Relay to be replaced

What to replace a relay with?

I read the above article and its links, and the main article is from 2002. I wonder if they haven't solved all/most of those issues with SSR's by now. Also I watched plenty of youtube videos on them working perfectly.

Point 5 raised in the article about liking the relays clicking sound, well I hated it.

It is part of a 120v ac outlet strip with a max of 15A on any one device. Obviously, I would want rated higher than 15A for safety and etc.

The problem is the relay wears out after only a couple years because the default is OFF and so the relays are ON 100% of the time till they die. There are 8 of these so they waste even more power. Also the manufacturer has a short warranty so we can't get them fixed.

  1. Anyway I would like to know what SSR (solid state) relay I should use to replace this traditional relay?

  2. Does a relay exist with 10x the lifespan?

Additional errata:

The max is 15A shared, but any outlet can draw whatever it wants to a total of the max. All of the outlets will be on 24/7, and the only time any of them will be turned off is during a power outage. Certain devices will be turned off remotely to give more run time (on the UPS) to important devices. The way the circuit is currently designed on the PCB is that the relay/ssr needs power to turn the outlets on, and so the replacement must be reliable when active 24/7 for years, and only occasionally off.

  • \$\begingroup\$ Many if not most relays have both a default ON and a default OFF pin to choose from, in case you weren't aware. \$\endgroup\$ – Bort May 19 '18 at 3:38
  • \$\begingroup\$ @Bort I am trying to repair an outlet strip, and it is design to work this way, and changing the PCB doesn't seem like a viable option. \$\endgroup\$ – cybernard May 19 '18 at 3:41
  • \$\begingroup\$ You will have to do major modifications to fit an SSR; they require heatsinking generally. \$\endgroup\$ – Someone Somewhere May 19 '18 at 3:57
  • \$\begingroup\$ If these are the only items wasting power in your dwelling then that is negligible... \$\endgroup\$ – Solar Mike May 19 '18 at 6:33
  • \$\begingroup\$ Solid state relays have not materially improved since 2002 (and well before that). A mechanical relay with significantly longer life, all other things being equal, will likely be significantly larger and consume more power. \$\endgroup\$ – Spehro Pefhany May 19 '18 at 14:12

Looking at the datasheet shows this relay to be very small for its load capacity.

When the coil is energized the relay dissipates about 400 mW. There would be very little additional dissipation across the switch contacts, perhaps only another 300-400 mW

An equivalent SSR will drop about 1.5-1.8 V when turned on, this would mean about 16-20 Watts dissipation within the device at 16 A.
You need to assess the physical space you have and how much room you have for a heatsink. If the unit is closed (ie no air flow) then this type of conversion may not be possible at all.

Crydom do make some small form factor SSRs like this, but it shows you the difficulty you are going to have in fitting and cooling them.

enter image description here

You could select much higher rated SSRs (20-30 A), and physically larger devices but doing so does NOT reduce the power dissipation in the SSR itself.

  • \$\begingroup\$ Except for the relays which I need to replace there are only the outlets, which means the entire center has a large amount of air space. Even between the outlet has a large of amount of air space. How such tiny pins handle 15A when you need 12g or 14g copper wire to do the same externally? Even the existing relays have tiny pins. 20w of heat doesn't seem like much especially as a max given the total power draw can't exceed 15A which means its divide 8 ways between outlets. \$\endgroup\$ – cybernard May 19 '18 at 14:11
  • \$\begingroup\$ @cybernard You don't divide by 8 ....the dissipation is the dissipation of the SSR! it does not matter which outlet is drawing the current...it's the total current through the SSR that counts. The SSR I showed has two wires per AC pin to get the current rating. \$\endgroup\$ – Jack Creasey May 19 '18 at 14:53
  • \$\begingroup\$ There are going to be 8 SSR because there are now 8 relays. \$\endgroup\$ – cybernard May 19 '18 at 15:22
  • \$\begingroup\$ You still don't divide by 8 ....if any outlet can draw 15A then that SSR will dissipate 16-20 W so will need a heatsink. If you can guarantee that the maximum current is less per outlet and the aggregate is a maximum of 15A ....then you may get by without heatsinks. Only you know the answer to that. \$\endgroup\$ – Jack Creasey May 19 '18 at 15:30
  • \$\begingroup\$ The house circuit breaker will trip at approx 15a(then I will have 0A) so yes I can guarantee I will not exceed that. I am willing to add heatsinks as needed. \$\endgroup\$ – cybernard May 19 '18 at 15:35

Unlike a mechanical relay, an AC SSR does not electrically isolate its AC load contacts when it is disabled.

They have a triac between their contacts with a leakage current, not an air gap. As an example, a 10 A mains SSR I just looked up has 1 mA 'off' leakage current. (It's 10 mA for the version with an internal snubber circuit fitted, but that's snubbers for you.) This particular SSR's 1 mA leakage current is not considered lethal but is certainly enough to give a painful and harmful electric shock.

So electrical isolation and safety requirements must be considered. Electrical safety is easy to overlook when modifying or lashing-up equipment.

If the load is already electrically isolated and its current cannot be physically touched or accessed as per CE/UL/CSA/etc requirements, then you can consider using an SSR. Otherwise, you cannot.

I have used AC SSRs in office equipment to control their AC motors. Using an SSR to switch on and off the internal motor was fine because the motor wiring and connections were all double insulated with sufficiently approved materials and it was all inaccessible to the user.

When I later needed to route either AC or DC to the motor for electronic braking, that AC-or-DC switching had to be done by a mechanical relay. I couldn't use a solid state circuit using my existing SSR, partly because the leakage current of the SSR wouldn't truly cut off the AC supply, mostly because an SSR failure to always-on would put AC into the DC supply circuit. Such a failure is considered a possibility by the safety standards the equipment must meet.

  • \$\begingroup\$ This is all useful information. Can I just add a bridge rectifier, diodes, or something else to block the AC returning to the DC circuit? The part are not user accessible, they are enclosed. On the AC side all the devices are computer related and all have bridge rectifiers and etc to convert and drop the voltage to DC 12v,5v,3.3v or etc. You seem to be pro relay, so if you were recommend replacement relay that has to be on 24/7 and last at least 10years compared to these which made like 2 years. \$\endgroup\$ – cybernard May 19 '18 at 13:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.