Testing Capacitor That Is Breaking Down When Under Full Operating Voltage.

Testing capacitor breakdown when under full load or voltage only can be done by using special meter. Normal digital capacitance meter and analog meter can’t test this type of failure. Have you encountered power problem where when you switch on the power supply, the fuse blow immediately? You have measured all the components in the power (primary) and secondary area and all the components seem to be working! Where is the culprit?

A fuse blown into dark color were usually caused by lightning strike, a shorted bridge rectifier, defective posistor in computer monitor and tv circuit, a shorted power transistor or Fet field effect transistor, shorted primary winding in switch mode power transformer, shorted secondary output diode and etc.

A Filter Capacitor In Switch Mode Power Supply

I’m frustrated as to where is the cause of the power problem. Every time when I switch on the monitor the fuse will blow immediately (the fuse became dark color and this indicates that there is a major short circuit in the power supply section or surrounding area which I’ve already mentioned above). I have tested all the components in the power supply section and can't find the cause of the problem. Then, I began to substitute all the suspected parts one by one and replaced with a good working component.

I eventually found the caused of the power supply problem. Guess what? It was the main power filter capacitor problems (220 microfarad 400 working voltage). After replacing the filter capacitor the fuse would not blow when switch on and the power supply worked perfectly fine. I begin my detective work to explore why this particular capacitor can caused the fuse to blow even though the filter capacitor already confirmed working with all my faithful meters.

The meters that I used to test the filter capacitor were analog multimeter, digital capacitance meter and the famous esr meter. About how to check a capacitor or testing capacitor and how capacitor work will not be explain in this article. You can read more about testing capacitor in this website. I assumed that many of you who read this article already know how to perform capacitor measurement with all the meters I’ve just explained above.

An Insulation Tester

Using an analog meter to measure this filter capacitor showed capacitor is charging and discharging (meaning the pointer will flick up and then gradually goes down), test with a digital capacitance meter showed around 220 microfarad (still within the capacitor tolerance range) and with the most powerful electrolytic capacitor tester which is the ESR meter, it showed a low esr reading which is also a good reading! Then, how do I know if the filter capacitor is faulty since all the meters tested ok on the filter capacitor?

What I’ve checked on the filter capacitor is just the value, esr and the charging and discharging test but another test you should not miss out which is the capacitor dielectric or leakage test. Use this test only when you want to check capacitors that have working voltage exceeding 250 volt. By using an analog insulation meter you will be able to test the capacitor leakage.

When i connect the filter capacitor to the meter and press the ready button-it showed a very low resistance and this proved that a short circuit occurred between the plates when high voltage applied! A good filter capacitor will just showed a charge and discharge in the insulation tester just like when you are testing a capacitor using analog meter. This proved that the bad capacitor breakdown when under full operating voltage.

You can get an insulation tester from any electronic suppliers. It comes in few ranges of 100v, 250v, 500v, 1000v and even 5000 volt! If you want testing capacitor of 220 microfarad 400 volt then you have to select 250 volt. If you select 500 volt, it will blow your capacitor that is under test because the voltage you have selected is higher than 400 volt! If possible ,you should get exact value and voltage for capacitor replacement.

Capacitor breakdown when under full voltage is quite rare and only happen to those capacitor that have 250 volts and above. If you come across some tough repair and could not locate the fault in the electronic board, I’m sure you will know what meter to use now. Go find the culprit.