Re: E-stop relay welds contacts for load dump resistor

Just a little more about relays.

Back when I was working on windmill controls, the early days when common systems were 24 to 48 VDC, we used relays for load controls. The problems didn't come with contact closure, but when the contacts opened, especially with an inductive load. As mentioned before, an arc can form if the contacts open slowly or the gap isn't wide enough.

Magnacraft and others sell contactors with blowout magnets that blow the arc away from the contacts, effectively lengthening it so it cools quickly. Another scheme that can be used with conventional relays is to wire the multipole contacts in series, not parallel. That doubles the speed and length of the switching contacts. Paralleling contacts is never a good idea.

The use of a diode across the relay coil in DC circuits to clamp reverse EMF when the relay opens will slow down the movement of the relay arms a little. A diode with a small resistor in series will help some. This isn't usually a big deal but keep it in mind for critical circuits.

Finally, an SCR is sometimes used as a 'crowbar' to dump a power supply quickly. Trigger current is very small and the SCR will turn itself off when the power supply voltage drops to zero. If you want to limit the surge current, add a power resistor to the anode of the SCR.

Earle Rich

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