I have a large LED light connected to a remote controlled relay. The relay is rated 10A, and the light is rated at 1.5A when on.

But I notice 2 things:

1. Sometimes the light does not turn off even though the relay got the signal to go off.
2. When I plug in the LED light into an outlet manually, there is a pretty big spark.

So what I think is happening is that the relay contacts get fused when there is a huge inrush to charge up the capacitors in the switching power supply driving the LEDs.

How do I deal with that?

 

Hello,

Just for curiosity... is that the behavior your device has always shown? Or did it start to happen after some time?

 

You can try placing a snubber circuit (RC Circuit) across the relay contacts. That assumes your relay is switching AC for the LED power supply? What voltage is the relay switching?

Ron

 

Is there any change you could get us the details of your switching power supply? I had a kind of similar problem once with a transformer which charged up a 38000 uF cap at 40v, which was solved by replacing my relay with a solid state one, which switches at zero crossing, and therefore there is much less (or zero) switching current.

 

Yes, it happened started happening after some time. It is switching AC. It is one of those typical WiFi enabled outlets..

 

Yes, it happened started happening after some time. It is switching AC. It is one of those typical WiFi enabled outlets..

Then I suggest a RC snubber or as mentioned going with a SSR but I would start with a simple snubber across the relay contacts. You can try a 22 or 47 Ohm 1/2 watt resistor in series with about a .47 uF cap.You didn't mention the switched voltage? Anyway I would start with a few snubber combinations and also consider a zero crossing SSR depending on what you want to spend.

Ron

 

I fully agree with Reloadron, RC snubber is definitely much cheaper and possibly easier. Excuse my ignorance of LED lighting supplies, but are they usually inductive? If so, RC snubber will be best. If they are only resistive the RC snubber wouldn't help, you would need something zero crossing.
@Reloadron please can you explain what the calculation for the resistor and cap sizes is? Or are they just common sizes? I have been having issues trying to impliment a snubber yesterday, but from these values it looks like my cap is too small.

 

I fully agree with Reloadron, RC snubber is definitely much cheaper and possibly easier. Excuse my ignorance of LED lighting supplies, but are they usually inductive? If so, RC snubber will be best. If they are only resistive the RC snubber wouldn't help, you would need something zero crossing.
@Reloadron please can you explain what the calculation for the resistor and cap sizes is? Or are they just common sizes? I have been having issues trying to impliment a snubber yesterday, but from these values it looks like my cap is too small.

There are formulas as can be reviewed here. Honestly though I generally use published charts and wing it. Before I retired we always kept a pile of them in stock.

Ron

 

If you put an RC snubber across the relay contacts it is not unlikely that the LED light will flash periodically when the relay is open.

Given the assumption that the problem is high current for charging capacitor, the circuit is likely designed so that it will not start until the voltage on the cap has reached some threshold. Once that threshold is reached, the switcher will turn on and remain on until the voltage on the cap has fallen below some lower threshold. Until the circuit starts, the load on the capacitor is probably nearly zero.

If you use and RC snubber across the relay contacts it forms a "trickle charger" for the light's capacitor that will allow the cap to charge slowly to the startup threshold. The light then flashes on briefly, discharging the cap. This repeats infinitely.

I've seen lots of LED lamps where this happens but also some where it doesn't, so testing is required.

RC snubbers usually are of little use for contacts for capacitor charging in any case. With an inductive load, the hazard to contacts is at opening when the inductance can force both high current and high voltage to exist across the contacts simultaneously, causing damaging arcing. No such equivalent exits in capacitor charging. There is no mechanism to discharge the stored energy through the contacts. The snubber will do nothing to limit the current through the contacts unless you have the situation where it provides that trickle charge path and the timing of closure just happens to work out advantageously.

 

There are commercial RC networks like this one

For specific calculations, there are some good applications notes around the web available.

...What if the relay is kind of deteriorated, and replacing it would be the solution?

 

I have some other WiFi enabled outlets. Some are older, some are newer. Some make a relay click, some don't. Is there a way to tell if any of them are solid state?

How does solid state handle high inrush currents?

 

As far as I'm aware you have to open them up to find out whether they are solid state or not. Even SSRs sometimes make a click when they switch, so I don't advise presuming from that fact.

SSRs detect when the voltage is crossing zero, and switch then. At that point there is no current flowing.

 

Even SSRs sometimes make a click when they switch, so I don't advise presuming from that fact.

That's interesting. What clicks inside a solid state component?

But I added a resistor-cap combo, with a 30 ohm resistor. The light now blinks 3 times a second, at full brightness. It's like I'm in a club.

So every time the light blinks I'm assuming the capacitor gets discharged to 0v? And if the relay turn on when it's not charged up there will still be potential to get a spark on the relay contacts?

 

That's interesting. What clicks inside a solid state component?

Maybe piezoelectric sounds from stress changes during switching while not at zero cross.

 

So every time the light blinks I'm assuming the capacitor gets discharged to 0v? And if the relay turn on when it's not charged up there will still be potential to get a spark on the relay contacts?

Yes.

High current transients like this are typically dealt with by use of an "NTC thermistor" made specifically as an inrush current limiter. They look rather like largish disk capacitors except they are usually dark grey and have fairly heavy leads. Surge-Gard is a brand name I've used. When cold the resistance is "high" but as their temperature rises from self-heating from current the resistance drops. Normally they run moderately hot. They do no perform well in circumstances where there are brief power outages because they don't cool fast enough. Trying to retrofit one into a circuit might be difficult, especially because they do run hot. They come in a range of ratings and need to be reasonably well matched to the circuit. You might find something rated at around 2 ohms "hot" and adequate for 2 or 3 A continuous. If you are running at 120 VAC, something in the range of 10 ohms cold resistance would probably be OK. I don't know if parts with these characteristics exist.

A solid state relay should be completely silent. If there is an inductor in series for filtering (not uncommon for some phase-angle control applications), it might make a bit of a tick or it might buzz slightly.

 

It would seem there is a large inrush current due to filter capacitor charging after a rectifier input of the LED power circuit, which is causing the relay contact to weld.
If so then a snubber circuit will do little to help, which are designed to suppress inductive kickback,

A current limiter NTC thermistor circuit, such as ebp suggested, is often used to suppress surge currents.

Or use a control circuit that uses an SSR (try one of the silent ones).
They often start at zero-crossing which reduces the surge current.

 

I built an SSR using MOC3061. The inrush current due to the filter capacitor charging after the rectifier, is pretty controlled now.

 

have you tried to test the relay with a smaller load?

 

You might improve matters with something as simple as a fixed resistor of a ohm or so in series with the light power input. It would need to be a wire-wound type to survive. This would still allow very high peak current, but perhaps sufficiently less than operating without it to prevent the relay from welding.

 

Switching an inductive load when the voltage is zero (and current is max.) is asking for trouble.

You misunderstand.
The zero voltage switching is at turn-on when the current is zero.
Turn-off occurs when the current reaches zero through the SCR.