solid state relays

[northwestmechanic]

whats the difference between regular relays and solid state relays

[soonera/c]

No contacts, only diodes and whatever else they decide to put in there.

[kuryakin]

SSRs have no coils or contacts.

They use an SCR, triac, or power transistor to switch the load.

[keepitsimplestupid]

SSRs have no moving parts and last a long long time. One typical black box may have a 3-32 volt DC imput signal at a few milliamps and switch a large AC load only at a zero ceossing of the AC line. RFI (Radio Frequency Interference) is reduced when doing this. Start up current is also reduced. SCRs and Triacs inherently turn off at zero volts, so they turn off at a zero crossing of the AC lne. Extra circuitry can be added to force the triac or SCR to turn on at a zero crossing. This circutry is not mandatory.

SSRs refer to a large class of devices which use SCR's, triacs, bipolar transistors and power Field Effect Transistors to swich a load. Since light can activate a junction, a light emitting diode (LED) is usually used as the input to the outside world. That input of 10 mA with a 1.2 volt voltage drop is usually conditioned to a voltage between 3 and 32 volts or 5 volts.

SSR's switching large loads should be protected with special I^2t fuses to protect the device. I use two fuses at times. One expensive I^2t fuse to protect the triac in the SSR and another lower rated inexpensive fuse to protect the load.

[Edited by keepitsimplestupid on 07-27-2006 at 08:32 AM]