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Old 27th Mar 2020, 10:06 pm   #1
RobertFM
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Default Measuring capacitance- meter problems

Ok, so on the basis that no question is too stupid, here goes.

Pretty much a newbie to all intents and purposes; I have a practical bent and usually manage to figure stuff out, but I'm stumped. I have been trying to measure capacitance of electrolyic capacitors out of circuit using my admittedly not overly expensive DMM on capacitance test mode . It seems to measure capacitors of 100uf or less fine - the ones I've checked give a reading, and checking the meter against new ones suggest it may be accurate enough. On larger value capacitors however - the ones I was looking at were marked between 220uf and 640uf initially the meter was reading them all about 1/4 of the stated value, new or used . Now it seems to flash up a number, then go to '1'. Have replaced the battery as the battery warning symbol was showing. New battery, no warning symbol, still the same. Am I doing it wrong, or is the meter broken ? It still measures sub 100uf accurately. Am using a Mastech ms8360e - it's at least 6 weeks old.

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Old 27th Mar 2020, 10:27 pm   #2
G8UWM-MildMartin
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Default Re: Measuring capacitance- meter problems

I've just had a look at http://www.mastech-group.com/product...ate=93&PNo=177
and downloaded the manual.
It is only specified as reading up to 200 microfarads, so it seems to be working within specification.
In which case you probably need to resort to connecting your test capacitor in series with a known one and calculating its value from the reading.
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Old 27th Mar 2020, 10:28 pm   #3
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Default Re: Measuring capacitance- meter problems

Max range is 200uF!

When meters display a "1" usually indicates over range, which would be correct for the ones you indicated.
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Old 27th Mar 2020, 10:31 pm   #4
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Default Re: Measuring capacitance- meter problems

You could test it to the limit by connecting a few caps in parallel.
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Old 27th Mar 2020, 10:34 pm   #5
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Default Re: Measuring capacitance- meter problems

A Uni-T UT210E or UT210D has a better capacitance range than the meter you're using, but you might have to wait a couple of weeks for UK delivery at the moment. (I used Amazon)

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Old 27th Mar 2020, 11:00 pm   #6
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Default Re: Measuring capacitance- meter problems

Run the cap under test in series with a resistance or a lamp from an a.c. supply with the meter connected in series on an appropriate current range, then remove the meter, make the circuit and measure the voltage across the capacitor. Then work out the capacitance from V and I: C = I / (2 x pi x freq x V)

Or use seperate meters for V and I measurements.
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Old 28th Mar 2020, 9:42 am   #7
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Default Re: Measuring capacitance- meter problems

Or for large electrolytics connect a 10K resistor in series, put the meter on volts range across the capacitor and connect to 10V. Use a stop watch to time how long it takes to get to 6.3V. Capacitance can be calculated from C=T/R.
For a 1000uF for instance T would be 10 sec.
If you do not have a 10V supply use a 9V battery. Measure the actual voltage and then find the time taken to get to 0.63 X the actual voltage.

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Old 28th Mar 2020, 2:15 pm   #8
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Default Re: Measuring capacitance- meter problems

I doubt there's anything wrong with your meter, and the higher the stated value of a capacitor, the wider tends to be the tolerance. A capacitance meter is much more useul with lower value close tolerance capacitors used in tuned circuits, so to move away a bit from your meter and to more broadly outline a few aspects of capacitors, the following notes might be useful.

Firstly, with electrolytic capacitors, the capacitance value is only one criteria and they will generally have a tolerance of either 10% or 20%. Because on physically small capacitors there isn't much space for text, the tolerance (if stated) is a single upper case letter. If the tolerance is ±20%, letter code is "M". If the tolerance is ±10%, letter code is "K". I've attached a pic of a 330uF electrolytic below which the letter 'M' is shown. Thus, it's tolerance is ±20% so this 330uF capacitor would still be in tolerance from 264uF to 396uF. A ±20% 47µF capacitor could have a measured value of between 37.6µF and 56.4µF.

Of much more importance (to me anyway), is the ESR (Effective Series Resistance), which I'll mention further down the page.

The capacitance value of an electrolytic capacitor also depends on the measuring frequency and temperature. The value at a measuring frequency of 1 kHz is about 10% less than the 100/120 Hz value, that being the frequency at which a reservoir/smoothing capacitor operates in a full-wave rectifier circuit at a mains frequency of 50/60Hz.

Electrolytic capacitors are often used for filtering and bypassing capacitors where they don't need narrow tolerances because they're not used for accurate frequency applications, such as in oscillator circuits. At the other end of the capacitance scale, those used in tuned circuits or timing circuits need to be close tolerance and need to be stable at a range of temperatures. For example, silver mica capacitors often found in the mixer oscillator circuits of radios will typically be ± 1%.

