Capacitor across relay switch

Discussion in 'General Electronics Chat' started by iinself, Feb 25, 2015.

  1. iinself

    Thread Starter Member

    Jan 18, 2013
    69
    0
    Hi,
    I am repairing an old receiver and observed that there is a 4700pF cap across the relay switch. I believe that is for protecting the relay switch from the inductive load (transformer in this case). My question is that when I measure the AC voltage across the switch and the neutral when the relay is open the meter reads 120V, is that normal? In the figure across points A and B. Currently there is no load connected. My guess is that the reactance of 4700 pf capacitor (about 565 Kohms ) is allowing the voltage to show up on the other side of switch even when it is open.

    Can someone throw some light on this ?

    Thanks
     
    Last edited: Feb 25, 2015
  2. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    Your meter likely has a 10 megohm input resistance so a 565 kohm reactance would allow most of the voltage to appear at the meter terminals.
     
  3. iinself

    Thread Starter Member

    Jan 18, 2013
    69
    0
    Yes, that makes sense, thanks. I was anticipating the meter to read 0 volts with the relay open but the voltage is appearing on the other side of the relay due to high input resistance of the meter.
    Any suggestion on how to check the voltage is not due to relay malfunction ?
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    Add a resistor load across the meter input, say 10k ohm or so.
    But beware of how much voltage you apply across that resistor. 120V will dissipate close to 1.5W..
     
  5. iinself

    Thread Starter Member

    Jan 18, 2013
    69
    0
    Ok, thanks.
     
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