I'm using an RFID switch that seems to be very sensitive to electrical noise. To protect it, I'm using it to switch a SPDT relay, and I'm powering it from the NC connection on the relay. I've tapped the NO side of the relay to the input so that once the relay is energised, it will keep itself open and the RFID circuitry will be switched off. However, the relay never makes it to fully open before it switches itself off, reenergises the RFID circuit and attempts to open again. It sits there buzzing away and never fully switching on. I believe I could use a capacitor between +12v and the input to keep the relay powered whilst it is switching, but I have no idea of what type or size I should be looking at. Any ideas?

Regards,
Dave

Quite a bit of guesswork here, but I'm assuming the RFID and relay take around 500mA from the +12V supply when switching, and the relay takes 100mS to switch (both probably a lot less than this, but safer to assume the worst...)

I would put something like a 4700µF 25V capacitor across the power input to the RFID board, after the NC relay contact. This should keep the board and relay powered for around 100mS after the NC contact has disconnected, and give the relay plenty of time to switch.

Ed.

PS: 16V capacitors would appear sufficient for the job, but I'd recommend at least 25V or 35V versions due to possible spikes on the +12V supply.

Thanks for that. Just been to Maplin and got one. It looks to be almost as big as the relay!

Regards,
Dave