I have a 12V 40A motor. I have a switch between the motor controller and the battery.

I want to put a pre charge resistor in the path. Like in the figure.

My questions are:

1) The pre charge resistor is connected directly to the battery, so that means the system is connected even the switch is off. How is this condition safe? How is this condition safer than having just the switch? And what should I worry about making this connection?

2) Is there in the market a switch built with this pre charge resistor? If so, what is the name of that switch?

I am thinking that I need some kind of two step switch.


I will follow Whyskeyjack solution, but I have one more question:

Is it OK to have the switch in parallel? Can the precharged switch be rated less power than the other switch?

This is the switch I got for the main path.


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


How about using a MCB + latching motor switch?

Switches look like this:


latch switch

MCB sits in between of power supply and the rest of the setup. Latch switch sits in parallel of the resistor:


To turn the motor ON, you lift the MCB and then press the latch switch after a second or two (depending upon the time it takes for charging). It provides additional protection in case of short circuit as well. In a power cut, the latch switch will automatically unlatch. Manually press it again to restart the motor.

  • \$\begingroup\$ I will follow this solution, I edit my question because I have one more doubt. \$\endgroup\$
    – Luis
    Apr 20, 2019 at 5:36
  • \$\begingroup\$ You can use the proposed circuit but you need to be careful about the order in which you close the switches. 5A switch is OK for the pre-charge circuit. Main switch is rated for 12V 40A and you mentioned that your motor consumes 40A. This is not a good idea. You need to use somewhat higher current rating main switch. 60 A or higher is something I will use for a 40 A motor. \$\endgroup\$ Apr 23, 2019 at 7:02

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