Using a 2 A, 48 VDC rated switch in a 8 A, 12 VDC circuit


I have a push button make/break switch rated at 2 A, 48 VDC. I want to use it on my aircraft in an 8 A, 12 VDC circuit. Can I use this switch in this circuit or will the switch burn up?

Best Answer

tl; dr: nope. The switch will fail.

I think you’re misinterpreting how switch ratings are derived. While the two proposals would appear to handle similar amounts of power (96W), switches are specified like wires: they’re rated for their insulation strength and current-carrying capacity.

  • Switch voltage rating means its insulation strength, both in the switch body and between its open contacts. That is, the max voltage that won’t result in arcing.

  • Switch current rating is based on acceptable heating in the switch when carrying current. That is, the max current at max ambient temperature that won’t result in the switch burning up.

In your case, since you’re switching only 12V, it’s the current rating that really matters. 8A is much more than your proposed 2A switch can handle: it will heat up and fail in short order. Further, the low-rated switch will have higher IR drop in it than a higher-rated one, robbing your load of adequate voltage.

To be safe, choose a switch that can handle at least 2x the worst case load, that is, 16A. Then you’re assured that if there is a short, it will be the fuse that goes and not the switch.

If your concern is panel space, you could use a relay, much like automobiles do for headlamps. Then you can use your 2A switch to control the relay, which would switch the 8A load.

If you’re unsure about any of this, and even if you aren’t unsure, consult a qualified aircraft mechanic or avionics technician.

Related Topic