Measuring Filter Capacitors

If you want to be sure the capacitors are ok you will need to isolate them from the rest of the circuitry (likely by physically removing them) otherwise your measurements will be effected by other circuit elements (or new ones you introduce) giving you strange results or masking the fact the capacitors are in fact faulty. From your test results it sounds like you are measuring more than just the capacitor.

Try un-soldering them from the printed circuit board to measure their properties directly.

MEASURING FILTER CAPACITORS I have isolated the two filter capacitors totally out from the circuit during the measurement but the doubtful readings were confusions. I understand that these capacitors were introduced at the input of the shunt current transducer to prevent spurious noises or RF energy getting into it and affect the accuracy of the output readings, 0 – 10 V DC. Actually the unit I am troubleshooting is a shunt current transducer, also known as signal converter. It can be configured to convert voltage to voltage or current to voltage, etc. My engineer has suggested that I use a DC and AC voltage injectors to test the transducer. At the same time, it will also test the function of the capacitors. He suggested that I inject 0 – 150mV DC and 0 – 150mV AC into the input of the transducer and I should get the result of permanent 0V and 0 – 10 V if the transducer operates correctly. Because the transducer is configured to receive the 0 – 150mV AC signal. If the capacitor works, it is blocking the DC signal but allows the AC signal to pass through them. But I am still not getting the simple method of measuring the filter confidently. If the filter capacitor is not functioning properly, I may not even know unless I measure the waveform at both ends, before and after the filter capacitors. I am writing to the manufacturer to check whether they can help. Have a nice day, Guru.

There are several ways to check- the easiest way is to remove part from circuit- to check whether o/c or s/c- put analogue multimeter on suitable range- to check actual cap use cap meter- to check whether can hold charge use 500 v Megger - should charge up & hold charge- so that s/c x term = spark. Other way is using ac thru light bulb- should be bright then dim if ok- stays bright= leakage current. Any doubt- replace device- they have a finite life.

The one thing I note is the term FEED THROUGH capacitor. Without going into details of the inner workings of the cap... Imagine a cylinder of a conductive metal (the case of the capacitor), with a SINGLE wire (the leads of the capacitor) which goes all the way through the cylinder and out both ends. This single wire is one plate of the cap, and the cylinder is the other plate. Thus you should always see zero (or very low) resistance across the leads at either end of the cap. If you put your ohm meter between either end of the cap and the case, you will see 0 ohms to start, but it will climb to infinity ohms. This was much easier to see when we had ohm meters with a printed scale and a needle to measure the resistance. Not these new fangled things with lit numbers on them. Actually, I would apply an ac signal to the wire, and see if I got an ac signal on the case using an oscilloscope.

Bill

I have also measured the capacitor with the analogue multi-meter and the reading indicated zero ohm and it stayed there indefinitely. The same result occured with another exactly the same type of capacitor. The two capacitors were in series for a shunt connected to a winding of a transformer in the transducer module. I have wrote to the manufacturer and I am waitiing for the reply. Meanwhile I am also attempting to buy the two equivalent capacitors because the present two were obsolete. Hear from me when I have the answer.

The manufacturer has reverted to me to buy the capacitors. It is US$65.00 each and there is a minimum order of 10 pieces. The delivery time is 8 months. My boss has sent the defective equipment to a company who specializes in repairing and re-building obsolete equipment. That is the end of the story, no clear answer to the condition of the capacitors. Thank you folks for the feedback. I greatly appreciate your kind gesture and help. Have a nice weekend ahead folks! Bye. Best Regards.