Help needed with solid state relay

I'm having a problem with a solid state relay. I'm in a paper converting plant with lots of dust. The original hardware was a mechanical relay with a 120VAC coil and switching 24VDC on one set of contacts. Because of the dust the contacts on the relay tend to corrode and burn up. I thought that a solid state relay might solve the problem. I replaced two of the relays with AD-SSR810-AC-28Z solid state relays (a comprehensive explanation of relays Basic Knowledge of Relay Electronics Tutorial with Video). for the first week everything was good. After a power outage, the PLC output, 120VAC, would energize the SSR coil but the contacts wouldn't switch. In an effort to troubleshoot, I used a jumper across the SSR contacts to insure the downstream equipment worked. After this the relays began switch every time with no issues. Until there is a power outage or a long duration downtime event. Then I break out the jumper and all is well until the next downtime event. How can I make this work? My original goal was to eliminate replacing mechanical relays every 5-7 days. Maybe a capacitor across the SSR output?
Thanks in advance!


  • PedalToTheMetalPedalToTheMetal Member Posts: 137
    Putting a snubber (do never use the capacitors alone as they will cause massive surges when a relay shorts them, no matter if it is a mechanical or a solid-state one) prevents the relays from arcing (or, for solids, from voltage spikes with possible breakdowns) when they open. They have no positive effect on the closing cycle however.

    As for your issue, read the datasheets first ;) You are using an SCR-based (thus, an AC-only) relay which is surprising to even be able to shut off when running DC. However it also has a zero-voltage detector which you trick by putting that jumper so it simply glitches out and pretends to work :) Replace it with a proper DC relay as soon as possible or it is surely some trouble brewing.
  • BlowbubblesBlowbubbles Member Posts: 5
    GOT IT! :) Thanks a million, friend, for your kind advice.
  • PedalToTheMetalPedalToTheMetal Member Posts: 137
    No problem! A little word of advice though: SSRs are very different from the mechanical relays. While upgrading to an SSR a good practice is to put a reverse diode across any inductive DC load as they are pretty sensitive to voltage spikes while switching off. They may have built-in snubbers yet such a precaution will be a good step anyway (otherwise it is a must).
    Also should you have trouble finding an AC-controlled DC SSR you can get a DC-controlled SSR and drive it via an enclosed mechanical relay from any low-voltage source (they don't need much current nor create any spikes so any small relay will go).
    Good luck!
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