# Triggering of TRIAC for Crowbar Protection

I am reading the datasheet of TRIAC (T1235T-8G). I am not sure how to trigger the TRIAC and what do they mean by maximum Vgt voltage (Page 2, Table 3).

Basically, I want to use that TRIAC for crowbar protection and I want to select a mechanism to trigger the TRIAC.

Note: Ignore the LM431 in the picture as I would be using some other reference IC.

Picture taken from here.

I want to use the TRIAC in Quadrant 1 (because of it's sensitivity). I have few queries:

1. Does TRAIC for Quadrant 1 will be triggered for any positive gate voltage (but gate should have minimum Gate current of 1.75mA)? So, how can I calculate minimum gate voltage required to latch the TRIAC?
2. What do they mean by Maximum Gate Trigger Voltage? Does it mean if I apply more voltage than the maximum, then TRIAC may get damage?

1. The gate requires at most 35mA to trigger at a junction temperature of 25°C. It could be more like 53mA at -40°C. That applies to Q1, Q2 and Q3.

It is guaranteed not to trigger with 1.75mA or less gate current. Again at 25°C. That could be around half that at high temperature. Q1, Q2 and Q3.

2. The current is the independent variable. They are saying that when you drive the gate with enough current to trigger the triac (exact conditions stated), at that current, the gate voltage will not exceed 1.3V (at 25°C junction temperature). It could be more like 1.65V max at -40°C.

You should drive the gate with a controlled current, knowing that the gate voltage at trigger can be variable. Usually that means a series resistor. In the case of the crowbar, the triac removes drive when it crowbars the supply, so provided that happens fast enough that the peak current does not exceed the maximum the triac won't be damaged.

In essence the way you use those numbers is as follows. Suppose you are driving the gate with a switch (such as a transistor) and series resistor R from a fixed supply voltage Vs. The switch has a voltage drop of Vsw

The gate current is Ig = (Vs - (Vsw + Vgt))/R

To ensure the triac will trigger under all conditions (Ig >= Igt) you need to look at the worst-case values for Igt, Vs, Vsw, Vgt and R.

That will likely occur at the minimum junction temperature for worst-case Igt, minimum Vs, maximum Vsw (at Igt) and maximum R.

• So, What will be Vgt at Min Igt? – abhiarora Sep 24 '18 at 19:39
• There is no specification for that. In practice if you keep gate voltage below 300mV you should be okay for it not being triggered (over temperature). – Spehro Pefhany Sep 24 '18 at 19:43
• In the picture that I have posted with my question, the TRIAC Gate is triggered with a reference. I am not sure how to design similar circuit for my TRIAC from st. – abhiarora Sep 24 '18 at 19:44
• If you use the circuit shown Q3 is used, which is the same sensitivity as Q1. Q4 (gate positive, MT2 negative is the problem child). So you want to pick a resistor that keeps the gate current below the trigger current. The LM431 can take more than 1mA to get into regulation, so you might pick something like 150 ohms (300mV @ 2mA). The other two resistors determine your trigger voltage and the details are well-described on the LM431 datasheet. – Spehro Pefhany Sep 24 '18 at 19:50
• You meant Q3 or Q4? – abhiarora Sep 24 '18 at 20:00