Capacitors in circut

[Evenflow]

Is there any way to test capacitors, diodes,transistors in circuit??

[mariushm]

Re: Capacitors in circut

You CAN measure in circuit, doesn't mean you'd get ACCURATE results.

No matter how you'd measure, as long as it's in circuit, a capacitor will "interact" with other capacitors placed in parallel or in series with it.

Lots of devices regularly have several capacitors in parallel to increase capacitance, decrease ESR and so on. Also, there are often low value ceramic capacitors in parallel with high value electrolytic capacitors, which helps due to the different properties of ceramic capacitors (but messes up the measurement).

Transistors, mosfets etc can often have resistors between base/gate and other parts of the circuit, which can trick you into thinking the transistor is shorted or faulty in some way.

Really, tools that advertise measurement in circuit really only mean to say they use a very low voltage to measure things, therefore there's very little chance they'd turn on a mosfet or transistor or diode (or whatever) and screw something up in the circuit.
Again, they don't say the measurement will be correct.

[Evenflow]

Re: Capacitors in circut

So unsoldering and testing out is only true way
Thanks

[momaka]

Re: Capacitors in circut

So unsoldering and testing out is only true way

Yes.
Key word here is "true way" because out of circuit there will be no doubt whether another component is affecting the reading.

That said, you can often measure components in circuit AND get conclusive results most of the time (varies by circuit the component is part of), but you'll have to pay attention carefully to what other components may be affecting the reading of the component you are trying to measure. You usually pick up on that as you get experienced and test components in a variety of different circuits.

The key to be able to do in-circuit measurements is to know the behavior of basic components very well and their typical failure mode. For example, resistors often go open-circuit, so if you do a in-circuit reading of a resistor, and it measures higher resistance than what is printed on its colors code (say much higher than 5%, that is... or whatever its tolerance is), then it is most definitely bad. However, if it's measuring lower resistance than what is printed on it, it likely isn't bad but it's not guaranteed (it could be open but maybe another component is pulling the resistance down across it, making it appear as if everything is fine).

Similarly, diodes and MOSFETs often go short-circuit when they fail, and very occasionally leaky. However, if a diode appears shorted in circuit, see what else it is connected to it. If it's across a coil/transformer to clamp down inductive kickback, let's say, then you will be measuring across the coil as well, and that might trick you to think the diode is shorted when it isn't.

BJTs can go short or open circuit (short most of the time, but not uncommon to go open either).

Capacitors should ideally measure open circuit with a DMM out of circuit, but in circuit that's rarely the case. Measuring caps in circuits (except maybe very simple circuits) is usually not very meaningful. Measuring the voltage across them when the power is applied, however, can be.

There's more to it than this. But like I said, some of it you'll pick up as you do more troubleshooting and a big portion is just analyzing the entire circuit and seeing what actually you can measure and conclude that you'll get a reliable reading.