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How do you test diodes, resistors, and capacitors?


Well-known member
With capacitors, it's true you can't measure capacitance in circuit. You can, however, find clues as to whether or not it's good with resistance and voltage measurements. You should see resistance across the leads if it's good, 0 resistance if it's a short. As far as voltage, you should see voltage rise out the cap as you apply power to the circuit. This might not necessarily tell you if a cap is bad or not, but if you replaced one, then you'll know if it's making good contact.

Just remember, you can take voltage and resistance measurements in circuit, in parallel, with a component, or in relation to ground. But not current. Current has to be measured in series. If you try to take a current measurement across a component in parallel, you can blow your meter.

Another thing to remember is that meters show a "difference in potential", not the voltage. If you have 100V leading into a resistor, and 95V coming out, placing your leads across the resistor will show you 5V. That is the difference in potential from voltage in to voltage out. If you touch it, you about to get zapped with 100V not 5V. That's why it's good practice to always take measurements in relation to ground. (black lead on ground, red lead where you're measuring.)

Safety first!



Well-known member
like everyone has mentioned, an ESR meter for caps, make sure you do this out of circuit for reliable results, compare the measured ESR to what that cap size/value/type is meant to be, as for diodes they generally have a forward voltage drop around .636v for silicon diode, though this can vary slightly depending on type/specs off the particular diode. do note that they shouldn't conduct voltage it tested with a multimeter in the reverse order. these can be checked in circuit assuming that they don't have any other components in parallel