Measuring a Component Inside a Circuit - Page 1
Forum section: electronics / projects

author: harerod
author_rank: Frequent Contributor
timestamp: March 15, 2022, 05:15:33 pm
content: How would one go about measuring a component (R,C,L) mounted on a PCB?I seem to have some dim memory of flying probe adapters that place some kind of "guarding" around the Component Under Test, which counters the effect of the other components. It would then proceed to measure the COT. Let's assume we know the layout and schematics.Could anybody enlighten me or give me a some search terms, please?

author: TimFox
author_rank: Super Contributor
timestamp: March 15, 2022, 05:19:58 pm
content: The search term is "guard". Essentially, you need a "driven guard", where a suitable buffer amplifier can drive "everything else" in the circuit to eliminate the voltage from the test point to "everything else".Some test equipment can provide such a guard.

author: harerod
author_rank: Frequent Contributor
timestamp: March 18, 2022, 02:34:50 pm
content: Alright, so I am not the only one who has heard of this. Could anybody provide additional pointers?"driven guard" only yielded the concept of driving guards in amplifier applications.Thanks.

author: electr_peter
author_rank: Unknown
timestamp: March 19, 2022, 08:06:34 pm
content: Key term is "6 wiremeasurement". Some notes fromNI on 6W technique

author: RoGeorge
author_rank: Super Contributor
timestamp: March 19, 2022, 09:32:13 pm
content: I don't think it's possible to measure in circuit components, unless the surrounding circuit is very simple and well known. How else to distinguish what current flows through the DUT without cutting the traces to insert a current sensor?Maybe using some very sensitive field sensors on the DUT (like a hall probe or a clamp meter coil, to detect the current going through the DUT terminals only), but that wouldn't be easy to do in practice.The 6 wires and the guarding idea can be useful in some particular cases, but I don't see how to apply that in a generic instrument able to measure in circuit RLC components.

author: harerod
author_rank: Frequent Contributor
timestamp: March 19, 2022, 09:47:57 pm
content: electr_peter, you just made my day. Thank you for jump-starting my memory.I had a vague memory of a discussion with a representative of a company that manufactured flying probe adaptors. The crucial point was an extension of the 6W-technique that you quoted. Their machine had a "large" number of flying probes, which could provide guarding even in complex networks. As I stated before - the layout and schematics are well known.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 19, 2022, 10:01:44 pm
content: Even if the circuit is known, it is not allways possible to reliably measure parts in circuit. It really depedends on the circuit. In some cases it works, in some cases active gurads can make it work to some degree and in some cases even with active gurads no useful accuracy is achieved.I may well need more than 6 wires, especially sometimes separate drive and sense for a single guard.In some cases it may need more indirect tests, and thus not only measuring the part of interrest, but also adjecent parts and than calculate back to get a resonable estimate for the part of interest. So this would be way more than ataching a special RLC meter, but more like sending different test patterns, record data and than fit the measured data to a model of the circuit. So it would need some engeniering and custom solution for the specific circuit.