Cheap AC Current Measurement


The easy way to measure
high AC currents is to use a clamp meter but these are generally quite
expensive and cost several hundred dollars at a minimum. Add-on
clampmeter adaptors can work well but they only work with digital
multimeters which have millivolt AC resolution. This is because the
output of most clamp adaptors is quite low, 0.1A = 1mV, for example.
This is no good for typical cheap DMMs which
have a lowest AC voltage range of 200V. This circuit can be built into a
low cost clamp meter such as the Digitech QM-1565 from Jaycar
Electronics. When dismantling this clamp adaptor, remove the label which
has the AC range conversion factors and then undo the two screws gain
access to the inside.

Cheap AC Current Measurement Circuit

Cheap AC Current Measurement Circuit Diagram

The two cross-connected transistors act like low voltage drop diodes
to generate a DC voltage which is proportional to the current in the
primary of clamp adaptor (ie, the circuit under test). The recommended
transistors are power germanium types such as ADZ16, AD162, AD149,
ADY16, 2SD471, OC16 and OC28. This approach gives lowest voltage drop
and good linearity, from 10 to 300A. Schottky power diodes can also be
used but the result will not be as linear. To calibrate, wind 10 turns
through the clamp adaptor’s jaws and feed a current of 20A through the
winding. This is equivalent to a single turn carrying 200A. Set the
trimpot to suit your multimeter, normally set to the 2V DC range. Do not
calibrate for a low current otherwise accuracy at high currents will be
poor.

Author: Gerard La Rooy – Copyright: Silicon Chip Electronics


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