# LED indication in triac switching

1. ## LED indication in triac switching

Hi,
I want to glow an led whenever triac switching and powers the load. So i gave a circuit as attached image.

The led is glowing properly on first switching. But when the triac is switching next time the intensity of led is diminishing and on next switching it is also flickering. After some 20 switching led died completely. I am switching and kept it ON for about 10 seconds and off it for about 5 seconds. Here the load is induction motor.
I put the voltage drop across each component in the figure. Why it is happening like this?

Before to this, instead of RC i just used a resistor of 55K but when triac is ON led glows properly but when triac is OFF also the led is glowing blurry.

What should i do for the proper led glowing?

Thanks,
Murugesh

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3. ## Re: LED indication in triac switching

Basically you want to drive LED with AC source... For this you can use Capacitor voltage divider instead of voltage divider. there will very less dissipation in capacitor voltage divider.
One important thing is...18V across single LED is too much...Thats why it is flickering and died in 3-4 turns. calculate capacitor voltage divider so that Iled =20mA max and Vled=3V....Normally...(LED current n Voltage also depends on colour of LED)....
Check out the link below for more details.....
Note that this design is for High power 1W LED...

http://www.circuit-finder.com/catego...e-powered-leds

Hope this helped you........

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4. ## Re: LED indication in triac switching

There is AC across the LED so it's probably the excessive reverse voltage that damaged it. If you want to show when the AC is being TOLD to turn on, connect the LED and a series resistor across the LED in the opto-coupler, this also isolates it from high voltages. If you want to show when the AC is ACTUALLY on, use your existing schematic but connect the LED across the DC arms of a bridge rectifier and the other components to the AC arms. If you are using a small indicator type of LED, you can make the bridge rectifier from four 1N4148 or similar, it doesn't have to be a high voltage or high current type.

Brian.

5. ## Re: LED indication in triac switching

Well thanks brian..

I had a thought of putting the LED in the input side of optocoupler, which you meant as "AC is being TOLD to turn on".

But i want to show when the AC is actually ON - So i put a resistor and capacitor before to LED to have net resistance of 68230 ohm for current limiting ((230 - 1.8 ) / 68230 = 3.3mA which is enough for me) for the 230V/50Hz AC supply.

The main doubt for me is how i am getting 18V across LED when i measured using DMM across LED terminals. Also i measured the voltage from the TRIAC as 224V(across triac output and neutral line) but the calculated values showing as Vr+Vc+Vled = 1.3+219+18 = 238.3V.

Where i am wrong?

6. ## Re: LED indication in triac switching

see the current rating, u may have exceeded the current rating of LED

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7. ## Re: LED indication in triac switching

It's nothing to do with the current rating. The reason you are measuring the voltage is the LED is conducting as a diode in one direction and breaking under the strain in the other. The asymetric result is the strange DC you see. LEDs must be run from a DC supply but you are using AC, on one half cycle of the mains it conducts and if not already killed, it lights up as expected. On the other half cycle the LED supply is reversed which means it does not conduct current until it reaches a voltage that causes it to break down. The 18V you measure is actually in reverse polarity to the voltage that lights it up. If you check the data sheet for most LEDs they are only rated to withstand about 5V in the reverse direction, you are trying to give yours 230V (about 320 at peaks). If you wrap a bridge rectifier around it like I suggested it will conduct on both polarities and not only light brighter but be a LOT more reliable!

Brian.

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