I'm working on a little car based project, where I have an inverter protected by a 40a internal fuse. I need this inverter to only recieve power when the ignition switch is on (i.e. when car is running, so I won't kill the battery). Unfortunately, the only existing accessory power sources are protected by a fuse rated for only 7.5a. So tapping into these sources will ultimately result in a blown fuse.

Constant power sources have much less restraint (40-50a), but would normally require either manually unplugging the inverter, or wiring up a switch, that I'd have to manually flip to save my battery.

So, in the name of laziness (and the rule of cool) I'd like to create a switch that works automatically based on whether or not there is power to an accessory source. I've tried Google of course, but the closest I've gotten is either a transfer switch (not applicable - it's only being powered by one source) or an automatic switch based on whether or not the main source has power.

  • \$\begingroup\$ if there an existing switch/button on the inverter, so that the added switching device wouldn't need to handle the full 40a? \$\endgroup\$
    – dandavis
    Aug 16, 2019 at 21:13

1 Answer 1


This is usually done with a relay.

The relay coil is fed by the ignition switch and the contacts by a supply, with a suitably sized fuse, directly from the battery.

  • \$\begingroup\$ Well that was a lot simpler than I was expecting. Thanks! \$\endgroup\$ Aug 17, 2019 at 0:00

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