For dummies question about current flow


I have a bit of a general question. Hopefully quite simple, my knowlegde of current flow is pretty shoddy.

I have a 24v battery which will be powering a dual motor controller. The motors will draw quite a high amperage. I want to be able to turn off the power using a key switch. The key switch is only rated at 1 amp and the motors will draw a total of 40amp nominal (More if stalled).

Obviously i cant just put the key switch directly into the circuit, So I plan to use the key switch to power a large relay which in turn switches the power.

So will the following work: +------------------------------------------------------+ | | 24v + ---------------- Key Switch -------- Relay + | | Relay GND - ------------------------------------- Relay - | | | | Motor controller + | -------------------------------------------------------Motor controller -

So basically I take two 24v wires from the battery, use the key switch to control the low voltage side of the relay. Then take two much chunkier wires and run them from the battery via the high amp side of the relay into the motor controller.

My question is, will this work. I.e will the current flow as expected. A little bit of current through the key switch and low side of the relay and a big bit of current through the other side of the relay into the motor controller.


Hello, What is the purpose of the key switch?? So someone can't drive off?? If so, why not have a small relay switch the power to the logic of the controller.

If you want "power off to work on the system" then how about a large circuit breaker (which you may want to have anyway) which you can switch off?

Its for a DIY Segway so yes the key switch is so someone can't drive off. The Battery powers a Sabretooth 2 x 25 motor controller which in turn give 5v out to the Arduino.

5v is not enough to drive a relay big enough to deal with 40+amps. Additionally I can’t just have the key switch turning off the power to the Arduino as the Arduino communicates with the Sabretooth using simple serial. Even if I turn off the Arduino the last command send to the Sabretooth will continue to be executed. So really I need to kill the power before it gets to the motor controller.

Hence my above question on doing this without going via the Arduino at all.

Ok, your original diagram is workable.. but will you leave power on to the Arduino? Does it come from some regulator from the 24V??

You need to find the right relay then...

I can't read your ascii art properly, but your description sounds correct.

A little bit of current through the key switch and low side of the relay and a big bit of current through the other side of the relay into the motor controller.

The low side of the relay, as you put it is the coil. This is an electromagnet that closes the contacts of the high power part of the relay. As long as you use a voltage close to the nominal voltage of the coil then the coil's resistance will ensure that it draws the right amount of current to close the contacts without drawing so much current that it gets hot and burns out the windings of the coil.

If you want a more definite answer about your specific circuit, can you send a better picture please. Draw it, scan it, upload it.

Thank you both, yes the power for the arduino comes from the Sabretooth 2x25 so if power to that is off, so is power to arduino.

As you suggest my proposed circuit is workable. The only problem I now have is that the battery puts out 24v. i need 24v for the motors but likely only 12 for the relay coil. Unless anyone can suggest a 24v / 24v 40amp relay?

Fear not! Look a little harder... a relay with a coil made to operate properly on 24v shouldn't be hard to find. Be sure it is designed for 24v DC... you will probably find plenty for 24 vAC, and yes, it matters.

Forgive me if I am stating the obvious, but... your design will have to overcome the natural inventiveness of Bad Guys... I suspect it won't be easy to fit that keyswitch in a way that isn't easily bypassed with a piece of wire. One way 'round that would be to fit some "distractor" wires, and fix things so that if the wrong connection is made (or broken), a fuse somewhere obscure blows, rendering the system dead until the fuse is replaced.