I've lost touch with electronics. I used to be an electronics hobbyist in my childhood.

I have some old small hobby project circuits, which I want to repair. One of which is a remote-operated garage gate. It has many (8) old electro-mechanical relays which are dead. What should I replace them with? I just read on wikipedia about the new solid-state relays, are they costly? Are they available?

  • \$\begingroup\$ In general, you shouldn't just go replacing one part with another unless there's a good reason to. Do the relays have part numbers? Could you take a picture of them? You could put that in the question and we could help find replacements instead. Or maybe they can be opened and cleaned. \$\endgroup\$ – endolith Oct 15 '10 at 13:58
  • \$\begingroup\$ It says: DSY2Y-212L, and I'm pretty sure they are burnt or rusted from inside.. \$\endgroup\$ – lalli Oct 15 '10 at 17:17
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    \$\begingroup\$ In the UK, Farnell have them, eight for a tenner. SSRs are around five times as much. There's no reason not to replace the old ones with the same model. If you're very worried about them going rusty again, buy spares and store them in a non-humid environment. You also might want to put the board in a better sealed box. If you need to replace the relays every ten years, are you expecting the board to last another 50? \$\endgroup\$ – Pete Kirkham Oct 17 '10 at 10:08

There are a few problems with replacing electromechanical relays with solid state relays.

1) Most SSR's cannot control an AC signal. They are only rated for DC. They will usually conduct all the time with a reverse biased drain-source current, due to the body diode of the MOSFETs.

2) SSR's can be damaged by exceeding the maximum current through the photodiode, which is usually no more than 10-20mA. A relay will take 100's of mA through its coil. So they aren't a drop in replacement.

3) They are faster than electromechanical relays, which can cause problems for some circuits (the circuit may be designed to reset in the time the relay is transitioning between the two states, for example.)

4) They are sensitive to ESD, electromechanical relays are not.

5) And finally, most importantly, they do not click when they change, which makes them kind of boring. I love hearing my scopes' relays click when you change the attenuation. Don't underestimate how awesome this is.

  • \$\begingroup\$ I kind of like the 3rd and 5th points you made... But over-all I see you point. \$\endgroup\$ – lalli Oct 15 '10 at 11:21
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    \$\begingroup\$ Relay clicks can have a functional use in terms of knowing the state of the system. Much like modems and fax machines, once you know what the relay pattern is supposed to sound like, anything "not right" really stands out. \$\endgroup\$ – John Lopez Oct 16 '10 at 1:54
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    \$\begingroup\$ That's a good point. I know exactly how the autoset function on the scope sounds when the relays change: click/click, 1 second, click/click/click, 1 second, click. Occasionally it is different which does alert me. \$\endgroup\$ – Thomas O Oct 16 '10 at 9:49
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    \$\begingroup\$ I don't agree with point 1. Many SSRs have a triac in them, and the NEED AC. \$\endgroup\$ – radagast Aug 18 '13 at 13:54
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    \$\begingroup\$ Point 5 is a point we should hear more of. There is something very comforting about hearing relays click. As Descartes said, "They clink therefore I am". \$\endgroup\$ – Ian Bland Sep 25 '16 at 7:40

Here's an article about EMR vs SSR: http://www.electronicdesign.com/components/electromechanical-relays-versus-solid-state-each-has-its-place

SSR are widely available and 'cheap', but why change something that 's guaranteed to work with something... less certain? I don't know the details of the board you want to repair, but I'd say: replace EMR with EMR.


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