EMI Sniffer/Checker

Many designs have been published in the past for various EMI probes. All have there merrits for particular applications. Here, a very handy small probe, especially suited to check enclosures of switch mode power supplies and similar circuits for EMI leaks. The probe has particular advantages for localizing the actual leakage spots, because it only captures fields in its close proximity, unlike many other EMI test probes.

The setup is very low-cost, using the venerable LM386 amplifier to directly drive a small speaker. Gain is set to 200, 46 dB. Sure, you can also hook this up to an oscilloscope, or to the line-in of a soundcard for waveform/spectrum analysis.

emi leak detector schematic

A 9 V battery is used to power the thing.

emi detector

The sense coil – a small ferrite rod, with a coil of about 150 turns, small-diameter copper wire.
emi sense coil2

To protect the coil, seal it with some heat shrink tubing.

emi sense coil1

Happy EMI sensing!

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