Relay contact voltages

[zotdoc]

I want to use a 12 volt automotive relay to switch 36 volts DC. I am using a 12 volt dc supply for the relay coil, but the contacts will cary 36 volts. My relay is rated for 30 amps at 12 volts. Do I need to get a bigger relay to run 36 volts at 30 amps or will the contacts on the relay I have work well enough? Thanks.

[haklesup]

It will work for a while but will fail sooner because you will get a larger arc inside the relay when the contacts open and close under power. This will erode the contacts faster.


In the shorter term, contact resistance will rise over time and at 30A, this may eventually become a problem.

Search on "Relay Contact Reliability" and you will find some information. This one was good. Fig 1 shows that higher voltage or higher current decreases lifetime as measured in switching cycles.
http://archive.evaluationengineering.co ... niques.asp

Worse case is an arc large enough to weld the relay contacts permanently closed. Current is the dominating factor in arc current but voltage will cause the arc to form sooner and last longer thereby increasing the amount of heat the contact needs to dissipate without being damaged. Normally this only causes pitting of the contact surface but if the relay is non-hermetic and operated in the presence of certain gasses, you can also get significant carbon build up on the contact surface.

[jwax]

Maybe some capacitance across the contacts will absorb some arc energy.

[haklesup]

It would make it worse. Thre charge from the cap can be delivered to the arc through a shorter lower impedance path than the primary source.

A series inductor would be a better choice as it would impede the rapid increase of current associated with arc ignition (large di/dt)

[jwax]

Sorry, a series resistor/cap combination is preferred.
Here's one article from Tyco-
http://relays.tycoelectronics.com/appno ... 3c3236.pdf