[SOLVED] TRIAC is not switching off when the Trigger is turned off

Hi all,

Is there need to have a resistor to ground from the gate of TRIAC in the schematic as some people have suggested in forums for more stability.


I tried the circuit for random phase triggering configuration according to above schematic but found that when tested for DC load of low current draw the load was still turned inspite the triggering being turned off after the initial test triggering. Does this have to do with any capacitance at gate not discharging or something.

Could someone describe what all are the function of resistor if any that connects from triac gate to the ground(or return or neutral). And would that solve my above problem.


and how to properly calculate the value required for it.
I would like to know the proper practice of solving for this resistor.

Hi,

when tested for DC load

Click to expand...

It seems you don´t understand the working principle of a triac.

Once a triac is triggered it stays ON until load current becomes near zero (below hold current). Therefore it is not suitable for DC applications.

Klaus

Oh K may be I would have done it wrong. Here is what I basically did. Could you please inspect the following procedure.

I tested the switching DC side upto MOC using powering only the dc part first. Since it was working.
next I tried to test if the triac was switching on or not.

Instead of testing the circuit with AC supply and load which would commute as the AC voltage would go to zero.
I decided and tried to test it with dc supply and load to reduce the risk of testing.

So I connected a dc supply through the Triac to power a light bulb in dc. And here I triggered the gate and the
Triac turned on and the Bulb got lit in the given DC power supply. So I removed the trigger and and then turned
off the DC power supply after it got lit. Which should have turned off the triac...but it did not.

Since the turning on part was right I decided to try the same with AC load and supply. This time too..the Triac turned on properly but did not turn off correctly even after removing gate current. Shouldn't it turn off as the voltage get backs to zero.

What might be the cause for it not turning off ?

It is basically two Thyristors back to back and therefore will adopt the same characteristics as a Thyristor.

What might be the cause for it not turning off ?

Click to expand...

With correct AC supply, either damaged parts or lack of snubber in a circuit with much wiring inductance.

Which load did you use for test?

Yeah I understand that. Could you give your diagnostics of what could be wrong in this situation. May it be something that I have done wrong or is it something like a bad component..I wanted to know what all are the probable situation that may cause this problem.

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Hi, I used a 40W Incandescent bulb as load in AC supply. Snubber circuit was present in the circuit for the Triac but not for MOC. So Should I testing the Triac...If so how.

I would prefer the circuit with additional coupler RC filter, as in post #1. But incandescent should regularly work without it. Are coupler and triac still healthy after the test (not internally shorted)?

Hi again, I will try again with a RC filter in the next testing PCB as you said. I will test the coupler as you said. But could you tell me how to properly the test the TRIAC...Above is the method I tested it. Could you correct me and point me to the correct testing method.

As I was testing the health of TRIAC by making it to switch a bulb by raw breadboard type circuit. Then found the triac was damaged and when I replaced the TRIAC the circuit worked back fine.