Schematic Reference

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.


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.