Why can't capacitors on PCBs be measured with a multimeter?

Question

While looking at this article, it mentioned that one cannot test the capacitance of a capacitor while it is on a PCB. The explanation given is:

The reason is, that when a capacitor is inside the circuit board, there are a lot of other components in series or parallel with it. So you get the equivalent reading, not the actual one.

Why is this specifically a requirement for capacitors? I don't quite remember this being a requirement for resistors and ICs when determining whether they are busted. Is there anything specific about the capacitance measurement process that makes this a relevant factor?

Please let me know if more information is required.

Answer 1

If you do any measurement on a PCB, you are doing a measurement on the "whole circuit" instead.

If you know the schematics of your circuit, you will find some components where the influence of other components is negligible, for example:

^{simulate this circuit – Schematic created using CircuitLab}

(The capacitor is used to change the DC bias of a symetrical AC signal: you can measure the value of this particular capacitor in circuit without much error.)

But often, particularly for decoupling capacitors, you have many capacitors in parallel, for example:

^{simulate this circuit} (If you try to measure C1 or C2, it is obvious that you also measure the other one in parallel, but less so that you are also measuring C4 and C5.)

So when measuring decoupling capacitors on a PCB, you usually just measure all capacitors on the bus in parallel (+ parasitic capacitance). Due to the high tolerances of most capacitors, it is usually impossible to detect even that there is an error in the total capacitance. If you do detect a short circuit, then you only know that it is somewhere on this voltage bus (and as soon as ICs or transistors are involved, the powered-off state is not really representative of the power on state).

So globally, any measurement in circuit is a long shot if you don't know the circuit well enough to know what you are measuring. You might still try some measurements without that knowledge, but expect a lot of false positives and false negatives.

Answer 2

Why is this specifically a requirement for capacitors?

It's not. It applies to any and every component.

Answer 3

You can't even measure a single resistor if it is in parallel with another resistor. It's just more obvious that it's impossible so usually there is no separate warning about it. Same goes for two capacitors in parallel.

The blog you link to is by about measuring capacitors so that's why they mention only how to measure capacitors, as the article is not about measuring anything else than measuring capacitors.

Nothing can be measured in-circuit unless the circuit is such that e.g. there is a possibility to have an open circuit by disconnecting connectors, removing a fuse, flipping a switch, or relay contacts, to isolate a component from other components for measurement.

Answer 4

It also follows that capacitave parasitics are generally "in range" more often than resistive. If you're measuring a resistor, say as part of an op-amp feedback circuit, the parasitics of the op-amp might be in the 100kOhm or 1MOhm range, which, in parallel, might not introduce enough error to make the measurement unusable.

However, I've found that parasitic capacitances are more often much closer to the values you'd like to measure (nF to µF), thereby polluting the measurements enough to make them worthless.

As others have said, each circuit is different and subject to caveats. But that's perhaps another reason for what you've been told.

Answer 5

It's not specific to capacitors, however electrolytic capacitors are frequently of concern in repairs.

They are promoting ESR meters so they are talking about capacitors. Since the ESR of a good electrolytic power supply capacitor is typically in the low single-digit ohms or less you can practically test it in-circuit, which saves time and prevents damaging the board if the cap is good. Testing the capacitance in-circuit is often not possible.

Answer 6

There are meters for measuring capacitors, but it is not a typical Digital Volt Meter.

A DVM can read voltage, current and infer resistance by passing a small current through a component.

A capacitor’s leads do not connect to eachother by design. They are seperated by an insulator of specific parameters. It will not normally pass DC voltage or current. Its resistance should be infinite. A capacitor only functions at frequency. It impedes lower frequencies and passes high frequencies. A DVM is, well, a zero hertz meter. In some cases when measuring resistance, you may get an initial reading, but it will decay as soon as the capacitor saturates with electrons.

You can create a capacitor meter with a DVM to measure the value of a capacitor, but it will involve passing a small current at a known frequency and measure the readings on another component such as a resistor in series or paralell with the capacitor. A function generator, a low tolerance resistor and a calculator will do. Buying a capacitance meter is probably easier.

Of course measuring ANY single component while installed in a circuit will need to account for the values in the rest of the circuit. Unsolder one lead and go from there.