Leds and capacitance...

HappyHippyDad · Oct 20, 2014

There are always quite a few threads where the problem is the LED staying on slightly even when switched off and I see that the solution is to fit certain resistors as a snubber.

I have never had this problem. I guess I have either been lucky or have found a combination of switches/lamps that works well for me.

I was wondering if anyone has actually fitted some resistors to stop the capacitance effect on the LED's? it would be good to hear if you have, exactly what you did and if you fitted them at each lighting point or just one at the supply end etc, etc?

I do like this post by MarkieSparkie (http://www.electriciansforums.co.uk/electrical-forum-general-electrical-forum/46668-led-glow.html) which includes the following:

"A simple solution is to permanently discharge the cable capacitance by wiring two 160K ohm, 1/4W, resistors rated at >=250V, in series across L and N on the final circuit, it is good electronics practice to do this as close to the effected lamp as possible, if this is done across the switched L and N at the lamp holder terminals it allows simple switching out of circuit (or disconnection) for testing. Do not try to use a single 330K ohm resistor because this may fail due to voltage breakdown.
If the final circuit has a large number of switched lamps it may be pragmatic to connect the series resistors between permanent L and N at a convenient point e.g. at the DB to reduce the number of resistors required, however the number of points that will require treatment will depend on how the cable capacitance is distributed on the final circuit. "

However I dont quite get the using x2 160K ohm resistors in series yet connected between L and N. This sounds like the 2 resistors are connected together in series and then connected across L and N (i.e in parallel)?

Also there seems to be a large variation in the size of the resistor recommended by various threads?

telectrix · Oct 20, 2014

markie is right. you want about 330K. the reason you use 2 x 160K in series is to halve the voltage across each resistor.

Richard Burns · Oct 20, 2014

I am just about to test this at my house as I have the perfect arrangement for LED glow.
I have two way switching for each of two lights at either end of the room and the current LED lamps in there will glow quite nicely when the other light is switched on, the only way to stop the glow is to have both gangs in the same position on one switch.
However I have not experimented yet.
The loading resistors above would be dependent on the size of the capacitance in the LED lamp as you are trying to ensure the capacitors never get sufficient charge to discharge to the lamp.
Using 100 ohm resistors would obviously work but would need 500W resistors to cope and the electricity use would be quite high!
Using 320 kohm (as 2x160 kohm) would allow you to use standard 0.25W resistors however the charge drain might not be enough (but should be if the glow effect is small) to eliminate the effect but would reduce it.

RobWansbeck · Oct 21, 2014

Two components in series are used so that the failure of one component doesn't put a short across the line.

HappyHippyDad · Oct 21, 2014

Richard Burns said:
I am just about to test this at my house as I have the perfect arrangement for LED glow.
I have two way switching for each of two lights at either end of the room and the current LED lamps in there will glow quite nicely when the other light is switched on, the only way to stop the glow is to have both gangs in the same position on one switch.
However I have not experimented yet.
The loading resistors above would be dependent on the size of the capacitance in the LED lamp as you are trying to ensure the capacitors never get sufficient charge to discharge to the lamp.
Using 100 ohm resistors would obviously work but would need 500W resistors to cope and the electricity use would be quite high!
Using 320 kohm (as 2x160 kohm) would allow you to use standard 0.25W resistors however the charge drain might not be enough (but should be if the glow effect is small) to eliminate the effect but would reduce it.