Usage of a small cap on a dc motor

Discussion in 'General Electronics Chat' started by timropp, Dec 5, 2011.

  1. timropp

    Thread Starter New Member

    Nov 3, 2011
    I'm playing around with modifying a small toy train motor (from the Lego trains, if you care). Inside the plastic box, there's a basic toy motor. It's running on 9V from batteries, with speed controlled by PWM from the control box. And of course, just reversing the polarity to go in reverse.

    For my usage, I need to switch the polarity on the motor - I planned to just swap the wires. when I got inside the box though, there's a small cap in series with the motor. I assume that's to reduce EMI? Does it matter which side of the motor it's on - there's not much space to work with, so I plan to just unsolder the wires from the motor, cross them, and solder back on, leaving the cap where it is, so it'd be connected to the other side of the motor.
  2. sheldons

    Well-Known Member

    Oct 26, 2011
    show us a picture....are you sure its not a small inductor/choke?
  3. timropp

    Thread Starter New Member

    Nov 3, 2011
  4. Adjuster

    Late Member

    Dec 26, 2010
    If it is in series with a DC motor, unless somebody has messed about with the circuit it can't be a capacitor (capacitors block DC, so the motor would not run). Small capacitors for EMI suppression are normally connected in parallel with the motor, where they reduce transient voltages across the brushes but do not affect the DC supply.

    It might perhaps be a positive temperature coefficient thermistor, intended for overload protection. These are not usually polarity sensitive.
    Last edited: Dec 5, 2011
  5. timropp

    Thread Starter New Member

    Nov 3, 2011
    Ah, thanks. It very well could be a thermistor... makes sense given the application. Lego is known to have thermal protection in their motorized stuff. Didn't even think of that.
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    It's not uncommon for those motors to also have a 0.01uF cap across the terminals to reduce RF interference.

    What is shown inline is a temperature fuse/PTC fuse, which opens when the motor is overloaded, but there isn't a cap in the picture for EMI supression.