Relay contact rating on DC
Relay contact rating on DC
(OP)
If a relay contact is used to switch raw (unsmoothed) DC obtained by full-wave rectifying AC line voltage, do I use the AC or DC contact rating. Since the voltage still drops to zero, does this mean I can use the AC rating, or is there a derating factor I can apply.
I am finding it almost impossible to find a relay to switch 10amps @ 120V.
I am finding it almost impossible to find a relay to switch 10amps @ 120V.






RE: Relay contact rating on DC
For pure ohmic resistance there is no difference, at least in first approximation.
For inductive load I would rather use AC rating.
For unrectified DC you can try solid-state relay.
RE: Relay contact rating on DC
RE: Relay contact rating on DC
RE: Relay contact rating on DC
Alehman is correct, DC tries to maintain the arc across the contacts, where AC passes through zero and helps extinction.
One method is to use DC contacts wired in series, which effectively increases the contact gap. Have a look at the information for the 700SP with four contacts in series, which may match your requirements, or use similar.
http://www.ab.com/en/epub/catalogs/12768/229240/229266/229661/3087128/229739/print.html
Regards,
KVKEV
RE: Relay contact rating on DC
TI 2CR Motor Starting Relays
http://www.ti.com/snc/products/controls/motor-2cr.htm
Power-IO H Family SSRs
http://www.power-io.com/products.htm
RE: Relay contact rating on DC
RE: Relay contact rating on DC
http://www.aecsensors.com/indust.html
RE: Relay contact rating on DC
For these reasons, we prefer to use solid state relays.
We had an EPA company quote the cost of hauling away an old control cabinet that had a bunch of MDRs. Wow.
RE: Relay contact rating on DC
RE: Relay contact rating on DC
What is the effect of a relay failure? Do you only have to replace the relay or could someone get injured?
USE THE DC RATING UNLESS A FAILURE WOULD HAVE MINIMUM CONSEQUENCE OR TESTING IS PERFORMED.
As has been stated in some of the other posts, if there is inductance in the circuit, the contacts will (if you are lucky) be good for the DC rating.
RE: Relay contact rating on DC