Which ceramic capacitor for small DC motor?


I have a project for which I have 'borrowed' a DC motor from another commercial device. The motor came with a small brown ceramic capacitor across the terminals and worked fine until the capacitor disintegrated. The motor now only functions some of the time but mostly not. I can't see what markings the old capacitor had as it was the potting that fell apart. The circuit runs on three 1.5v AA batteries in series. What is likely to be the value of the capacitor I need to replace the old one.



by PPI
May 21, 2020

You've given precious little information about whatever you are using to drive the motor so I or anyone else can only give wild guesses as to the problem. My current guess is that the drive is some sort of electronics and that the capacitor was used to filter out brush noise from the motor so now the noise is disturbing the electronics. If so, I would try a few different capacitors in the range .47, 1, 4.7 mfd to see if anything helps. I ran into a similar problem once trying to drive a very small dc motor from an Arduino UNO pwm output.

by Foxx
May 21, 2020

2 Answers

Answer by PPI

Hi Foxx,

Thanks for your reply.

Sorry about the lack of info: Lockdown+kids off schoolĂ—hours in the day = rushed post.

I'm actually trying to fix a baby bouncer that has a music function and a vibrating base. Yes, the motor is controlled by a small pcb. It has a cam weight on the shaft to give the vibrations rather like a mobile phone / cell phone. The motor has two speeds, controlled by the pcb: one button press = high speed, two presses = low speed, third = stop. The previous motor was identical but one of the supply tabs fatigued and broke off flush with the case and I replaced it with the current one, which came out of a toy. Both had the parallel capacitor but the old motor is long gone now otherwise I would have checked that cap.

I read about the parallel cap being utilised to suppress interference but also wondered if it helps with the initial high torque / demand of the start.

I'll try those cap values and let you know.


+1 vote
by PPI
May 21, 2020

Answer by Foxx

Any sort of starting assist capacitor would have to be quite a few mfd so beyond what a ceramic capacitor would supply and would probably be electrolytic. So I still think the best guess is that it's there for noise suppression. I've assumed that the dc motor is true dc and not "brushless dc" which is actually an ac motor with a chopper of some sort built in. Try the values I've given but don't be shy about trying others, possibly double or half what I've suggested; it's not critical. Also, it may be worth while trying a 100 ohm resistor in series with the capacitor which is often done to convert noise to heat and so reduce or prevent ringing. Just don't use a wire wound resistor.

+1 vote
by Foxx
May 21, 2020

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