Where electrolytics are used as in reservoir/smoothing circuits there are other important considerations. Notably, the ripple current, which is often not stated, so it's best to buy from reputable companies who provide such data. Also, the operating temperature, which may be 85C or 105C. There's a sketch below which explains ripple current and ripple Voltage. In Vintage radios, reservoir/smoothing that lose their capacitance over time cause hum, at either 100Hz in a full-wave rectifier circuit, or at 50Hz in a half wave circuit

Turning to ESR, a capacitor with a high effective series resistance, (which may be just a few Ohms), will behave as though it has a resistor in series. It's therefor useful to be able to measure the ESR. Because electronics enthusiasts want to measure capacitance, ESR, and other things such as inductance, to test transistors and identify their pinouts, 'Multi-Testers' such as the ubiquitous 'MK328' have become widely adopted. They can be bought for under £10 as a 'bare bones' circuit board and display, or ready boxed up for about £18.00 from China, or presently, for £21.95 post free on Amazon.

https://www.amazon.co.uk/ROKOO-Trans.../dp/B07429JHJ5

To give an example of the display, the 330uF ±20% tolerance capacitor pictured below under test on my cheap multi-tester which I boxed up myself, shows that its capacitance is 287.4uF (- 13%) and hence is within tolerance, with an acceptable ESR of 1.2 Ohms.

Leakage, (in the electrical sense), becomes an important consideration in vintage radios, where the role of the capacitor is to allow signals to pass, but to block DC. If DC is allowed to pass, in some instances it can lead to costly damage. For example, the audio coupling capacitor in the output stage of a valve radio can overdrive the valve and burn out the primary of the output transformer. That's why it's never a wise idea to plug in an old radio to 'see if it works'. There are countless threads on this forum and elsewhere which typical read: 'I bought this Bush DAC90A radio and plugged it in. It worked fine for a while but then went dead'. Likewise 'It went bang' which will probably be the RF bypass across the mains input of the radio.

Leakage in a high voltage capacitor can't be tested at low voltage on a multi-meter - only ate its rated voltage on an insulation tester.

Pic 1: Shows the capacitor is marked 330u-M, which means 330uF ±20%.
Pic 2: Explains ripple voltage, ripple current and how reservoir/smoothing capacitors smooth out the unsmoothed DC from a full-wave rectifier.
Pic 3: Likewise in a half-wave rectifier circuit.
Pic 4: 330uF capacitor on test showing 287.4 uF with an ESR of 1.2 Ohms.
Pic 5: Sketch to explain ESR and leakage.

Hope these rather lengthy notes are of help.
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Old 28th Mar 2020, 2:58 pm   #9
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Default Re: Measuring capacitance- meter problems

The above is very informative David.

I have been looking around for a cheapish ESR meter, the MK-328 looks interesting.

I have had a quick look through the MK-328 manual and don't think it answers my following question. When you say that leakage in a high voltage capacitor cannot be measured on a multi-meter, what do mean by high voltage, i.e. what sort of maximum voltage electrolytic's could one expect to get accurate/reliable ESR measurement on the MK-328 ?
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Old 28th Mar 2020, 3:18 pm   #10
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Default Re: Measuring capacitance- meter problems

Capacitance, ESR and electrical leakage are different things. An ESR Meter does not measure leakage, at least not directly. Ideally you should test for leakage at the capacitor's full working voltage.

A couple of analogies I use are these. If a steam boiler was designed to work at 100 PSI there'd be no point in testing it at 10 PSI, it might show no leakage, but burst at full pressure. If a door was unlocked, but jammed you might not be able to move it with your finger, but a kick with your boot might do the trick.

Leakage changes the characteristics of the dielectric, so sometimes a capacitance test will show leaky capacitors as having a greater capacitance than expected.

Where vintage valves sets are concerned I never bother measuring ESR. Leakage is the problem.
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Old 28th Mar 2020, 10:22 pm   #11
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Default Re: Measuring capacitance- meter problems

Quote:
Originally Posted by DMcMahon View Post
The above is very informative David.

When you say that leakage in a high voltage capacitor cannot be measured on a multi-meter, what do mean by high voltage, i.e. what sort of maximum voltage electrolytic's could one expect to get accurate/reliable ESR measurement on the MK-328 ?
As I said in my earlier post, (reaffirmed by Graham) you can only test a capacitor for leakage at its rated voltage, for which you need an insulation tester, often referred to as a 'Megger' in the same way that vacuum cleaners are often called 'Hoovers' and vacuum flasks are called 'Thermos'. You can't test for leakage on a multi-meter as it only applies a small voltage across the capacitor.

Leakage tests are most relevant with high voltage (250V plus) capacitors seen in old radios. Typically waxed paper capacitors, Hunt's Mouldseal capacitors and pitch encapsulated capacitor. To give an example, the first pic below shows a Hunts 'Mouldseal' capacitor marked as .05uF, but when measured the capacitance was 0.1 uF.

Second pic is the cap connected to a multi-meter on the 20 Meg Ohm range. The meter shows '1' meaning no leakage.

Third pic shows the same capacitor subjected to its stated working Voltage of 250 Volts on a Robin insulation tester. You will see on the top red scale it shows 250K Ohms.

The fourth pic shows how the capacitor is behaving as having a 250K Ohm resistor in parallel.

The last pic shows a new polypropylene capacitor tested at 1,000 Volts showing no leakage.

