Have you ever taken a voltage reading with a digital meter
and gotten something that really did not quite make sense? With high impedance
digital meters, it is common to get readings where you don’t expect them. This
can be very confusing for students. Volt-meters are basically electrical loads.
Normally, for them to read anything, they have to be in a complete circuit. The
impedance rating of a meter determines the amount of current required to
operate the meter. The higher the impedance, the less current required to
operate the meter and get a reading. High impedance meters are preferred for
electronic work because they don’t add any load to the circuit they are
testing. However, high impedance meters can behave differently from older
analog meters. A phenomenon called capacitive coupling can cause high impedance
meters to read voltages that low impedance meters don’t read. These voltages
are sometimes referred to as ghost voltages. Unenergized wires and devices in
close physical proximity to energized wires and devices are charged by the
energized wires in their proximity. This is a static voltage which cannot move
any appreciable current. Since high impedance digital meters really don’t require
a current flow to operate, they can read the ghost voltage. To differentiate
between a ghost voltage caused by capacitive coupling and a hard voltage that
will move current, place a load across the voltage. When loaded, a ghost
voltage disappears. Some digital meters actually have settings for both high
and low impedance to allow technicians to distinguish between a ghost voltage
and a hard voltage. Fluke makes an adapter, SV225, that can be used with high impedance
meters to effectively turn them into low impedance meters by placing a small
load across the leads. Low impedance analog meters do require a small current
to operate, so they typically will not read the ghost voltage built up by
capacitive coupling.
No comments:
Post a Comment