contactors?
 

so i have seen a lot of designs with SSR's switching 120v hot legs, and have recently read up on some 220v setups that seem to be using SSRs + Contactors. is the contactor simply to drive 2 separate hot legs for 240v? do i need a contactor to run a heating element at 120v? :confused:

Since the 120 elements are less wattage, typically 1500 or 2000w, some run them at 100% power w/out a controller. This is a bare bones system but will work.

This is a safety thing. When SSRs turn off they don't actually disconnect the lines. The contactors are there to completely cut current when the switch is off. If, however, you switched the contactor via the PID (as if it were an SSR) it would destroy it. So basically people use the SSR to control the element from the PID and then the contactor to completely cut the connection when it isn't needed.

that makes perfect sense. thanks EFaden! it didn't look like the contactor could handle being switched rapidly any better than a regular relay, so i didn't understand why it was downstream of the SSR, but it's really being activated by a switch.