justjed:
Just a followup. This SSR showed up. It's a Kodak, PN 611490. I have duly searched for a datasheet and come up empty. The input resistance is about 891 ohms, so that's good news. But what I hadn't thought about was whether I need a big heat sink for the load. From the faceplate of my A/C unit, the load will be ~ 8A @230VAC. The MPJA page for it says,Opto-Isolated, "0" voltage turn-on, internal snubber
which tells me not much.
So, anyone have some experience or memory to draw on here?
@JimG. It's going to be inside an enclosure, but your point is well taken.
Most of those industrial SSR are pretty much the same. The input side is straight forward, an internal LED with built in series current limiting resistor. You can control it directly with a arduino digital output pin and a ground connection. The internal snubber part is actually good for you. Most SSR don't handle inductive loads well (fail to turn on, or fail to turn off) when driving inductive loads (like motors). Snubber circuits help deal with this kind of problems, however it kind of depends on the amount of induction and the specific value of the snubber components in your SSR. Worst case you may have to add external snubber components if needed.
As far as heatsink needed, yes you will. The maximum current ratings for SSR only apply if you can also keep the device below it's maximum operating temperature rating. Past experience I've has is that most SSR will 'drop' about 2vac across it's switching device, so at your 8 amp current load, that's 16 watts of heat dissipation that will surely overheat the SSR unless bolted down to a large enough heatsink using proper heatsink paste, etc.
So how big a heatsink? Probably larger then you wish, have room for, or have avalible on hand. 