Relay contact resistance
I’ve been using a. wireless Tado to switch an Ecodan ASHP off if the house temperature gets too hot. The switching circuit is a 12v 1mA supply which needs shorting to turn off the heat pump. This is done by the Tado relay. This has worked well for over a year but now the heat pump will not turn off. I suspect the relay contacts have got oxidised and do not self clean with only 1mA current. Any ideas to make the contacts self clean under these low standing currents?
Answers
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The relays do stick on, even when switching boilers at mains voltage. Maybe you could automate something to tap the casing.
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I think the problem is the opposite of this, it is not making contact on closing. Tapping the case is worth trying though.
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Ok, I see what you mean. You ought to be able to hear it move the relay.
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The relay is clicking correctly but it does not always give a low resistance short when closed. I think the on current of only 1mA is not enough burn off any tarnish that builds up. Gold is usually used for low current contacts but I can’t imagine Tado using expensive contact metals! I wondered if anyone knows the make and model of the relays Tado uses and if they have a minimum current spec. The heat pump uses weather compensation and so the Tado may go several days before the house gets too hot, plenty of time for the relay contacts to tarnish.
Maybe I should program in regular on-off cycles to exercise the relays? Another idea is to put a capacitor across the contacts to zap off any clag when the relay closes.
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