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# How to in circuit test capacitance and resistance

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#### Jamesscaggs

##### Newbie
I am looking for recommendations on the best way to do in circuit testing of capacitors and resistors.

I've read about ESR meters but cannot find any quality ones online.

I've also read you can use a scope and a function generator.

I'm really looking for something we can use when diagnosing and refurbishing a lot of the same board. Quick and repeatable is more important than customization in my case.

I've also seen some smart tweezers but it's unclear if they really do in circuit.

Anyone have experience with any of these and can make a recommendation?

#### KlausST

##### Super Moderator
Staff member
Hi,

From my experience in most cases it's not possible to do precise part value measurements in circuit. Mainly because they are connected to other parts that will influence the measurement result.

It 's like you want to measure the weight of a fish in a box of water.
The amount of water will influence the result, and the weigth of the box.
Also the weigth of the water may be much more than the fish. Let's say 100g fish in 10kg water.
Now if your measurement accuracy is +/-5% = +/-50g. So the measured weigth of the fish may be 50g ... 150g...not very accurate.

Klaus

#### FvM

##### Super Moderator
Staff member
In some cases, the component under test can be isolated by an active shielding circuit.

Jamesscaggs

### Jamesscaggs

Points: 2

#### crutschow

Unfortunately, you typically need to isolate the component you want to measure by removing one lead from the circuit.

Jamesscaggs

### Jamesscaggs

Points: 2

#### betwixt

##### Super Moderator
Staff member
...or by active guarding (applying the potential at the measurement point to all surrounding circuit nodes to eliminate their influence). That's how it is done on automated board testers but it requires extra circuits and connections to the board.

Brian.

Jamesscaggs

### Jamesscaggs

Points: 2

#### Taihung

##### Member level 4
As my experiment work with Automatic Test Equipment (ATE), we can test in some case but need special tool.
1 - Special LCR meter with frequency configurable, exciting voltage < 0.1V (this help signal is not leak via body diode of IC pin).
2 - Special Multimeter with Ohm meter function, exciting voltage < 0.1V, measure 02 times 1->2 pin and 2->1 pin.
3 - Indirect Measurement. If know design circuit, like C1-L-C2 circuit, have to measure C1, L, C2 then re-calculate real C,L,C. Use special Impedance meter with 1 excitation signal and multiple sensing/ probe. Normal kind has 01 excitation and 01 sensing probe. Input impedance of meter need very high to ensure accuracy.
4 - Active Mode. Some switching power when operating will make ripple on Capacitor, measure it also estimate quality, ESR, capacity and estimate Inductor. With Linear power can make small exciting signal to see what happen to output.
5 - Reduce/ Stable measurement enviroment temperature. Because lower temperature make semiconductor higher inpedance and low leak current. The measure board need take time to balance temperature after it hold by hand.
6 - Don't touch/ measure with some sensitive point like RF path.

The metered value is never correct as independent component, but it is close expected value help to estimate faile case.
The measure position depence so much on design.

#### D.A.(Tony)Stewart

AS a former Test Engineer and Test Engineering Mgr at Burroughs/Unisys for over 10 yrs, we had a dozen or so old ICT, but my favourite was the CA Marathon Functional Tester which required the Test Engineer to have design expertise to work with designers on new projects to ensure DFT occurred in parallel. Then the test engineers job of FT design was easier using the transfer functions with specs of each section within tolerances. Pinpoint single component issues can be done with a "gold or silver std " board otherwise known as a KGB Known Good Board and define the actual values with tolerances in circuit which may be from Thevenin equivalents in the circuit. Fault detection must exceed your yield requirements like >99%. while Fault isolation must achieve your repair time goals with a lower value depending on complexity. As most failures are solder related or wrong parts, rather than component tolerances, fault detection is more binary or extreme.
The functional test was like a suite of instruments with various signals with constant current to measure a impedance or generate pulses to detect responses or generate fault conditions etc.

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