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Tek A6302 Current Probe: Reason for Being

The question occasionally comes up as to why one would want a Tektronix A6302 Hall effect current probe and AM503 amplifier. The answer is simple: non-contact, essentially non-invasive current monitoring.

I used the venerable dead-battery Astable Multivibrator to check out the rebalanced A6302 probe:

Tek A6302 Current Probe vs Astable Multivibrator

Tek A6302 Current Probe vs Astable Multivibrator

The scope screen in the background shows the two base voltages at the top, plus the overall battery current along the bottom:

Tek A6302 - Astable multivibrator - LED current 1 mA-div

Tek A6302 – Astable multivibrator – LED current 1 mA-div

The current at 1 mA/div shows plenty of noise, but the 200 ms LED pulse is barely 1 mA tall. The two AA alkaline cells have faded to 2.5 V, so the “wearable” white-LED-with-dyed-overcoat runs far under its nominal 3.6-ish V spec.

There’s basically no other way to get that result, because inserting a current-sense resistor into the circuit will alter the results, plus be intractably difficult to measure, particularly if you need the current in a non-ground-referenced branch of the circuit.

The AM503 has terrible thermal drift, by contemporary standards, but after the first half-hour or so it’s manageable for short durations. I’m thinking of epoxying a small knob to the screwdriver-adjustable twiddlepot to simplify the baseline adjustment.

Alas, even non-working probes and amps have become eBay collectables. You could, of course, buy new.

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