HI Stephen

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.

Was the controller new (or sold as 'new').?
Maybe the dodgy relay is why it was on ebay ?


Does anyone know the schematic for these boxes of tricks? I presume
the relays isolate the electronics from the motor?

Probably

Is the switch side
of the relay connected to the mains on one side and the motor on the
other? Is the relay output open circuit when he door is stationary?

Very likely

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?

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!

If all that your switch is doing is to short out the relay contacts then
it shouldn't do any harm.... but do you know how the power to the motor
is cut when the door is at 'full open' or 'full shut'. If this is done
by the control box then adding a manual control across the relay could
be very dangerous...


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. Possibly this is welding the relay stuck.
Do you think I should add diodes?

You're confusing two things here, I think.
The reverse emf from the relay coil (assuming that the relay coil is fed
from a low DC voltage) will appear on the low-voltage side of things -
and should be absorbed by a diode connected across the coil.

If you're seeing sparks then they are coming from the contacts
themselves - which are on the mains side of the relay. Don't fit diodes
to these contacts - they won't last very long..... There might be some
point in fitting a high-voltage capacitor across the relay contacts....
but make sure it's one that's rated for mains use.

Have you mentioned it to the seller - might be a plan before trying to
re-engineer it yourself....

Regards
Adrian