Normally Closed Circuit Question

Discussion in 'General Electronics Chat' started by RanHam335, Mar 10, 2011.

  1. RanHam335

    Thread Starter Member

    Dec 4, 2008
    Hi, Im an electrical apprentice and I was troubleshooting a emergency stop control switch circuit that went into and I/O board. It was a normally closed switch on a 24vdc circuit. I ended up finding that the contacts on the switch were intermediately dropping out when the control panel had vibrations going to it and it was tripping the estop bus. I found that out by putting my meter in parallel with the switch, at first I got 0 volts (which i knew was good) but then I would get 24v to show up for a less then a second and go away. I replaced the contacts and everything ran great for the rest of the day.But when I was troubleshooting I notice that when I put my meter to one contact of the switch to gnd I got 12vdc and then when I tested the other contact to gnd I got -12vdc. So obviously they split the power supply up to be on both sides of the switch, has anyone else ever seen this, is this common practice?, and if so is there any reason why they do this?
    Last edited: Mar 10, 2011
  2. Commander#1

    New Member

    Jan 22, 2011
    Hello, RanHam335 -
    No, this does not sound normal to this old geezer. How many wires are attached to the switch? More than two
    would indicate the switch is being used for two separate circuits. With power DISCONNECTED, wring out the
    switch contacts and verify with the appropriate schematic. " . . . replaced the contacts . . ."? That's rather unusual.
    Generally, the whole switch is replaced. Vibrations great enough to cause the points to bounce would be:
    A. Horrendously large - or
    B. The holding spring inside the switch is weak.
    Either way, this problem is going to happen again until the situation is corrected.
    Hope this is of some help.
    :) Phil Potter :)