Sticking Relay

[rtbcomp]

On a DC energised relay the problem is often residual magnetism in the armature (core) of the coil

Shouldn't really happen, cores of DC relays will be made from soft iron or some other material that doesn't retain its magnetism.

 

[awemawson]

Tell that to the armature of the emergancy cut out on my shaper <G> - it was laminated soft iron, but stil has enough to hold the beast 'in' until vibrations freed it. I put an aluminium shim on the end to slightly increase the gap solving the issue.

 

[tigler]

out of interst then, what's the science behind it actually sticking then once the control voltage has been removed? as the coil is no longer energised and pulling the contact arm down, so whats keeping the contact "in contact" as the spring is clearly trying to pull them apart?

More than likely it will be micro welding at the contact point. The only force pulling the contacts apart when power to the coil is removed is a small spring, contact welding will easily defeat that.

 

[rich r]

Also if it has been subject to heating (from arcing or high currents) the copper loses it's springiness and tends not to be able to pull the contacts apart as easily.

 

[M@tt]

New relay arrived Friday (farnells are cheap and have free delivery!!!), fitted it yesterday and normal service has been resumed

many thanks for the help again