Using capacitor working voltage to find out the line voltages value 


Have you ever come across problems where you do not know the exact voltage at certain part of a circuit?


Assuming you do not have the schematic and similar unit of equipment for comparison, how are you going to know that the voltages that you are measuring is correct or not? Very simple-by looking at the working voltage of the capacitor that is connected to the voltage line that you are measuring. For example, a monitor came in with high voltage shutdown when power on, you want to find out whether is the output voltage from the power supply is the source of the problem or at the flyback transformer side?. You don not have any record about this particular monitor. You also not sure whether

all the output voltages is according to the spec of the monitor or not.


To solve this problem what you need to do is to look at the filter capacitor working voltage for each rail of the output voltages. If you are measuring the line that have a filter capacitor of 25 volt then you can guest that the line voltage is somewhere between 12-18 volts! The line voltage cannot exceed the filter capacitor voltage because if exceeded the capacitor will blow or smoke will come out from its casing. Similarly if you are checking the B+ LINE of a tv or monitor. If you see a 100 microfarad 160v filter capacitor at the B+ line, your common sense will tell you that the line you are measuring have a voltage between 80v to 120v. If the result that you measured show 20 volts only automatically you will know that the line is under voltage!


True case example: Nec monitor came in with high voltage shutdown, upon checking found the b+ line too high. How?-the filter capacitor is a 220 uf 160 volts and the line voltage measurement that i measured also around 160 volt! This can't be and the voltage should be lower than 160v. If i continue to switch on the monitor for few more times i guest the capacitor will blow due to the peak voltage that the capacitor cannot handle. The caused of the increase of the b+ line voltage was a bad electrolytic with high esr located at the feedback circuit. Conclusion-The working voltage of a capacitor is a good indicator to find out the voltages of a particular circuit. Though we can't get the exact voltage at least it save our time in the guessing game as what the output voltages should be.


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Author By : Jestine Yong


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