TRIAC circuit

[прямо]

Anyone is familiar with a TRIAC circuit?

I came across this schematic, and I built it, but I can't seem to figure it out why I'm getting a low AC voltage output at the TRIAC terminals.

With 230V AC hot side at the TRIAC MT2, I measured 60V AC between MT1 and neutral side when the comparator output LED indicator is on, and 20V AC when it is off.

Same result if 230V AC hot side is at MT1, and I measured between MT2 and neutral side.

My heating element doesn't work with 60V AC. It's not even getting hot and the thermocouple amplifier output is a constant low, therefore it will not trigger the comparator to drive the optocoupler to turn off the TRIAC.

Can someone explain it to me what is wrong?

[redwire]

Your load is your multimeter - triac's have some leakage current so your voltages will not be the same as if you used a light bulb load. BTA136 needs 2.2mA to stay on, leaks 0.5mA when off.

With LM358 powered from 5V, it can never output 5V. The most is 3.3-3.5V out. So your opto-coupler+LED+resistor will not have much current at all. I would move the LED and change resistor to 220Ω.
Using IC1B as a comparator, it needs a hysteresis resistor added like 1MEG. How many degrees difference between heat on and off?
C2 is in wrong spot, should be across R3+R6 as a filter. Triac gate R8 etc. should be around 470Ω total, not 2x560Ω. Where is the RC snubber?
For thermocouple use, better op-amp like OP-07 is used. Add a safety pullup resistor so if the thermocouple wiring goes open-circuit, the heat is not stuck on and burn your house down.

[harp]

I not sure about LED1... try bypass
R2 maybe too high
Use BTA12

[прямо]

Here's a short video to show what I meant.

Red DC voltmeter: thermocouple sensor DC mV amplified by 220 times to give a temperature reading in Celcius.

Green DC voltmeter: adjustable temperature threshold (set at 200°C)
LED indicator is on = optocoupler is on = TRIAC is on.

As I mentioned before, measured between TRIAC MT1 terminal and neutral = about 60V AC.

And when the heat that I applied to the thermocouple sensor reached/passed the temperature threshold, LED indicator turns off = optocoupler turns off = TRIAC also turns off = AC voltage drops to 20V, which is exactly what I want.

What I don't want is when it is not off. Why 60V AC output if input is 230V AC? I just don't understand.

harp, I don't have BTA12 but I have a BTA41.

[redwire]

Your load is your multimeter - triac's have some leakage current so your voltages will not be the same as if you used a light bulb load. BTA136 needs 2.2mA to stay on, leaks 0.5mA when off.

With LM358 powered from 5V, it can never output 5V. The most is 3.3-3.5V out. So your opto-coupler+LED+resistor will not have much current at all. I would move the LED and change resistor to 220Ω.
Using IC1B as a comparator, it needs a hysteresis resistor added like 1MEG. How many degrees difference between heat on and off?
C2 is in wrong spot, should be across R3+R6 as a filter. Triac gate R8 etc. should be around 470Ω total, not 2x560Ω. Where is the RC snubber?
For thermocouple use, better op-amp like OP-07 is used. Add a safety pullup resistor so if the thermocouple wiring goes open-circuit, the heat is not stuck on and burn your house down.

[прямо]

Ok, changed the comparator output resistor to the value you suggested and now it is 230V AC across MT1 terminal and neutral side.

Thanks!!!

[прямо]

Using IC1B as a comparator, it needs a hysteresis resistor added like 1MEG. How many degrees difference between heat on and off?

I tried the circuit with my hot air station handle.

Thermocouple reading has 10 degree higher than the temperature I set.

And again you're right about the hysteresis.
Without it, the comparator turns on and off too quick which the LED indicator follows after, and its flickering gives out a negative impression that something is not stable/dodgy when it's actually not. I was able to use the hot air station handle to remove and solder a component without any issue (I had the fan hooked up to a DC PWM circuit to control the airflow)