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When trying to debug a non-working circuit on a PCB, it can be difficult to trace the circuit flow visually, as described in the answer to this excellent and related question:
How can I reverse engineer a simple through-hole board?

Traces on the top layer can be hidden under bulky components like transformers, heat sinks and capacitors, so sometimes you have to take an educated guess and use the continuity setting on a standard multimeter to check where a signal is going.

However, this will also "beep" when the signal goes through an inductor. Missing that could lead to a time-wasting misunderstanding of the circuit.

It might also beep when going through a low-value resistor, but I'm not so worried about those since it's easier to see traces around and under an axial resistor.

Question

Is there a simple way to check for continuity that does not signal positive for inductors of "reasonable" sizes?

Obviously every PCB trace has stray inductance but I don't want to limit any answer by setting an arbitrary cutoff.

Let's assume I have access to a basic multimeter, power supply, oscilloscope, and function generator.

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  • \$\begingroup\$ What kind of (or sizes) inductors are we talking about here? \$\endgroup\$ – Voltage Spike Oct 1 at 18:31
  • \$\begingroup\$ If it is in circuit, how would you know you are measuring a single trace/component and not all other parallel traces/components as well? \$\endgroup\$ – Huisman Oct 1 at 18:34
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You can detect inductors of arbitrary size if that is the only thing on the circuit (or close to the only component). RLC or other un-lit up components (transistors) can produce unpredictable results and all bets are off as to what components are there. So I would say that this could be done on a net to detect inductance, if the assumption was that the inductor is or is not there and there were no other components (or everything on the net was high impedance and no caps).

A waveform generator and oscilloscope is usually what I've used to find values for inductors (using methods similar to the ones described here), and this could be done to find a resistor vs inductor vs capacitor. Phase changes can be detected, and even the value of the inductance obtained.

The problem is you are probably looking for chip inductors (big ones are easy to find), and they have small inductance and resistances, which would make them hard to find.

Another thing that I have done is used smart tweezers that can detect LRC detection. The results are limited in circuit and suffer from the same problems that ohm-ing out resistors do with other portions of the circuit in parallel.

One success that I have had in reverse engineering boards is taking a board in a dark room and shining a laser or bright light through the board to illuminate the PCB under components and see what traces are there, this works for 2-layer boards well. With 4 layer you can only see one side, so it's not as useful.

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  • \$\begingroup\$ Technically, you could probably get decent results on mixed RLC circuits with a good impedance analyzer, but given the price tag of even a mediocre impedance analyzer it's quite unlikely you'll just have one to hand. \$\endgroup\$ – Hearth Oct 1 at 18:56
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    \$\begingroup\$ I agree, but you'd have to know the configuration. And most things on a PCB are connected to a transistor, which is really hard to find the impedance of, especially when the transistor is not powered. A scope and a waveform generator make up for a good RLC meter, the problem is in the parasitics, which add additional unknowns. \$\endgroup\$ – Voltage Spike Oct 1 at 19:10

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