Relay Shield Failure - Relay LED lights but circuit (NO) doesn't close - tapping the relay makes it work

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I’m using a photon with a relay shield. I am powering the relay shield with a Mean Well DIN rail mounted 12VDC 0.88 Amp external power supply. I use all four relays in my application, only one relay is triggered at any given time.

I installed a new relay shield last week and only 3 out of 4 relays worked properly. Relay 3 failed about 75% of the time. The relay would be engaged, the red LED would come on, but the NO circuit wouldn’t close. If I tapped the relay (the black box) with a small screwdriver the NO circuit would close.

So I swapped out the relay shield for new one. I got the same kind of failures, but on a different relay. I swapped out for another new relay shield and got the same results. Strange, huh?

I also tried to power the relay shield with a 12VDC 30Amp power supply with the same results.

The very first one of these that I built I used a 5VDC power supply and saw this kind of behavior but it totally went away when I properly sized the DC power supply.

I have built dozens of these systems with no relay problems, then three failures in a row. Has anyone experienced something like this? Suggestions?

Thanks for your help!


Yes but possibly not with the same sort of relay. What voltage and load are you switching?

High in-rush currents can make relays ‘sticky’ because the contacts get micro welded together unless the contacts are tolerant.

I have been using a Hongfa HF115fa relay then for a batch of boards could not source that particular relay and had to use a Schrack brand with the same specification. Virtually all the relays in a batch of 20 had to be replaced because they went ‘sticky’.


Thanks for the reply.

Two of the relays are wired to a 90 vdc power supply with a max output of 1 amp. The other two relays are wired to a 12 VDC power supply with a max output of 30 amps, but the expected max load is less than 20 amps.

I’m seeing this stickiness on both types of circuits.


Relays usually have a lower DC voltage rating than AC since the arc wouldn’t break after 10ms but may stand longer causing damage to the contacts more and quicker.

Unfortunately the Panasonic relays on the store image and my board don’t have the DC rating printed on them but here is the datasheet from the Particle hardware repo

which puts the DC rating at 24V or 30V with 1A or 3A depending on the version of the relay.


For 12 VDC and 30 amps I would suggest using an automotive relay rated at 50A. On a 12V battery stack these have proven to be robust and reliable.


or if the cost is acceptable - a solid state relay like this


Good call - suggest looking at application notes for SSRs.


Thanks for the advice everyone - not exactly what I wanted to hear but much appreciated.


Are you getting enough drive current for the relay coil? Sometimes if the drive is low, tapping it will assist the magnetic field to pull in the relay, and then it will stay held in. Does it chatter any?