Solid state relay (SSR) - various brands - off the shelf. Cheap. Load is 110VAC light(s), say, 3A max.
Anyone know how to preclude failure of the SSR if an incandescent bulb's filament goes to zero ohms for a fraction of a second as it burns out? And the current in-rush at turn-on?
Thermistor?
According to Crydom you should use an ultra fast fuse or the SSR will be destroyed by slower means..
Yes, it HAS to be fast. Inrush current on an incandescent lamp can be 2X operating current but last only a few line cycles. Self resetting fuses are not fast enough. Most regular, even standard "Fast Blow", fuses are not, either.
For inrush, its a lot easier to just size the relay for the expected inrush rather than for the operating current. You really need to do this because you don't want the fuse popping when the lamp turns on.
Jim
I'd prefer not to use a fuse - that has to be replaced just because the incandescent bulb burned out with a brief zero-ohm condition. Better to use a relay than a fused SSR.
We've all had solid state switches and dimmers fail due to a light bulb burning out. How can I do better and use an SSR without a fuse?
The critical value here is i2t - rate of change of current over time. When incandescent bulbs blow you can get an arc formed that is much lower resistance than the filament thus the only thing that limits the fault current is the impedance of the supply and wiring. A high speed fuse is usually much more expensive than the triac it protects, so it is not an economical solution. I used dual scrs in my commercial light dimmers as these had a superior i2t rating to triacs and a superior price! This allowed me to use thermal-magnetic breakers to protect the circuit - these breakers are slow to trip so you need a robust scr/triac to withstand the fault current. A more modern solution would be ti use mosfets or igbts and electronically limit the current. These device switch in the uS region so you can interrupt the supply very quickly before the current rises to levels that will damage your semiconductors.
You can understand why the manufactures of light dimmers/switches don't incorporate extensive protection due to the cost. Plug in triacs??
Current inrush I addressed by having a ramp up time in my dimmers. Thermal shock causes a lot of failures in incandescent light bulbs.
Any off the shelf SSRs for 300W, 110VAC that incorporate what you speak of, say, dua-SCRs, fast current limiting? And of course, not wildly expensive?
