Relay failures in lighting controller

Sort of off-the-wall...

Years ago I handled such an inrush issue by using TRIAC's at turn-on which were then bypassed by the relay. ...Jim Thompson

Now I know someone is going to ask, "Why bother with the relay?" Answer: Heat. ...Jim Thompson

Which wouldn't work if Mr. Horde can only change the relay part number, alas.

Having one relay switch in with a resistor, followed a few cycles later by a shorting relay, would work like your TRIAC idea, if someone resisted the idea of TRIACs.

That would eliminate having significant electronics.

Maybe the first relay (with resistor) could also power the coil of the second relay. Might be just enough of a delay to avoid burning the contacts. ...Jim Thompson

Would something like this help?

or something similar?

First place I'd look... Electronic ballasts require an EMI filter in front of them to prevent the conducted emanations. What you have there is a capacitor across the AC mains followed by some husky inductance. That makes the load look incredibly inductive, not capacitive - with main impact as you turn it off. To envision, simply unplug your PC very slowly from the AC outlet. Even with it turned off, you'll draw an impressive arc. All due to the stored energy in the EMI filter. I suspect it is THAT energy that is welding the contacts together.

Not sure, you can place effective arc suppresssors at your relay contacts. Especially, with such an energetic arc occurring just as the contacts start to open. HP invented a circuit that is the opposite of a zener - dead short until a maximum of current is reached. IC made by local foundry in CA, Supertex? You could place THAT component in series, then in parallel to the load place a tranzorb to take the voltage and the combination should completely protect the contacts. From memroy there is some voltage drop across HP's component Don't know how much, but less than 1 V, probably Vbe drop like in a 2N3904 [again form memory, that was the bipolar process for making the die] I have a schematic of the IC somewhere.

It would depend on how long the main relay took to close, I suppose, and how long the inrush surge lasted.

Sizing the pilot relay and resistor would be a challenge: if you make them big enough to stay on continually without the main relay engaging, then they'll be way big. On the other hand, if you size them for the thermal stress of the turn-on only, then if the main relay fails open one or the other will go up in a large puff of smoke.

OTOH, a plain ol' RC snubber may do it, if that's really the issue. You don't need to prevent a voltage spike -- you just need to prevent it from arcing.

It sounds like a good reason for experimenting. I had thought about inductive arcing on contact opening, but I kind of assumed that while such arcing may pit the heck out of the contacts, it would be events happening on closure that would weld them shut.

"Robert Macy"

First place I'd look... Electronic ballasts require an EMI filter in front of them to prevent the conducted emanations. What you have there is a capacitor across the AC mains followed by some husky inductance. That makes the load look incredibly inductive, not capacitive - with main impact as you turn it off. To envision, simply unplug your PC very slowly from the AC outlet. Even with it turned off, you'll draw an impressive arc. All due to the stored energy in the EMI filter. I suspect it is THAT energy that is welding the contacts together.

Inrush surges with the SMPS associated with LED lights ( and CFLs) are often huge and with multiple loads on the same switch quite disastrous.

With contacts closing and current surging simultaneously, spot welding is the outcome.

Relays have no ability to pull such welds open like a regular switch has.

... Phil

I think there is something they are not telling you..

Just a thought

Jamie

contacts can't weld closed while they are open. therfore the damage is occurring when they close capacitor derived surge current through the bouncing contacts heats them up to melting and the weld closed.

They most certainly can weld together while they are in the process of opening with induction loads. Which is why special designs are out there just for handling induction loads.

If you don't use the correct contact design for these types of loads, the relay will not last long.

Jamie

I have hard time seeing how it can happen when opening, arc start when they are already moving apart so unless you close them instantly after being melted by the opening arc I don't see how they could weld

at closing it is more like spot welding, melt and press together

sure arcing at opening will eat the contacts, but that is different

-Lasse

Many relay designs do a lot of surface sliding while they are pulling apart or coming together. The pulling apart is worse because you are increasing R as you do this. Since the arc has already started before the R gets too high, plasma burn is already in the process and heating is severe. When closing the contacts, you get only the initial bounce and if the load is inductive, the contacts will most likely be closed before enough energy is store to cause critical damage.

Simply put, you get a lot more plasma burn on the surface while contacts are opening because the initial plasma has started while the contacts where still touching but under a load and arcing, this generates a malted metal vapor cloud which is highly conductive and perpetuates the process. If the contacts have enough damage to them already over time from doing this, the contacts will be pitted and carbonized and at some point, the next plasma burn it goes through will weld them. Repetitive action on the contacts will heat them and they will weld anyway, most likely from incorrect use of the device.

Induction loads just makes things much worse because they can arc at greater distances when opening and cause more damage.

Relays with spring arms are known for this kind of damage in the wrong environment. You need contact designs that pull and push straight and fast. Some have rounded heads to help vent off the metallic vapor along with using tungsten, etc..

We have discharge contacts we use for draining off HV in the 20KV or more range, from doing HI-POT test on large cables that can hold a large charge. These units have 2 round balls that come together that form the contact to ground and a air supply that blows fresh air across the surface area. These units have been in service for years and have never been replace as far as I know.

Jamie

You've never done any arc welding, have you?

All the times I've stuck the rod to the work were exclusively when closing the circuit. I can safely say I have never, ever had a rod stick to the work when pulling it off!

Tim

Sigh. How is the metal transfered from the rod to the work?

nt

All

I have done plenty of welding, stick and tig

have you ever got the rod stuck while pulling it off?

-Lasse

Makes sense that while being heated the contacts are having pressure applied, thereby enhancing the welding process.

I'll 'retreat' to simply pointing out that the EMI filter is one of the worst loads possible for relay contacts.

So, How about 'trying' to time the closurre so that the contacts hit at the zero crossing? Hitting within 1mS, on the 'shy' side should help a bit. Whoever suggested, now has me sold on the solid state in parallel with the relay.