Proper use of a 'continuity tester'

Discussion in 'General Electronics Chat' started by steveray, Sep 20, 2008.

  1. steveray

    Thread Starter New Member

    Jul 14, 2008
    7
    0
    I may not correctly understand how a 'continuity tester' works, but if the tester's alligator clip is attached to one prong of a standard AC plug, and the tester's needle is brought into contact with the other prong, shouldn't the continuity tester bulb light up?

    Thanks.
     
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    I think you're refering to a line-voltage sniffer. A continuity tester merely test whether there is a conductive path between the leads. This path should not have voltage present, or you may damage the meter.

    Steve
     
  3. the_guth

    New Member

    Sep 8, 2008
    8
    0
    "Prongs" to me suggests that you are testing the male end of an appliance plug. As scubasteve_911 suggests continuity meters only indicate whether there is a low resistance electrical circuit that is closed.

    As you suspect your item should be continuous - after all it does use electricity and return it to the supply.

    If the appliance has a switch and this is in the off position then this will open the circuit and will not be continuous. Alternatively, other switching mechanisms (be they physical or electronic) can also open the circuit.

    Another possibility is that your meter does not have the capacity to read through a high resistance circuit or is not on the correct setting.

    Only trained professionals should meter mains supply electricity socket outlets as this can result in damage to property, injury or even death.
     
  4. steveray

    Thread Starter New Member

    Jul 14, 2008
    7
    0
    Thanks for those replies.

    Sorry. I wasn't clear. This is just a "self-powered circuit tester." It has no meter, just an alligator clip on one lead and a needle on the other, both attached to a barrel-like container with a battery and a bulb that lights up when continuity is present.

    I'm trying to determine whether the problem with several torchiere lamps is with the bulb, the lamp's contacts (where the bulb is positioned) or the switch.

    But using the tester I was getting no reading (i.e., the tester's light was not going on) anywhere, even with another such lamp that works. I know that the tester itself works because if I touch its two leads together, the light comes on.

    I expected to get a reading if I touched one of the tester's leads to one of the lamp plug's prongs and the other lead to the other prong (this, btw, is with the lamp unplugged from the AC).

    But the tester's light didn't come on even with the lamp I know is good.

    Am I misunderstanding how to use the tester, and/or how to figure out where the problem with the lamps is?

    Thanks.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Was the good lamp switched on? The path of continuity has to go through the switch contacts and the lamp filament.
     
  6. steveray

    Thread Starter New Member

    Jul 14, 2008
    7
    0
    Maybe not. So, if I understand you, I should make sure the lamp switch is 'on' and then check the plug prongs for continuity -- even though the lamp won't be plugged in to an outlet?
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That is correct. With the switch on, there will be a path from one prong through the switch and the filament back to the other prong (but not the round grounding prong, if present).
     
  8. veritas

    Active Member

    Feb 7, 2008
    167
    0
    If the continuity tester lit up with connecting it to the plug of an appliance, it would indicate that the appliance has a short, which it should not.

    Continuity testers (generally) only give you a "closed" signal if the resitance between the leads is quite small (< 100 ohms for digital ones I've used)
     
  9. veritas

    Active Member

    Feb 7, 2008
    167
    0
    Now if I actually read the rest of the posts: For a lamp, the resistance should probably be low enough, and beenthere's suggestion is sound.
     
Loading...