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wrote:
Stephen wrote:

Hello,

I bought a roller door from a well known auction site ;(

The problem is that the box of electronics controlling it has got two
relays. I imagine one for up and one for down. The "up" relay is
sticking so that the door will not open. I have opened the box of
electronics and given the relay a tap and it is working for now but I
expect it to stick again soon. How can I prevent it from sticking in
the future? Is there anything I can do? I guess I really need to get
out the soldering iron and replace it before it fails completely.

Sticking is localised welding of contacts, caused by the contact
surface being rough and oxidised, which is normal wear & tear of
relays.

take the cover off the relay, use a little piece of medium-fine
sandpaper to smooth the contacts.

Does anyone know the schematic for these boxes of tricks?

Dont think theyre standardised.

I presume
the relays isolate the electronics from the motor? Is the switch side
of the relay connected to the mains on one side and the motor on the
other?

usually yes

Is the relay output open circuit when he door is stationary?

you tell me. Probably, but really you shouldnt need to ask that

If
so I was wondering whether I could put a switch in parallel with the
relay so that I have an emergency override? Is this possible?

cant you figure out how to answer that? We cant see the circuit
working.

My worry is that if the relay output is connected to ground when
stationary, I don't want to blow a fuse when I switch the override!

I notice there is a spark when the relays open. I'm not sure whether
that's the reverse emf in the coil or just the mains jumping the
contacts as they separate.

the latter

Possibly this is welding the relay stuck.

yep

Do you think I should add diodes?

no, that only works for dc loads.

The box is branded Nico IIRC.

Thanks,
Stephen.

NT

http://www.wiki.diyfaq.org.uk/index.php?title=Snubber