Smallest Magnetic Relay and how to choose the Ampere rating

I am building an automation system, where I don't have much space, so I require really small relays, 3.3 or 5 VDC controllable relays.

I need to connect it to say a lamp, or a fan, equipment's that draw less than ampere and work at 230 VAC. The 5 V 10 A relays are really big, and I need some advice on choosing the smallest relay and want to know the minimum ampere rating they should have.

Please share your ideas, thanks.

Answer

SSRs are usually rated for more current than you need (I gather you need \$\le 1\:\textrm{A}\$) and their packaging is often designed with a heat sink in mind. So they are usually not small devices -- perhaps rather larger than the size of an equivalent-purpose mechanical relay. They also tend to drop up to \$2\:\textrm{V}\$ across their leads, so in use you might see dissipation of \$\le 2\:\textrm{W}\$. Which probably means that you actually don't need a heat sink, of course. But they tend to build them with that in mind, anyway. So they usually aren't small.

It's possible to make an SSR using a pair of SCRs and tiny 8-pin ICs like the MOC3023 or MOC3063. But even if you source some smallish parts quite capable of the modest dissipation you need, doing all that just complicates matters and you'd also need to work out the packaging and safety details, too. So I think that's off the table here.

That said, you can find SSRs in your current rating. They will look something like these:

But I'm not sure why you can't find a small relay. Here's one:

This can be found for sale at Digikey: Potter & Brumfield Through-Hole Relay SPST-NO. The contacts are rated for \$250\:V_{AC}\$ at \$2\:\textrm{A}\$, which seems to meet what you say you need. They are also small, at \$10\: \textrm{mm}\times 6\: \textrm{mm}\times 5.65\: \textrm{mm}\$. And not terribly expensive, either.

(I haven't checked if that relay is UL approved. You need to check that out, if it matters to you.)

The idea of a reed relay is also possible. Here's what one of those would look like:

This can also be found for sale at Digikey: Cynergy 3 S2-05EU. The contacts are rated for \$300\:V_{AC}\$ at \$1\:\textrm{A}\$, which also seems to meet what you say you need. They are also small but larger than the above telecom unit, now at \$7.62\: \textrm{mm}\$ diameter and \$22.86\: \textrm{mm}\$ length -- about 3 times the volume as the earlier device. They are more expensive, too. But in this case, designated as UL approved.