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Old 3rd Dec 2017, 2:57 pm   #1
Oldelectronics
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Default Should an electrolytic capacitor have measurable resistance?

Hi Folks this may be a silly question but should an electrolytic give a resistance reading when measured on an AVO I dont have a capacitance tester and I measured a physically leaky 40uf 16v from a Hacker Democrat out of curiosity and got a reading on the x100 scale measuring a 'new' one I also got a reading that was not unlike it see photo. I dont get a reading on ordinary polypropylene capacitors? I must buy a capacitance checker next stop E Bay!

regards

Kev
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Old 3rd Dec 2017, 3:01 pm   #2
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Default Re: Should an electrolytic capacitor have measurable resistance?

Connect meter +ve to capacitor -ve and meter -ve to capacitor +ve?

Lawrence.
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Old 3rd Dec 2017, 3:12 pm   #3
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Default Re: Should an electrolytic capacitor have measurable resistance?

Lawrence is pointing out that the actual voltage on the prods of a meter from the internal battery are the opposite to the what the colour of the prods suggests, electrolytic caps show a larger leakage when the voltage is reversed.
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Old 3rd Dec 2017, 3:16 pm   #4
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Default Re: Should an electrolytic capacitor have measurable resistance?

A good electrolytic will have a small leakage current. The AVO 8 uses a 15 volt battery on the ohms * 100 range but i don't know what the terminal PD is on that range. However if we assume a terminal PD of 10 volts and your meter reading of 40K ohms (a bit over) this equates to a leakage current of 0.25mA, that is not a lot. As has been pointed out above the leakage will be greater if the capacitor is connected reverse polarity.

That method of testing an electrolytic capacitor doesn't tell you anything useful I'm afraid.
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Old 3rd Dec 2017, 3:22 pm   #5
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Default Re: Should an electrolytic capacitor have measurable resistance?

Thanks for that guys I didn't realise that about the AVO prods

Kev
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Old 3rd Dec 2017, 3:28 pm   #6
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Default Re: Should an electrolytic capacitor have measurable resistance?

It was just idle curiosity Colin and thank you for the explanation.

Kev
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Old 3rd Dec 2017, 8:15 pm   #7
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Default Re: Should an electrolytic capacitor have measurable resistance?

And just to add that the reversed polarity seen on the ohm ranges of the AVO is typical of many 'traditional' analogue meters, whereas digital meters have the test voltage assigned to the correct terminals.

It's something to bear in mind, but once you have used both types you make the switch mentally without even thinking about it.
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Old 3rd Dec 2017, 9:07 pm   #8
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Default Re: Should an electrolytic capacitor have measurable resistance?

It was logical to me when testing transistors on the Avo. Black prod on the base and red prod on Collector & Emitter ... NPN. Black being ‘negative‘.

Well it was logical to me as a boy......��
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Old 3rd Dec 2017, 11:47 pm   #9
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Default Re: Should an electrolytic capacitor have measurable resistance?

AFAIK all analogue meters have reversed polarity when set to a resistance range (DMMs do not) Whilst, as ukcol says, testing a capacitor on the resistance range of a multimeter doesn't provide any useful information unless the capacitor being tested exhibits a fixed resistance withe the prods applied either way round, a capacitor being tested in this way should show a very low resistance at the start, but this should gradually rise to (near)infinity as the cap. charges.
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Old 4th Dec 2017, 12:33 am   #10
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Arrow Re: Should an electrolytic capacitor have measurable resistance?

With your AVO, what you are effectively doing is measuring the leakage current at the voltage that the AVO produces across its test prods. (When I say '"leakage current", that is the current flowing when that current has finally stopped rising).

Quote:
Originally Posted by Oldelectronics View Post
I must buy a capacitance checker; next stop E Bay!
The measurement of leakage current is one useful assessment of the serviceability of any capacitor - and is easy to do. All you need is a source of adjustable d.c. voltage and a current indicator (such as a milli-ammeter) with a switch across it to short-circuit that meter. You need to know the working voltage of that capacitor and adjust the output voltage from your d.c. voltage source so that it is a bit less than that figure - say 15v. for a 16v. working capacitor. (That leaves a bit of margin for instrument tolerances and reading errors).

Procedure:
1. Set the test voltage accordingly and operate the switch across the meter to that the meter is short-circuited.
2. Connect the capacitor (make sure the polarity connections are correct) to the d.c. supply and switch it on.
3. Wait a few seconds and then operate the switch across the milli-ammeter. When the current has stopped decreasing, that will indicate the leakage current at the test voltage.
4. Disconnect the capacitor and discharge it. (Warning: if it is a large capacity type, use a low-value resistor to do that, not a screw driver!).

