Measuring capacitors in circuit

The honest answer is No.  You cannot measure the actual value of the used capacitance while it is in a circuit unless it is written on it.  

What you measure with the multimeter probes will be the total capacitance at that point in the circuit (then make a rough guess to nearby standard cap value but this depends on your circuit). e.g. If there some other capacitive devices nearby you will see a summation value or something like that.

Or you can try to understand the circuit and why the cap was used there? Is it a blocking cap or bypass cap near power device then they have pretty standard values.

Cheers

I was hoping you wasn't going to say that, but I had an idea that was going to be the answer.

Hi Andy,

I used to cut traces and wires temporarily in order to isolate the part in question. It is only necessary to break the circuit on one side. After the test one just patches the break and moves on to the next questionable component. Generally, while not impossible to fail, the non electrolytic capacitors rarely fail.

John

Another way would be to remove it physically from the circuit (if possible) and then measure it and solder back a new one 

And also not to forget, if your cap is a very low value e.g. pF range or something you will actually add capacitance to it with the multimeter probes. Hence you may end up measuring the capacitance of your probes or the probe connector but not the cap in the circuit.

So again it is not the best way to measure cap with multimeter

I'll explain my issue. I've been sent a circuit that I have to copy (All will be made apparent once i post my newest blog post) Anyhow, I need to check the resistor and capacitor values on the board and make a clone of this board.

The schematic is below

Yeah, I had already thought of that, but you'll see from my post further down that I have to test a whole PCB... I don't really want to break tracks etc on a whole PCB

Andy,

There are some techniques that can be used to minimize the effects of parallel paths around components that you are trying to measure.  I spent 10 years of my career working on in-circuit or 'bed of nails' testers that  were used to measure each component on a board to validate value, function and/or operation.  We used a method of guarding to break parallel paths, effecting the component that we were attempting to measure.  Depending on the device that you are using to measure the component, you might be able to perform a similar test.  The concept is base on the measurement device having stimulus on one lead (for capacitors this might be either an A/C or D/C signal) and the measurement is performed on the other lead.  Guarding would attempt to inject the same stimulus signal into the parallel path to prevent current flow (same potential on either side of the parallel path meant no current would flow).  Guarding will not always work.  It has difficulty with low impedance paths and also some active paths.

From your schematic, it was not clear which component(s) you were attempting to measure.  Can you provide any additional information?

Gene

Hi Andy,

Why not accept the nominal values of the capacitors as stated on the schematic?

John

Thank you for this goo question I have read great explanation and good answers.

Enrico