Electrical doodad's Question

[daplatapus]

Because electrical is probably my weakest field of knowledge (go figure with all I'm trying to build lately) can someone help me determine where I would want a mechanical relay as opposed to a Solid State relay?

So say I have my heaters, Ca reactor, main return pump, skimmer, ATO, sump lighting, and whatever else you can think off of the top of your head.

[blacknife]

I have not shopped for relays before, but would price be more for one or the other?

[makana]

http://en.wikipedia.org/wiki/Solid-state_relay

scroll down to the advantages/disadvantages section near the bottom of the pate.

[ScubaSteve]

I use them for a number of things. Where they really kick butt is high speed switching... Which I assume isn't what you're looking for. Really, it doesn't matter which one you use for your purposes. The SSR will run hot but is silent. If you need to switch big currents the SSRs can get pretty toasty and can sometimes need heat sinking (I don't imagine you'd need to for your purposes). The mechanical relays are the flip. The only reason I'd maybe use the mechanical over the SSR for certain applications is that they tend to fail open/shorted, which if it is being used on a heater, could be problematic.

[daplatapus]

Originally Posted by makana

http://en.wikipedia.org/wiki/Solid-state_relay

scroll down to the advantages/disadvantages section near the bottom of the pate.

Ya, I had read that earlier (and about 12 more like it) but because of my seeming inability to grasp electrical language, sentences like:

"Voltage/current characteristic is not linear (not purely resistive), distorting switched waveforms to some extent. An electromechanical relay has the low ohmic (linear) resistance of the associated mechanical switch when activated, and the exceedingly high resistance of the air gap and insulating materials when open."

might as well be written in Greek.
The fact that SSR's appear to fail typically in the closed position makes me tend to steer away from them for heaters, but with skimmer's, pumps etc, that might be a good thing.
I know with an Apex system, the EB8 has some SSR and some electromechanical relay's there don't they? And I thought people were making sure to plug some equipment in SSR controlled outlets, and other equipment in the electromechanical controlled outlets. Can anyone speak to that?

[daplatapus]

My bad, upon further reading and searching I mis-understood. The EB8's are SSR and TRIAC controlled. I read the Wiki on TRIAC and got a headache I doubt I'll be having any of those.

[mike31154]

I would not base my decision on which type of relay to use by their 'typical' failure mode. I think I saw a statement in one of those wikis where they said electromechanical relays typically fail in the open position. In my experience over the years, quite a few electromechanical relays fail in the closed position due to the contacts fusing together especially where a higher current is switched. Kind of like welding in a way. Same thing that happens to many heaters where the contacts on the bimetallic strip that turns it on/off fuse together & the heater does not shut off.

I think it comes down to cost for most applications. If you can afford the few extra $$s for a SSR, it's probably a better choice for most of the applications we use.