Sunday, 30 November 2008


Some time ago I blogged that the GM3 gearmotor generates a lot of RFI, which was interfering with TV reception in our house and corrupting I2C comms on HydraRaptor. I designed a simple suppressor that fixed the problem, details here.

Recently Zach Smith designed a nice little PCB version of it and produced a kit. He gave me a sample to test. Here it is installed on a GM3: -

To test it I wired a GM3 with no suppressor to a bench power supply with a pair of jumper cables about 30cm long. I viewed the noise on both motor terminals with a scope grounded at the PSU. This is what it looks like in the time domain.

It is massively noisy producing about 50V pk-pk. And here is the spectrum in the frequency domain: -

Although this is the 12V version of the motor it looks similar to the 6V version I tested before.

I repeated the same test with the suppressor fitted, measuring the voltage at the terminals of the suppressor.

The noise is vastly reduced, now only about 700mV pk-pk.

The spectrum is reduced drastically as well: -

Compared to my Vero board version, tested under the same conditions, it seems to work a bit better, but that could be down to variations between motors.

So the kit version works well and also gives convenient screw terminals or 0.1" header robustly anchored to the motor.


  1. Please give some information about the capacitor. What type of capacitor should I use?

  2. They are disc ceramic capacitors.