Notes:
1. The purpose of the 'switch across the meter' is to protect it from the large surge current that will initially flow into the capacitor as it charges.
2. You can use your AVO as the current-measuring device. I use an AVO 8 on its 10-amp. range. Selecting that range removes the need for the switch across the meter. As the current decreases, I change the current range of the AVO accordingly.
3. There are other tests that can - and under certain usage conditions of a given capacitor should also be carried out - such as the determination of its ESR and the measurement of its true capacitance. There are also other tests that may need to be considered.

Al.
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Old 4th Dec 2017, 1:25 am   #11
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Default Re: Should an electrolytic capacitor have measurable resistance?

For less than a tenner, buy a Chinese Magic Box. It will tell you more about a capacitor than equipment costing 10 times as much.
And it will tell you what sort of component you have connected from a resistor through capacitors, inductors, diodes, transistors to Fets and whether it is any good.
Will not test caps to a high voltage but its invaluable for a quick ESR and leakage check.
Strangely I have noticed that when testing some electrolytic caps, it sometimes makes a small difference which way round it is connected. The 9v battery is safe on practically any component, mine is on the original PP3 after four years use.
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Old 4th Dec 2017, 9:46 am   #12
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Default Re: Should an electrolytic capacitor have measurable resistance?

I second your comments about 'Chinese magic Boxes', Sam. I have two, one of which has yet to be built into a box! They are extremely useful, bearing in mind thelimitations you point out. Thise ZIF sockets they come fitted with are by awful after a bit of use, though. I must fit alternatives, possibly 4mm sockets to take standard test leads, to mine (Wnen I get 'a roundtuit!!)
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Old 4th Dec 2017, 10:15 am   #13
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Default Re: Should an electrolytic capacitor have measurable resistance?

I find the ZIF handy for transistors but for in set use where I have only disconnected one end of a component I use 16" croc clip leads from the solder pads on the right of the ZIF onto which I soldered thick wire loops for the croc clips to bite on. These pads go to the first 3 ZIF locations.
Mine's not boxed either! Its been rattling around for 4 years like this and it is fine.
An astounding piece of kit for the money.
What should we be calling them? I coined the "Chinese Magic Box" and folk seem to know what I mean. CMB for the initiated ( and my wife's initials!)
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Old 4th Dec 2017, 10:26 am   #14
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Default Re: Should an electrolytic capacitor have measurable resistance?

Quote:
AFAIK all analogue meters have reversed polarity when set to a resistance range
They do not. Well, all the ones designed by me do not for sure - a feature.

The only way to be sure about capacitor leakage is to test at the rated voltage. Leakage is not usually ohmic, i.e. it does not usually obey Ohms law so expressing it as a resistance is misleading. Double the voltage and the current is likely to increase more than the double you would expect for a resistor. It is also very temperature dependent.
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Old 4th Dec 2017, 10:31 am   #15
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Default Re: Should an electrolytic capacitor have measurable resistance?

True, and I did point that out in post#11. There is no real substitute for a full rated voltage test, I find a megger handy for a fast go-nogo test @500v on suitable caps.
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Old 4th Dec 2017, 11:39 am   #16
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Default Re: Should an electrolytic capacitor have measurable resistance?

Getting back to your original question, is it possible that the original cap simply had its shrink-wrapped cover put on 180 degrees out, so the capacitor was actually OK but the polarity was wrongly indicated? (With predictably catastrophic results, since you would naturally assume the polarity was marked correctly and insert the part accordingly).
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Old 4th Dec 2017, 12:43 pm   #17
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Default Re: Should an electrolytic capacitor have measurable resistance?

Thanks for the input folks I found it interesting and enlightening. Hi SirusHardware the original cap was changed as it was leaking (or had leaked) A crumbly yellow material almost like it was sitting on a felt washer (it was fitted positive down both the sleeve, groove and board marking tallied with being positive I was surprised they didn't use a radial but they haven't been used at all I will change C2 the cap nearest the bottom of the picture as it looks as though something has leaked and may as well change the pair of 400uf 10v ones as well as I cant see the ends without taking them out.

Regards

Kev
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Old 4th Dec 2017, 3:03 pm   #18
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Default Re: Should an electrolytic capacitor have measurable resistance?

PS the crumbly yellow thing is whats left of the seal!!

Kev
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Old 4th Dec 2017, 3:39 pm   #19
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Default Re: Should an electrolytic capacitor have measurable resistance?

What you have to remember about multimeters is, an analogue one measures resistance by applying a constant voltage and measuring the current through the unknown resistance (which is why they have a reciprocal-law scale, since R = E / I); whereas a digital one applies a constant current through the unknown resistance and measures the voltage across it.

With an analogue meter, the current flows into the red lead and out of the black; but it means the black lead is at a positive potential WRT the red one.
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Old 4th Dec 2017, 4:26 pm   #20
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Default Re: Should an electrolytic capacitor have measurable resistance?

Thanks for that Julie.

regards

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