Capacitor check

There are many tests you can make on capacitors depending on there capacitence and voltage rating.

For instance with a 100uF 450v capacitor you could feed in 1mA from a 500v source via a 500K ohm resistor and note how quickly the votage rises in it and to what value it reaches. (measured with a 10M ohm voltmeter), this would enable you to calculate both the actual capacitence and the leakage.

Capacitors come in all manner of shapes and sizes and there are many other techniques

I would just use an LCR bridge and measure the capacitance and loss factor or dissipation factor, which is a guide as to the quality of the capacitor.

If you need to measure absorption factor etc... then you will need to use other methods.

If you just need to know whether a capacitor is 'bad' then you will first have to know what you call 'good', 'ok' and 'bad' so that you can then decide the best method for testing them...

A bad capacitor in one application maybe fine in another, the dielectric varies so much between capacitors, you will need to know what you are looking for first - So your question should be.

"How do I determine what would be a bad capacitor in this particular application?"

John.

I made the assumption that no sophisticated test gear was avaiable.

I made the assumption that the questioner doesn't know what a good or bad capacitor is...

So that's why I said he first needs to find out what his application requires the capacitor to do...

John

Most troubles with Capacitors -- either open or short.

A ohmmeter (multimeter) is good enough. A shorted C will clearly show very low resistance. A open C will not show any movement on ohmmeter.

A good capacitor will show low resistance initially, and resistance gradually increases. This shows that C is not bad. By shorting the two ends of C (charged by ohmmeter) momentarily can give a weak spark.

To know the value and other parameters, you need better instruments.

Yours was the best and most accurate answer, and was perfect for someone with little knowledge and very little test gear...

Won't work in all cases. The assumtion you made was that the RC circuit of the meter internal impedance and the capacitor is long enough to for the capacitor charging action to be notiable by the individual (an RC of in order of at least couple 100 milliseconds).

If the capacitor's capacitance value is small, the RC time will be too short for a person to notice any difference between an open capacitor and a good one with a VOM or DMM.

Could I add a caveat to my suggestion that you test fairly large capacitrors to a relativly high voltage.

Make sure that they are discharged before you attempt to use them, always discharge thru a robust resistor of 5 ohms or so NEVER with a short this can destroy the capacitor and or printed circuit wiring

NEVER short circuit a charged capacitor, although this will make a satisfying bang you have greatly increased the chances that it will go open circuit in service!

Andy, what makes you think the questioner has little knowledge and very little test gear???

He might not speak English very well... been in a rush or just want a quick answer...

The problem is that we don't know, so the answer to his question has to be as vague as the question, as we don't know what he/she means by good or bad capacitor, do we?

John.

You did not read and understand EXACTLY what I wrote I think, read it again! I does not assume that he has little knowledge or test gear!!! (But I think that was possibly also true!)

Yours was the best and most accurate answer, and was perfect for someone with little knowledge and very little test gear...

Andy, so in other words your answer was of no relevance to the questioner's problem?

As it wasn't an answer at all, not even a suggestion i.e. of no help whatsoever...

What was the purpose of your post then?

John.

John,

did you actually read and FULLY understand what I wrote AND the original query?

I have the impression that you are not concentrating on the words fully and are making up something similar in your mind that has not got a lot to do with the Blog....!! I have been guilty of that myself from time to time, but when I read the text a second time, I usually get it straight....

The original question implies that the person concerned has little knowledge in this area and they needed help from CR4 (if he was a real whiz with electronics, I doubt if he would have posed the question in the first place, do you?).

The answer given was accurate for someone with little knowledge, assuming that they had at least access to a multimeter of some sort......as we also did not get informed just how big the cap in question was.....the multimeter test is valid for a wide range of, but of course not all, caps....but small caps that are not shorted are 99% of the time OK. so a short test with the ohms range would have found out most of those small cap short problems....

I do not understand where you are coming from with aggressive manner, but as an outsider, what do you think of the human race?

I can still remember when I was a school kid our beautiful lady teacher showed us how to check capacitor up. She connected one capacitor, one AA battary, one 2V light bulb and one switch in series, then "1,2,3!", the switch closed, the bulb flashed. Then she put the bulb on the charged capacitor to get the bulb flash another time...actually I did not actually see the bulb flashing. She was so beautiful that I could not move my eyes away from her...

Hi: Don't Laugh, I use a pair of dark safety glasses,pair of safety gloves an a old large screwdriver with plastic handle and some cotton in my ears. The Idea is to charge the cap up then remove the voltage source, then Waite a few seconds count 5, next discharge the cap with the screwdriver . Example a 300 VDC cap @ 250 will make a bang sound as loud as a shot-gun going off, and that will scare the heck out of everyone in the place,also it will vaporise a small part of the screwdriver tip. this is a good quick test for holding power. Next if your still alive, Connect a 1 watt -1 meg resistor in series with a 1 watt- 1 k resistor; then connect a 10 amp amp-meter across the 1 k ohm resistor, Connect the positive lead of amp meter between the 1meg, 1k junction( the place where the two resistors are connected together), connect the negative lead of amp meter to the other side of 1 k ohm resistor (the open end) . Now connect the 1 k ohm resistor open end to the negative side of cap, next connect the 1 meg ohm (open end) to the positive side of cap; Next apply charge voltage for 5 seconds - watch amp meter close. next remove charge voltage and observe the current across the amp-meter take notes of the reading. The current reading will vary due to voltage and mfd rating. However, this information Will give you an idea of RC and current info which you can use to formulate the curve of the cap performance.

Mr X ps be careful

Ah ha cool! I'll be pleased to witness such a test and be more than pleased if the test is to be done by a beautiful lady...still can not forget my school teacher...

The best is to set up a cascade event. gets kind of smelly though. I would like to see Elvira do it .

Mr X

Will this work on super high voltage capacitors?

just learning

Charge it up and touch the terminals

There are multimeter with capacitive test.

Or charge it up, connect to a load and measure current and voltage.

Pineapple

Most of the replies seem to be assuming that the capacitor that needs testing is a large electrolytic...?

What if its less than 0.1 microfarads?

Possibly only 100 pF... you can't test that by watching the charge and discharge times, well, not easily anyway.

John.