checking if capacitor is burned out

[Kardo22]

Hi
Some time ago I saw someone check if a capacitor is OK with an ohmmeter. He measured the resistance of the capacitor (on the board, I think the capacitor was ceramic 10uF with 7V rating).
If the result was a little over 1kohm, he said it has fried. I didn't think much about it then but recently I got interested in it again. Unfortunatelly I can't get in touch with the guy to ask.

How can I check if a capacitor on a board is ok using an ohmmeter?

Thanks

 

[KrisBlueNZ]

If you measure a capacitor with an ohmmeter, you should measure open circuit. With small capacitors (1 µF or less) the meter should indicate open circuit immediately; with larger capacitors (say 100 µF and higher) it may take a few seconds or minutes for the meter to read open circuit, as the capacitor charges up.

If the meter does not show open circuit after the appropriate amount of time has elapsed, the capacitor is either shorted (if the meter reads a few ohms or less), or leaky. Some leakage is normal and acceptable with electrolytics and other high-value capacitors, but not with lower capacitance types, so any capacitor less than 1 µF that doesn't read open circuit is most likely faulty.

These measurements need to be made with the capacitor removed from the board. Other components on the board will normally affect the measurement.

 

[Gryd3]

He may have been pulling your leg, or he had a deeper understanding of the board he was working on.
As Kris had said, making measurements across an item on the board will often affect the reading.
Check to make sure there is no visual indication of a bad capacitor. Deformations, discoloration, or burn marks. Then to make certain, you can disconnect one of the leads to create an open circuit and measure across the capacitor.

Your multimeter will apply voltage to the part that you are measuring resistance on. This will charge a capacitor which means that the resistance shown on your meter will vary from 0 to infinite (overload, or open) as you are making the measurement. Smaller capacitors charge quicker than larger capacitors.
(Remember that a capacitor stores voltage, it would be wise to measure voltage across the capacitor on the board first before attempting work on it. You may need to drain the capacitor first... or let it sit for a while before working on it. This is not always required based on the capacitor you are working on.)


While we are on the topic of measuring items on the board... Resistance and current measurements should not be made with items on the board without first removing the part. (at least partially)
Voltage can be measured across individual items on the board, or from GND to a specific point, but the numbers you will receive may not be as expected. This is usually only useful when you are familiar with the part you are measuring, or are familiar with that particular branch of the circuit. Doing this measurement will slightly alter the voltage in the circuit, this is usually negligible (but not always)