TRIAC/Optotriac doesn't turn off

Thread Starter

Fayçal-TEB

Joined Mar 25, 2022
5
Good evening everyone;

I'm actually working on a circuit using ne555 ic, and triac+optotriac to make leds modules blink (about 500leds), a power supply if going to be used instead of L1, but i ran into a problem while testing it with a simple 80W bulb,the ne555 seems to send the right impulses to the optocoupler , but the triac doesn't turn off, i tried removing R9 (because the first time i plugged it, the resistor burnt, so i replaced it with a 10K, but the same problem persisted, the triac doesn't turn off), after removing it, the triac doesn't turn on at all.

I tried a second circuit using only ne555 without the optotriac, same problem, the triac doesn't turn off.
The third option that actually worked but is useless, is by manually plugging and unpluging the gate to +9v , in which case the triac turned on when plugged, off when unplugged.
Did anyone ran into a problem like that and can help me out ?
Thanks
 

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Ian0

Joined Aug 7, 2020
6,267
First suggestion would be to find a power supply with an enable input.
Power supplies generally don't like being switched on and off repeatedly.

If the load on your triac is a rectifier followed by a smoothing capacitor, it will tend to defeat the zero-crossing detector. It usually results in the triac not switching on (rather than not switching off)

If you are driving the triac directly as in the first diagram, then you should drive the gate with a negative voltage. The diodes and capacitor in the "alimentation" section should be reversed, pins 8 and 4 should join to the common AC terminal, and pin 1 should go to the output of the rectifier.

Which resistor burned? If it was R4, I bet your triac was faulty.
 

Thread Starter

Fayçal-TEB

Joined Mar 25, 2022
5
The load on the triac is actually a simple light bulb (purely resistive load) for the moment, i'm using the second diagram, the one with the optotriac, and the resistor that burned was "R9" but when i placed a 10K resistor instead, it turned on, but doesn't want to turn off, so now i removed it and the output of the optotriac is going directly to the gate or the triac.


The only way i found to make it work is by removing the MOC3041 and using a jumper between pin 8 of the ne555 and the gate to turn it on/off by hand, i also tried the first diagram with only NE555, but instead of not turning on, it truns on but doesn't turn off.
 

Ian0

Joined Aug 7, 2020
6,267
The load on the triac is actually a simple light bulb (purely resistive load) for the moment, i'm using the second diagram, the one with the optotriac, and the resistor that burned was "R9" but when i placed a 10K resistor instead, it turned on, but doesn't want to turn off, so now i removed it and the output of the optotriac is going directly to the gate or the triac.
Do you have the triac the right way round? MT1 and MT2 are not interchangeable. The load connects to the middle pin.
 

Thread Starter

Fayçal-TEB

Joined Mar 25, 2022
5
That's my very first triac circuit too haha, i didn't know that would create such a problem, again, thank you very much sir !
 

Ian0

Joined Aug 7, 2020
6,267
To anyone else who is following this:
Can the good folks of AAC think up some better names for the terminals of the triac than MT1 and MT2, so that it's obvious which is which, and which is the one that the gate voltage is measured from?
Where were all the Latin and Greek scholars when the terminals of the triac needed names?
 
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