Separate Power Supplies for Motors/Logic Circuit?

I’m just starting out on controlling small DC motors via PIC micro controllers (via the SN754410 Dual Motor Control and the Pololu 210:1 micro metal gear motor, pn 1096) and I’ve read that the power supply for the motors must be separated from the power supply for the logic circuit (possibly sharing a common ground). My question is this: how separate, exactly? I have a 1A 9V DC regulated wall mounted power supply that I’d like to use for the whole project (using a 7805 voltage regulator for the logic circuit) and I’ve soldered 0.1uF caps to the motor leads, but will I still have trouble with electoral noise? If I can’t use the wall mounted power supply for the whole thing, I’ll have to use a 9V battery for the motors but I’m worried that they’ll drain quickly.

Hello, Ross.

How to separate them depends on your reason for separation. Do you have some voltage regulator that is regulating the 9V down to logic voltage? If so, that is possibly enough separation depending on your requirements.

You are correct that a 9V battery is not a good supply for motors.

- Ryan

Thanks Ryan. Yes, I have a voltage regulator that is regulating the 9V down to 5V logic voltage. I was also thinking about adding a 3rd 0.1uF cap between the motor terminals plus further isolating the power supplies as outlined here


A voltage regulator is usually one of the best ways of decoupling logic and motor voltage. The motors you are using are not particularly noisy, so your setup might be fine just as it is. I suggest you worry about further decoupling/noise suppression only if you notice power-related problems.

- Ben

Good advice - thanks Ben. But is the only thing I should look for is the PIC resetting?

Yeah, that or strange problems with other electronics in your system. If you have a power problem, it will generally get worse as the load on your motors increases, and it will often show up when you make abrupt changes the motor speed (e.g. commanding it to go full speed from rest or to go full-speed reverse while it is going full-speed forward). The first thing I would check in such cases is that the power supply can actually deliver the current the motor is drawing. Once you’ve ruled that out as the cause, you should move on to trying to reduce motor noise.

- Ben

Perfect, thanks Ben!