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TRIAC based on/off circuit showing flash problem with LED lamp

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kiranlaleu

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I had made a circuit to turn load on and off using a TRIAC.
It all worked fine until the load was resistive or inductive.

As soon as I connected a LED lamp to the circuit all problems started coming in to the circuit. When the TRIAC was off. The LED lamp started to flash every few seconds with each flash becoming
brighter than the one before it.

eevblogforum3.jpgeevblogforum2.jpg
So when I read about it in eevblog forum. I found a post where Mr. SeanB had said " LED lamp with a TRIAC can cause issues of flashing when off, as the slight leakage current through the snubber capacitors can be enough to charge the power supply in the lamp up to startup, which will make the lamp either flicker when off, or flash every few seconds."

But I could not find a solution to it being supplied in the post or across the forum that I could find.

So if you guys could help me with the same. I really appreciate it and thankful for your time.

Here is the modification I did from the schematic. I used the earth as the return for voltage resistor circuit for voltage sensing even though I knew the earth is such a bouncy return path.
Also I removed one of those 100nf capacitors in the 5V stability correction.

Also instead of the 360 ohm resistor and 470 setup changed it to the MOC3021 datasheet recommended 180 ohm and 2.4k combo resistance along with the 0.1uf X rated capacitor.

Looking forward for you instructions to rectify the issue.
 

Hi,

I´d try this:
add a RC in parallel to the load (LEDs). Where R and C are in series.

Use a 100 Ohms resistor rated for at least 300VAC and 0.5W. Here a carbon one is a good choice, because of the current pulses.
And a 470nF film capacitor. Also rated 300V AC at least. You could try X2 saftey capacitor.

Be carfull, working with mains voltage is dangerous.



Klaus
 
here is some clues
https://www.fairchildsemi.com/application-notes/AN/AN-9745.pdf
..remember, triacs were never meant for use with led lamps......for a led lamp to be triac dimmable, it needs minimal input capacitance and needs dampers and bleeders to be added.

- - - Updated - - -

also this
https://www.onsemi.com/pub/Collateral/TND398-D.PDF
..remember, nobody wants to make a triac dimmable led lamp......you just end up having to modify led lamps to make them triac survivable because the customer may end up putting the led lamp to a triac dimmer...unfortunately..........if led lamps were the only lamps to have ever existed, then triac dimmers would never have been made.........trailing edge dimmers however, can be used with led lamps....or at least, some led lamps.
 

Hi,

I´d try this:
add a RC in parallel to the load (LEDs). Where R and C are in series.

Use a 100 Ohms resistor rated for at least 300VAC and 0.5W. Here a carbon one is a good choice, because of the current pulses.
And a 470nF film capacitor. Also rated 300V AC at least. You could try X2 saftey capacitor.

Be carfull, working with mains voltage is dangerous.



Klaus

I will purchase the resistor of that rating and try and report to you. Thanks for the advice. Could you possibly tell me what could be reason behind the leakage current. And is there anyway to reroute it to either earth or Neutral(I have the earth shorted to the digital ground of microcontroller).
 

Hi,

Could you possibly tell me what could be reason behind the leakage current.

On first step I´d consult the triac datasheet.
As far as I can see you don´t have a snubber across the triac. Is this true?

Klaus
 

I will purchase the resistor of that rating and try and report to you. Thanks for the advice. Could you possibly tell me what could be reason behind the leakage current. And is there anyway to reroute it to either earth or Neutral(I have the earth shorted to the digital ground of microcontroller).

The complex snubber impedance Z = R + j/(2*pi*f*C) creates a shunt path across the optocoupler which allows current as dictated by Ohm's law: I = V/Z

This is a very, very common problem, not only with LED but also with CFL lamps.

The solution is very simple: just add in parallel with your lamp, a resistive load which will shunt away the leakage current. I've used small incandescent lamps for that purpose.
 
The solution is very simple: just add in parallel with your lamp, a resistive load which will shunt away the leakage current. I've used small incandescent lamps for that purpose.

I used this remedy as well many times.
 
Hi,



On first step I´d consult the triac datasheet.
As far as I can see you don´t have a snubber across the triac. Is this true?

Klaus

I did look into datasheet to see how is leakage current affecting the triac. I have a snubber across the triac as could be seen in the schematic I gave across phase and return. Please do clarify the same.

- - - Updated - - -

I used this remedy as well many times.

Okay. Do you have the specs of the incandescent lamp used for the same.

- - - Updated - - -

The complex snubber impedance Z = R + j/(2*pi*f*C) creates a shunt path across the optocoupler which allows current as dictated by Ohm's law: I = V/Z

This is a very, very common problem, not only with LED but also with CFL lamps.

The solution is very simple: just add in parallel with your lamp, a resistive load which will shunt away the leakage current. I've used small incandescent lamps for that purpose.

This is the first time someone gave me the maths equations that would explains the leakage current. Thanks so much for the same. Let me try to work it out using the same. Many have suggested the resistive load in parellel method.
Is it the industry standard method and also one other person had suggested to use a Potted transformer(2 VA) across it and then just either open the secondary or drive an LED as indicator in the secondary.
What do you think of the both ?
 

Hi,

I find your s hematic difficult to read...
But now I see the snubber.

With: Xc = 1/ (2 × Pi x f x C)
And: I = V / Xc
You can calculate the current through the C.

This current may cause the LEDs to glow/flicker.

The triac leakage current (vector) will be added to this current.

***
Did you try the RC in parallel to the load?

Klaus
 
Hi,

I find your s hematic difficult to read...
But now I see the snubber.

With: Xc = 1/ (2 × Pi x f x C)
And: I = V / Xc
You can calculate the current through the C.

This current may cause the LEDs to glow/flicker.

The triac leakage current (vector) will be added to this current.

***
Did you try the RC in parallel to the load?

Klaus

Yes I did try with the value of capacitor as you told along with the resistor I had of 47 ohm since I could not get the 100 ohm resistor and the LED flickering got stopped. And now it completely stays
off. But When I checked the voltage across the gate and earth still the optotriac was giving voltage of around 31V AC. But when I removed the snubber this voltage changed to 0V. When the snubber
was put back on. Again the voltage became 31V ac. Is this acceptable behaviour of a snubber circuit.
 

Hi,

I checked the voltage across the gate and earth
I can't find "earth" in your schematic.

Is it running as you whish now?
Can you post a schematic of the running circuit?

Klaus
 
Hi,


I can't find "earth" in your schematic.

Is it running as you whish now?
Can you post a schematic of the running circuit?

Klaus

I am sorry I forgot to mention it. I had changed the reference to the Voltage divider to Earth instead of return(when switch on or neutral) which is used to
measure the voltage of AC line that is coming through the board to load when the triac is on.

Presently it is running with LED not flickering when done as you told. Schematic is the same as the one that is shown except for the change in the reference of the voltage
divider to earth instead of return. I will try to update it and upload it again now.
 

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Hi,

Is there still a question not answered?
* snubber current is explained in post#6
* RC in parallel to load is explained,
* voltages and currents can be calculated according Ohm's law

I can't comment about voltage divider and the use of earth, because there is no additional information, no circuit, and I don't know about your country's power grid configuration.

Klaus
 

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