Testing Capacitor-How To Test A Capacitor When Breakdown Under Load

Have you ever come across power problem where once you switch on the power supply and the fuse blow? You have checked all the components in the power (primary) and secondary section and all the components seems to be ok! Where is the fault? A fuse blown are usually caused by a shorted bridge rectifier, defective posistor, power transistor or FET, shorted primary winding of switch mode power tranformer, shorted secondary diode and etc. But in this article i'm gone to show you another secret of electronic troubleshooting tips.

I got frustrated as to where is the cause of power problem. Everytime when i switch on the power supply the fuse blow immediately (the fuse became dark color and this indicate that there is a major short circuit in the power supply). I have checked all the components in the power supply and can't find the culprit! What i do is i desoldered all the suspected parts one by one and replaced with a known good component. I eventually found the caused of the power supply problem. Guess what? It was the main filter capacitor (220 microfarad 400 volt). After replacing the filter cap the power supply worked perfectly fine. I begin my detective work to find out why this capacitor can caused the fuse to blow eventhough i already confirmed it ok with my meters.

The meters that i used to check the filter cap were analog meter, digital capacitance meter and esr meter. In this article i will not explain about how to check capacitor or testing capacitor and how capacitor work. I believed most of you know how to check capacitors and also generally using this type of meters. Measured with analog it showed capacitor charging and discharge, with digital capacitor tester it showed around 220 microfarad and with esr meter it showed low esr reading!

This proved that the bad capacitor breakdown when under full operating voltage. Then, how do i confirm that this filter capacitor is faulty? By using an analog insulation tester. When i connect the faulty cap to the meter and press the go button-it showed a very low resistance and this is the proved of short circuit between the plate when voltage applied! There is nothing to do with bad electrolyte. A good capacitor will just showed a charge and discharge in the insulation meter just like you are checking a capacitor using analog multimeter. In the market there is quite a number of ranges that you can buy. It has the range of 50v, 100v, 250v, 500v, 1000v and even 5000v! If you want to test a capacitor of 100 microfarad 160v then you have to select 100v. If you select 250v, it will blow your capacitor that is under test.

If you have the SENCORE TEST EQUIPMENT such as the sencore lc meter LC102 OR LC103, these meters have the capabilities of checking any type of capacitors with four tests:
-testing for capacitor values
-checking for leakage
-equivalent series resistance (ESR) and
-Dielectric absorption

It can check aluminum electrolytic capacitor, film capacitor, ceramic, high voltage capacitor and etc.

Conclusion-Different capacitor manufacturer produced different type of quality of a capacitor. Perhaps the bad capacitor that I encountered are from the lowest grade one. A capacitor failure when under load is very rare. Using ESR capacitor meter alone can solve most of the electrolytic capacitor problem.

Jestine Yong is a electronic repairer and a writer. For more electronic repair information please visit his website at <a target="_new" href="http://www.noahtec.com/electronic-repair-articles.htm">http://www.noahtec.com/electronic-repair-articles.htm</a>