Most restorers wouldn't bother with these tests on old capacitors - I only carried out the test to show that if anyone doesn't have an insulation tester, don't assume that if the capacitance on a meter looks OK, the capacitor is healthy. Where these types of capacitors (waxy paper, Mouldseal or pitch) are encountered, it's best to replace them on sight with for example, polypropylene ones.

I wrote the notes to try to explain that there's rather more to capacitors than simply measuring the capacitance on a meter. If you wanted to test a capacitor which is beyond the range of your meter - say 220uF, when you put capacitors in series, they behave in the opposite way to resistors, so whereas two 220 Ohm resistors in series would equal 440 Ohms, two 220 uF capacitors in series would be 110 uF, so would be in the range of your meter, notwithstanding the tolerance of the caps. (Two in parallel would be 440 uF).

Hope that helps.
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Old 28th Mar 2020, 11:04 pm   #12
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Default Re: Measuring capacitance- meter problems

Thank you David & Graham.

I do not have a Megger but for my recent vintage R2R paper capacitor (Wima, Frako, Hunts etc) replacements, I just did a DC voltage pass through test (my terminology) by connecting one end of the capacitor (after capacitor removal) to the most suitable voltage on the R2R I could easily access and measured how much voltage got through the capacitor.

I only did this as an interesting exercise, because virtually all the paper capacitors on my last 2 old Grundig's were in a very poor visual condition, I decided to replace all of them with Polypropylene.

I found there was a strong correlation with increased capacitance and leakage.
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Old 28th Mar 2020, 11:29 pm   #13
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Default Re: Measuring capacitance- meter problems

Isn't the Robin showing 125k? 250V range is reading x 0.5.

That cap's twice as bad as you think!
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Old 29th Mar 2020, 12:13 am   #14
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Default Re: Measuring capacitance- meter problems

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Isn't the Robin showing 125k? 250V range is reading x 0.5.

That cap's twice as bad as you think!
So it is - thanks for spotting that!

I’ll get my coat.
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Old 9th Apr 2020, 3:04 pm   #15
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Default Re: Measuring capacitance- meter problems

Thank you for all the help - I've been off line for a while, but now firmly back in the saddle. David, your notes are v useful, and I will have a look around at alternatives. I should of course have realised my meter didn't measure above 200uf. One of the few bits of advice I recall from school was from my physics teacher who used to say to me "if all else fails laddie, read the instructions !!" Many thanks. More stupid questions will no doubt follow in due course.

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Old 9th Apr 2020, 9:00 pm   #16
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Default Re: Measuring capacitance- meter problems

I use an old Hunts resistance and capacitance bridge for cap checking. It's an amazingly useful bit of kit. It's accurate to about +/- 2 per cent from 10pF to at least 100uF, and tests for leakage at up to 400V. It also has three resistance ranges, but I use an ordinary DMM for that.
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Old 10th Apr 2020, 10:15 am   #17
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Default Re: Measuring capacitance- meter problems

I have at least three bridges. A GR1615A that goes from 1aF to 1.1uF at audio frequency, 0.01%. A Marconi TF1313A, again audio frequency, to 100uF 0.1% (also does L and R too) and an HP250 RX meter which is low C (~100pF) and L, but goes up to 250MHz. Useful in particular for working out whether resistors are going inductive at high frequency.

Most recently the Marconi has worked very nicely to measure interwinding capacitance and leakage inducting in mains transformers (to confirm snubbing values).

I also have an ESR meter to check the condition of electrolytics, and a Lafayette TE46 CR analyzer, which I actually use for reforming caps and measuring leakage current at the operating voltage.

I tend not to use a DVM to measure capacitance. If I wanted half way decent meter to measure C and ESR of electrolytics I'd go for the Peak ones.

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Old 10th Apr 2020, 10:12 pm   #18
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Default Re: Measuring capacitance- meter problems

Quote:
Originally Posted by RobertFM View Post

One of the few bits of advice I recall from school was from my physics teacher who used to say to me "if all else fails laddie, read the instructions !!" Many thanks. More stupid questions will no doubt follow in due course.

Robert
I guess as a teacher he had to be restrained in his language .....

There's a well known four letter acronym "RTFM!" I'm sure you'll figure it out.

Remember a stupid question is better (not to mention safer) than no question at all!
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Old 13th Apr 2020, 2:33 am   #19
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Default Re: Measuring capacitance- meter problems

Anyone built a "Mr Carlsons Lab" low voltage capacitor leakage tester?
Looks interesting.
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Old 26th May 2020, 9:23 pm   #20
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Default Re: Measuring capacitance- meter problems

Quote:
Originally Posted by DMcMahon View Post
The above is very informative David.

I have been looking around for a cheapish ESR meter, the MK-328 looks interesting.

I have had a quick look through the MK-328 manual and don't think it answers my following question. When you say that leakage in a high voltage capacitor cannot be measured on a multi-meter, what do mean by high voltage, i.e. what sort of maximum voltage electrolytic's could one expect to get accurate/reliable ESR measurement on the MK-328 ?
I now have the MK-328, I am impressed !
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