value).

#### robotDR

Joined Mar 17, 2020
76
It's a diode, so it has resistance.

The signal you show controlling the SCR is confusing. Once triggered, subsequent pulses will have no effect.
Yeah that’s exactly what I want. I want it latched after the first state change.

#### robotDR

Joined Mar 17, 2020
76
It has the forward voltage drop and resistance of a diode, so the current through the gate must be limited to below its maximum rating (typically with a series resistor of the appropriate value).
ok I tried looking for a rating but I couldn’t find it in the data sheet. I’ll take another look. Thanks!

#### robotDR

Joined Mar 17, 2020
76
It has the forward voltage drop and resistance of a diode, so the current through the gate must be limited to below its maximum rating (typically with a series resistor of the appropriate value).
ok it looks like peak gate current is 1A at junction temp of 110C.

it looks like I need to drive this gate at 200uA to trigger the SCR on. And it says 0.8V trigger? Does that mean the forward voltage is 0.8 V at 200uA?

#### crutschow

Joined Mar 14, 2008
34,019
Does that mean the forward voltage is 0.8 V at 200uA?
The maximum voltage is 0.8V @ 200µA.
The nominal would be a little less.