Blown relay?

Hello everyone,

I managed to build a control for motorized window blinds. I was using a standard no-name two-channel relay-card (to be precise: this one: I'm using one relay to let the blinds go up and one to move them down.

This works great until some time passed. Then only one direction will activate and the other doesn't react. The relay was the whole time connecting the „normal open“ switch and didn't fall back. You can hear the relay „clicking“ but the connection doesn't change.

So I thought that something happened with the relay and used another very similar card. The same thing happened. In both cases it was the relay for the down-direction in my case located on the same side of the card. Maybe that's random, maybe it has a reason.

Has someone an idea why this could happen? I'm not using the isolation-feature but simply using the VCC and GND of my microcontroller (strictly speaking a Photon, no Arduino, but shouldn't matter in this case). The driven motor runs with 230 V. Unfortunately I do not have any information about the current or actual motor used.

Thanks in advance for any help and/or advice.

If it use to work. Now it does not. You can hear the relays click.

The contacts may be welded together. ;)

Hey Larry,

thanks for your response – although I don't quite get it. Yeah, it worked now it doesn't. It isn't switching anymore. In both cases. It remains „closed“. I hear the clicking when toggling the relay.

The relay cards itself look fine. Now blown transistor, diode, … Everything seems to be in place. I don't mean what you mean by „welded together“ – there seems to be no physical connection between the two pins.

Heinzelchen: I don't mean what you mean by „welded together“ – there seems to be no physical connection between the two pins.

"Welded" mean that the internal contacts are melted together. It could be because of the high voltage, which causes arcing when the contacts are opened. Adding a small capacitor across the contacts may be all that's needed.

Thanks a lot for clarifying that. As I'm not quite comfortable experimenting with high voltages: could you please be more precise and tell me what capacitor I should apply and where? Connecting the motor and power line directly?

And I guess there is no saving the relay anymore?

Thanks again for any help, it is really appreciated.

According to this datasheet for the Songle SRD-05VDC-SL-C (from the image you linked to):

That relay (see the second page) is a Form C (the -C in the model name) relay - and so has a contact rating for an inductive load (a motor) of only 3A @ 120VAC (see table 7 - Contact Rating).

Most likely you -are- burning out your relays in some manner (the exact manner would need to be determined by taking apart the relay).

Notice those voltage/amp ratings on the picture of the relay? Those numbers mean little to nothing; the manufacturer appears to take the "max ratings" for both the Form-C and Form-A relays (and both resistive and inductive loads), and slaps the silkscreen on there. Refer to the datasheet, always, for the true numbers (if you have the ability to do so - prior to purchase).

What I would do is locate some relays with the specifications you need (likely something by Omron, if you want reliability) - ie, 5v coil, whatever amps (you -need- to figure this out), and 275-300 VAC contact rating; then desolder the relays off those boards, and solder wires from the pads to the new relays (because it is unlikely that the new relays will fit).

That - or use the smaller relays to switch the larger relays.

Thanks a lot for your reply – that would be clarifying a lot. I don't exactly know, what Motor is inside there. It runs off 230 V directly, the fuses are set for 16A. But thats the same for all fuses in the house. It must be lower than 6A because I can drive three motors the same time.

I'd like to try it out with a more powerful relay then. As I'm not comfortable with soldering something for 230 VAC – 5VDC is fine with me – I'd rather buy a ready-to-use board to be safe.

I found a „big brother“ to my first relay with a ready to use board:

Unfortunately the datasheet does not state how large the inductive load could be – or maybe I just missed it. Do you habe anything to say against this module?

As I'm also tight on space (I want to fit it all inside the wall near the shutters) – is there any compact two-channel-module you know of? I would currently go for something like this:

Thanks again for all your help!

That's a much better relay. You would have to try them out to see if they will work. Buy spares. .

Normally a snubber circuit across the relay contacts would be used to prevent arcing damage. Exactly what component values are best depends on your motor - its current and inductance ratings.

Motors should have plates on them with the rating stamped on.

Hello everyone,

just a quick update: I now bought proper relays that are used to repeat commands from a controller unit for those motors. These are controlled by relays themselves that work with 230 V but do not switch any heavy loads. Works fine so far! Thanks to everyone for your input!