EX330 No Contact Voltage Sensor

[geo_leeman]

Hello all.  I picked up an Extech EX330 for a cheap meter in my daily carry bag.  I've a PhD student and work on lots of analog circuitry, but a lot of the lab equipment is operated on the mains.  The no contact voltage sensor is a great way to ensure the equipment is dead before we tear in.  While playing with the meter I noticed an odd thing.  When holding down the button for NCV measurement if you tap the top of the meter the beep occurs and the red lights flash like there is AC voltage....  Any ideas on why this happens?

[thomastheo]

I also have one of those, and just tried it on my unit. It behaves just as you described, so I doubt its just a problem with your meter. Weird...

[geo_leeman]

I opened up the unit last night and can't see anything that should be shock sensitive.  It's just the metal plates, a spring contact, and not much more.  Maybe I'll see if shocks to the unit cause variations in measurements when just measuring DC voltage/current?  I'm glad to hear it's a general problem...

[Robomeds]

Your body is a capacitor.  The movement of your hand probably causes enough of a voltage due to moving the capacitive plate that is your hand to cause the NCV sensor to momentarily detect a voltage.  I would think this is somewhat similar to how waving your hand over the meter without leads can cause non-zero readings. 

[geo_leeman]

I'm fine with the idea of the human capacitor.  I really think this is a physical shock effect though.  I've tapped the top of the meter onto wood, plastic, and other surfaces.  All give me the same effect.  This thing has to detect AC, so that seems to no line up with your idea either.

[ConKbot]

When you shock it mechanically the metal sensing plate inside moves closer to and further way from the plastic (higher dielectric constant than air) as it vibrates, which changes its capacitance, which produces a voltage which it detects

[geo_leeman]

When you shock it mechanically the metal sensing plate inside moves closer to and further way from the plastic (higher dielectric constant than air) as it vibrates, which changes its capacitance, which produces a voltage which it detects

That makes sense.  I guess it's just the marginal construction on these lower end meters.  Thank you for the reply!