Building a Clamp Meter From Scratch

A clamp meter measures AC current without breaking the circuit by using a current transformer — a toroidal ferrite core that the conductor passes through, acting as the single-turn primary of a transformer.

Wound 100 turns of 30AWG wire on a split ferrite toroid. The secondary voltage is proportional to dI/dt, so the output requires integration to recover the original current waveform. For a simple RMS reading, skip the integration and use a true-RMS converter IC (AD737) on the secondary voltage — the RMS of the secondary voltage is proportional to RMS current.

Calibration: pass a known AC current (measured with a traceable reference) through the clamp and adjust the gain. My finished clamp reads ±2% of a reference meter from 1A to 30A at 50Hz. At very low currents (under 0.5A) the core permeability variation causes nonlinearity.