# capacitor help

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#### Mdkanz

##### New Member
Is there an easy way to test a capacitor?

#### colin55

##### Well-Known Member
Yes. Measure it with a mutimeter set to HIGH ohms and see if it is "shorted."

Put it in a 555 circuit and measure the output frequency via a LED blinking.

#### Mdkanz

##### New Member
Thanks,

I was wondering if a capacitors rated dc voltage would be the same as ac

#### Hero999

##### Banned
DC only capacitors can't be used on AC without a DC bias or they'll explode.

#### Willbe

##### New Member
If it's over 10uF, put a 9v battery and a 1 Megohm resistor across it. Remove the battery and time how long it takes to drop to 37% of initial voltage. Time in seconds = capacity in uF.

#### Hero999

##### Banned
In fact you don't even need the 1M resistor, if you know the impedance of your DVM you can work out how long it will take to get down to 37%.

Another thing to consider is ESR, a 100:mu:F capacitor might read 105:mu:F but if the ESR is 2R then it's useless.

#### chris54

##### New Member
What would be a good place to buy or find details on a meter or device that would read esr in a cap

#### Willbe

##### New Member
"A typical test for ESR involves passing an AC current of around 100 millivolts AC at 100 kilohertz through the capacitor. The test signal can be sine, square, or pulse waves. The low voltage prevents damage or interference with associated circuits for in circuit tests. It also limits the reverse voltage applied to electrolytic caps. At 100 kilohertz, the AC impedance is nearly zero for larger caps. When measuring caps below 10 uF, the impedance due to the capacitance should be accounted for."

#### Hero999

##### Banned
100kHz is the standard test be beware that larger capacitors will become inductive at lower frequencies than that. A typical 10,000:mu:F capacitor will resonate at under 5kHz.

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