Testing capacitor in series with resistor

Discussion in 'General Electronics Chat' started by ebeowulf17, Dec 22, 2015.

  1. ebeowulf17

    Thread Starter Distinguished Member

    Aug 12, 2014
    Where I work we use snubbers on 240V motors to reduce electrical noise making it into a 5V system. In general, this has made a world of difference for us. Occasionally we still see noise related problems, which has me wondering how I can test if the snubber is good. The snubber in question is listed below:


    It's just a 100ohm resistor in series with a 0.1uF capacitor. I've got a multimeter with a capacitor measurement range, but I don't know if it will work with the resistor in series.

    Any advice on how to check an rc snubber, or if it's even possible?

    I'm sure many of you are chomping at the bit with advice on other ways to help with the noise, and we're working on that too (with lots of help from these forums) but in this thread I really just want to know how to test a snubber. Can a regular dmm do be any good? If not, could I use a signal generator with an oscilloscope to find the frequency where the filter action kicks in? (Probably not worth the effort in many cases, but just curious if the approach would even make sense.)

  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    You can test it on ohms, it should start off at 100 ohm then ramp up to near infinity, or measure its capacitance with an appropriate meter.

    At 50Hz the cap will have a reactance of approx 32K ohms.
    Last edited: Dec 22, 2015
    ebeowulf17 likes this.
  3. #12


    Nov 30, 2010
    What the Dodger said. Then reverse your probes and watch again if you need to test twice to feel certain. The higher the Ohm scale, the slower the change will be. If you start with a 1K scale, it might charge so quickly that you can't see the meter changing.

    The 32k number (31.831K) can be used by connecting the snubber to a 50 Hz voltage source and checking to see if the right amount of current flows.
    ebeowulf17 likes this.
  4. ebeowulf17

    Thread Starter Distinguished Member

    Aug 12, 2014
    Thanks for explaining the 32k number. I had no idea how to go about testing the reactance; now I do!