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I am building a battery powered trike. My battery is 72v. I have a three position key switch ( off, on, accessory) I have purchased a 72v contactor coil to operate my dc controller, the controller is rated for 0v to 100v. My confusion is in the switch, the switch is rated at 28vdc. My question is:Can I use a voltage divider to drop to 12 volts, and then go through my key switch using the 12 volts to turn on the contactor to supply the 72 volts to my controller? My knowledge is limited on the types of components needed to achieve this and I did not want to burn up the switch with 72 volts. I also failed to mention that my battery does go through a 30 amp breaker on my positive line, before the contactor. Any help would be appreciated.

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  • \$\begingroup\$ I actually looked at that exact one this morning! I ended up going cheap and got this ebay.com/itm/… So... do I need something between the 12volts from the key switch to the coil of the contactor ? \$\endgroup\$ Commented Apr 4, 2020 at 23:59

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Voltage Divider is not a good option as it will HUGELY consume current and most probably cause Fire Hazards. You can use a voltage regulator and choose one that supports 12 Volts. Try to choose one where the voltage conversion is in the low power range of the voltage regulator.

Another option is to use a DC/DC buck converters and there are options in the market ready for the specific voltage 72 Volts to Volts 12.

For example: https://www.powerstream.com/dc-72v-12v-5a-extreme.htm

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The easiest way would probably be to just upgrade the switch to a 125V type. These are common for industrial controls, so should be easy to find / order from the usual places like Mouser or Digi-key.

Also, search for '72V key switch' and a lot of e-bike stuff comes up.

I would also recommend a kill switch / deadman's switch for your setup that will open your contactor. Fail in wide-open is a thing, as my friend the motorcycle mechanic can attest (a Zero got away from him and launched him at a tractor-trailer.)

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  • \$\begingroup\$ I would change to a 125v switch if I had to do it all over, But The original key switch is in a tight location, so I cant put a larger switch in its place. The key switch I have is mounted in a 19mm hole. \$\endgroup\$ Commented Apr 5, 2020 at 0:23

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