# limiting current in an ac for triac optocoupler interface

Discussion in 'The Projects Forum' started by Clinton Brits, Dec 30, 2015.

1. ### Clinton Brits Thread Starter New Member

Aug 8, 2015
15
2
Hello Everyone.

I was working through a few youtube videos showing how an optocoupler can control a triac to allow a mcu to control high voltage device. When I did the maths I needed to limit the current going through the optocoupler and the gate of the triac to 40ma. After doing all the calcs I ended up needing 5K resistance to make it happen and given the power equation it would have needed to be a few 2W resistors in series.

I took a solid state relay circuit I had from another supplier and it did just that. The only problem is that it had only 2 resistors. Which looked to be 2 W resistors one of 450 ohm and another of 550 ohm. This is far less and my calculations work out to 0.44 amps going to the triac which would probably burn it out. Not to mention that the current will cause the 2W resistors to burn.

The only problem is that the circuit works and works well. I put together a simulation of the circuit using a multimeter to represent the gate current and a switch to represent the triac conducting to provide enough current for the lighr to burn brightly.

I attached the circuit and was hoping that someone could spot the concept this works on because everything I do says it shouldn't work.

2. ### #12 Expert

Nov 30, 2010
16,671
7,314
No drawing visible. Please try again.

Aug 8, 2015
15
2

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4. ### #12 Expert

Nov 30, 2010
16,671
7,314
There are no MCU's or opto-couplers in those drawings. The two resistors are obviously not doing what you think they are doing, but the drawings are not leading me to what you want.

5. ### sailorjoe Member

Jun 4, 2013
361
63
Cool circuit! Close the switch to blow the fuse. Very handy. Sorry, Clinton, but you have to try again.

6. ### #12 Expert

Nov 30, 2010
16,671
7,314
I think, "LA1" means, "light bulb".

7. ### sailorjoe Member

Jun 4, 2013
361
63
Oh, thanks. That makes a big difference.

8. ### BR-549 Well-Known Member

Sep 22, 2013
2,177
418
First you will need to select a triac that will handle your "high voltage device" voltage and current requirements.

Next you want to find an optocoupler that can be driven by your mcu and can output the needed current for the triac, using available supply voltage.

Then you get to study datasheets in order to meet each of the components requirements.

It will involve considerable study.

9. ### Clinton Brits Thread Starter New Member

Aug 8, 2015
15
2
Thanks for the reply. I found a post on experts exchange that asked something similar and it went into calculating the value for the resistor on the triacs gate and how the gate is triggered in AC and how it differs from DC.

10. ### Clinton Brits Thread Starter New Member

Aug 8, 2015
15
2
Turns out I misunderstood the datasheet for the triac. The gate maximum gate current is shown as 100ma but they also state a peak gate current of a lot more. Apparently the 1K resistance they use is just that giving +-140ma through the gate and since it only has current through the resisors for a really short time when opening the gate, the power rating on the resistors aren't as high as what would be needed if it had a constant current.