I have 4 SSRs on my board with a steady state current rating of 2A and a peak current rating of 9A (Relays are CPC1976). This product is meant to be a commercial one and the relays will be powering bulbs (CFLs, Incandescent etc) upto 100W @ 240VAC
My main concern is short-circuits. A lot of CFLs that I've used in my house short circuit for a small time when they fuse. The short circuit is serious enough that the circuit breaker trips. This will obviously fry my relays.
I can easily include a fast-acting fuse but that will be have to be replaced everytime the lights fuse! I could consider a resettable fuse but most of them seem to be too slow to act to save a semiconductor device.
What are my options here to have a resettable solution yet save my relays from damage?
One idea I had was to do away with the relays and use opto-coupler + triacs instead. Triacs are cheap and I can easily find some that can handle large amounts of current (obviously a short circuit will be more than they can handle). These may survive such short circuits with the addition of a resettable fuse?
Best Answer
You need to monitor the current using a low resistance in series with the load (bulb). Or possibly, the drop across the relay can be used to calculate the amount of current taken by the load.
Once the drop exceeds a threshold, the relay should be turned off. Make sure that the detection mechanism and relay is fast enough to allow for abrupt termination.
Otherwise, go for a fast blowing fuse in a holder; solid state stuff has low thermal mass. Replace the fuse while replacing the bulb.