Confirming PSU spec for steppers.

[irving2008]

Voltage on relay contacts is generally not important, its the current carrying capacity. If you need more current then you wire the contacts in parallel. The reason why contacts have a lower DC voltage rating is to do with the arcing that happens when the contact opens. Generally the contact gap is very small, maybe <0.5mm or so. With a DC current the arc ionises the air in the gap and causes a plasma to develop. If the voltage is too high this plasma will continue to pass current until the contacts have opened sufficiently that the voltage cant sustain the plasma, which seriously shortens the contact life.

One solution to the arcing is to put a snubber across the relay contacts - a capacitor and resistor in series - the idea being that as the contact opens the capacitor charges up thus drawing current off the contacts and stopping the arc forming until the contacts are far enough apart to prevent the arc being sustained. This alows the contacts to be used at a higher voltage at the rated current.

This isnt a problem with AC contacts as the voltage returns to zero every cycle thus quenching the arc, thus they can be rated at much higher voltages.

Now, in your situation, using it for the bleed resistor isn't a problem. The current passed is tiny (30mA) so the voltage rating of the contacts isn't an issue as very little arc will be generated.


Incidentally, where did you get the 10v from? If you look at graph H55 on the spec for those relays, you will see that the 4PCO version is rated at 30v 6A. At 70v its derated to 0.5A... but thats only to meet the contact lifetime figures at 1800 cycles/hour. If you switch a dozen times a day it'll be a lot!!! On the AC side graph F55/1 shows that at 900VA the contact life is 200,000 cycles at 1800 cycles/hour so its not going to fail in your lifetime (about 45years at 12 cycles/day) lol.