Hi, all
Recently, I read an article How to Test Capacitors by Three Tools (spam link removed by moderator), and I confused about something. If its working the resistance will slowly increase if you connect the probe in the correct polarity and decreases if you connect it in reverse. Is this true?
I tried doing this on a power supply board, almost all of the capacitor behaved as described above, except two capacitor, does this mean these two capacitor are not working anymore?( I did not desolder the capacitors and just test them on the unplugged board)
I also did this test on a good capacitor and a slightly bulging capacitor (both were desoldered) and gave the same reading of slightly increasing resistance, does this mean the slightly bulging capacitor is still working?
Is this a reliable test to figure out which capacitor to replace? (I don’t have an esr meter or capacitance meter.)
Setting a multimeter to measure resistors and using that for a capacitor is not a good way.
The capacitor could be very large and its leakage current could be more than what the multimeter uses. The capacitor could be very small, and then you can not see it. If the capacitor is still in the circuit, then anything is possible.
What you can do is to charge a big capacitor to a certain voltage, then wait for 10 or 30 minutes, and then measure the voltage. If there is still enough voltage, then it has a low internal leakage.
Do you know what the best way is to deal with capacitors ? If your are not sure if they are okay and if they are from a unknown brand, then replace them with good quality capacitors.
There was a time when cheap capacitors failed. See Wikipedia about the Capacitor Plague.
As you can see by this Amazon page, they maintain their value. For good reason. No digital DVM can do the same things as the Simpson. They are simply too slow.
I can test any capacitor with my Simpson. If you watch the needle when you touch the leads to the capacitor, you will see a small dip.
A "slightly bulging" capacitor might still work but it should be replaced. (But if you are troubleshooting a circuit board this might not be the only problem.)
Good! They do have to be out of the circuit to measure correctly.
In fact... Once you've removed a component it's almost always better to replace it with a new one unless you are 100% sure it's good, or unless you've already found the "real problem" is something else. And if your goal is troubleshooting/repair it's almost never worth setting-up a "special experiment" to test a component. (It's OK to do a quick-check you can with a meter or with whatever you have handy.)
There are some multimeters that can measure capacitance.
Right. When voltage is applied (from the Ohmmeter) current flows and it will read some resistance. The current decreases as the capacitor charges-up and when it's fully-charged current stops flowing (except for "leakage") and you should read very-high or infinite resistance.
The charge timing depends on the capacitor value and the meter. If you have a known-good capacitor of the same value you can compare. Otherwise, checking with an Ohmmeter is a very-rough check.
Of course if the capacitor is shorted (nearly zero Ohms) or if it reads "permanently" low resistance, it's bad.
Reverse operation of a polarized capacitor is "undefined".
If you suspect a capacitor on a board, a quick-and-easy check is to stick another capacitor across it in parallel. (Usually you can just hold it place without soldering.) In most applications a higher-value cap is OK so it doesn't have to be identical. If the capacitor is open or has simply "lost it's capacitance" the circuit should start working. Of course if it's shorted, this won't work.
The only type of capacitors that commonly fail are electrolytics and tantalums (polarized capacitors).
Ceramic capacitors, film capacitors, etc., almost never fail in "normal operation". Of course you can kill one with excess voltage but that's usually the result of a bad/failed experiment or a bad design and you virtually never see it on a good-working board. These small-value capacitors usually charge quickly so they normally just read open (infinite resistance). The leakage on these is usually super-low so they usually do measure open.
(But if you are troubleshooting a circuit board this might not be the only problem.)