DC motor causing noise on the power line

Question

I've been building a circuit where I'm controlling a motor using a bluetooth module + phone. I've been having some problems, which mostly are fixed (see topic: previous topic).

The main problem I'm facing at the moment is that the DC motor is causing quite some noise on the power line (even if I power the motor directly from V+). I've managed to reduce this by limiting the motor current (using resistor R13) and put in some capacitors + a diode (D3) over the motor. However there is still siginificant noise.

Therefore I would like to know if there is a way to reduce the noise on the power line even more beside having these capacitors and the diode(D3), when the motor switched on (or have additional capacitors)? And what capacitors are best in reducing noise?

See schematic below:

Answer 1

Solder a 100nf capacitor as close to the motor brushes as you can, and if the motor has a metal case, also 100nf from each brush to the case. Similar to what is shown at the bottom of this page on the Pi2Go build instructions. Keep the capacitor leads as short as possible, even a large SMT ceramic capacitor would be a good option. This is assuming the noise is RF, interfering with the bluetooth. If its interference with an MCU, you could increase the capacitor value maybe 10x.

Answer 2

You could put a pie filter in front of the motor. This could reduce your overall load spikes on the system.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

I've been testing with bigger capacitors, and a 1000uF (C5) solved all the issues. Unfortunately I won't have enough space in my casing for such a big capacitor. But even a 100uF made it a lot better. I will see what I can squeeze in.

@ Sean Houlihane, I'm not able to solder directly to the motor or very close to the brushes. The motor is placed in a very tight space in the casing and a couple cm away from the print. Luckily replacing C5 did the trick well enough.

@JWL, unfortunately the pie filter won't be an option, since the motor is also powered from the 3.3v so they share the same power source. I don't want to power it directly from the LIPO battery since the current the motor draws at 4.2 volt is too high. At 3.3 volt the current stay nice and even.

Answer 4

Unfortunately the motor is alternatively pulling energy from the power line and dumping it back in. I would might try the following approach: put a low ESR cap (ceramic is best) as big as you can across the motor to knock out the high frequency. Then put an electrolytic across the processor with a resistor in series to give you some isolation. But most importantly, make sure the battery or power source is connected to the motor terminal and MOSFET source such that the current from the power source going to the motor does not pass through or under the processor circuit. Use wide traces when you can, but it is most important to keep the motor current separate from the low power stuff, including processor power and signals.