Inaccurate reading with th2822c?!

[Aido4ever]

Hi everyone...

I just bought the Thongui TH2822C (100khz) LCR Meter.

This is my FIRST LCR meter, never had one before and i'm kind of new to it.

I've tested bunch of capacitors with it, from few pF to 1.5mf.....

I've found that it has inaccurecy in the uF range.... especially in that range!!!

for example:

a 330uF capacitor shows as 305uF on the LCR (pal/100hz) while on the UT71E it's 328uF and on the fluke 289 it's: 326uF.
I found out i have it manly on the uF range....

Did i just waisted 420$?

[amspire]

The Thongui TH2822C is more likely to be correct - the multimeters will be in error.

The multimeter-type capacitance measurement is usually at an unknown frequency, and you only get an accurate reading when parts behave as near ideal capacitors. It could be that if you look at the frequency across the capacitor for the multimeter measurement, it is high enough for the ESR of the capacitor to increase the reading.

The Thongui is measuring properly at a known frequency and it is separately measuring the capacitive and resistive components of the impedance.

Richard.

[Aido4ever]

Thanks richard.

But the multimeters reading are much closer to the actual value of the capacitor then the TH2822C so logically they are more accurate.

I've tested also with a cheap capacitor tester and it gave a reading closer to the multimeters as well.

[Harvs]

Do you have a basic sine wave generator and scope?  If so it's easy to make a low pass RC filter with a resistor, find the -3db point and calculate the capacitance based on the frequency and resistance.

Keep in mind most electrolytic caps have very wide tolerances. So all of the values you've measured are well within what I would expect.

But it is strange that two meters are quite close and one markedly different.  I'd suggest it probably has a lot to do with the way they're measuring as amspire was saying.

It would be interesting to hook a scope up to the cap and look at the test waveform from each meter.

[robrenz]

I think most multimeters are measuring the capacitance with strictly DC and just the RC timing so they would give a less meaningfull measurement.

[amspire]

305uF is well within the tolerance. Harvs is right - you can easily test using simple test rig.

I will just suggest a different method in case you do not have signal generator. Put a 1K resistor in series with the electrolytic and then attach it to a transformer secondary - something like 10V - 20VAC. Measure the mains frequency, measure the voltage across the resistor and the voltage across the capacitor.

The capacitor value is

C = V_{1K_resistor}/(V_capacitor x R_{1K_resistor} x 2 x 3.14159265 x f_mains)

Anyone checking this formula will notice it is not exact, but the errors are better then 1 part per million. No need to make it more complex.

As long as the meter is accurate to better then 1%, you should be able to measure the capacitor to at least 2% this way.

The method is limited by the distortion of the mains, but the mains waveform is hopefully good enough. If the capacitor value is 305uF, then with 10V is applied, there will be 0.87 volts across the capacitor for a 60Hz mains. 326 uF will give 0.815 volts.

Richard.

[Aido4ever]

I have RIGON singnal generator and Hantek DSO1200 scope...

what exactly you want me to do?

I've tasted with the scope and with the ut-71e and the TH2822C give accurate frequencies. I've tested the Fluke 286 while the 27nF capacitor connected to it and it was hard testing with the scope and the freq is about 5.8Hz (the scope shows 6Hz which sometimes jump to 4Hz).

When trying to test the ut71e freq i get Noise with unstable frequency... maby it tries to lock on a frequency while autoranging? i'll test more and post.

I didn't use just those two multimetter for my test. i did not think it was importent to write them all but those are the list of multimeters i've used:

fluke 289
flue 11
ut61e
ut71e
kt-7115 (cheap capacitor meter)

it seems that for some capacitors the TH2822C is less accurate. and if it's true that the way is testing is more accurate it's not what it seems, since the others seems to be more accurate in the nF range while the TH2822C is more accurate in the pF range.

if you want i can write a list of capacitor testing with the results for comparing..

I know the testing are withing the tolerance, but when someone pays 420$ for a meter he expect it to be a reliable meter which he can relay on it and not having to test everything with more then one meter just to be sure....

can someone test with branded LCR metter and see if he gets better results then me? let's say testing a 330uF capacitor with agilent for comperation?

[amspire]

Do the test I suggested but with the signal generator running at 100Hz. The generator may not go as high, so use a series resistor of 220 ohms, and my formula will still work. You should get an accurate measurement of the 330uF capacitor.

So all you need is a 220 ohm resistor, the signal generator and a good DMM to measure the 220ohm resistor, the voltage across the resistor and the voltage across the capacitor. WIth the signal generator, the result should be within about 1% with a good DMM.


Richard.

[Aido4ever]

Hi amspire, i did exactly what you asked me to do and the result is 301uF. So by this test you wanted to show me that my TH2822C actually was the one with the correct readings of the capacitors?

[amspire]

Hi amspire, i did exactly what you asked me to do and the result is 301uF. So by this test you wanted to show me that my TH2822C actually was the one with the correct readings of the capacitors?

Absolutely. You have just discovered the whole reason for owning a proper LCR meter.

One more test. Connect the cap back to a multimeter and check the test frequency with you scope. It would be interesting to see the result. It could be a very low frequency like at few Hz or it could be 1 kHz or more that puts you down into the lead resistance and ESR error zone.

The more you use your LCR meter, the more you will find that many capacitors and inductors just don't behave anywhere as nicely as resistors. They are lossy, they change with frequency. The multimeter type capacitor meters only give good results with very well behaved capacitors.

Richard.

[saturation]

Or if you don't measure LC that frequently or can spend more time doing the measurement, such as if you are a hobbyist, you can save $400 as the simple methods Harv and ampsire work without buying more gear.

Hi amspire, i did exactly what you asked me to do and the result is 301uF. So by this test you wanted to show me that my TH2822C actually was the one with the correct readings of the capacitors?

Absolutely. You have just discovered the whole reason for owning a proper LCR meter.

One more test. Connect the cap back to a multimeter and check the test frequency with you scope. It would be interesting to see the result. It could be a very low frequency like at few Hz or it could be 1 kHz or more that puts you down into the lead resistance and ESR error zone.

The more you use your LCR meter, the more you will find that many capacitors and inductors just don't behave anywhere as nicely as resistors. They are lossy, they change with frequency. The multimeter type capacitor meters only give good results with very well behaved capacitors.

Richard.

[Harvs]

If capacitors were ideal elements, all of the meters would agree.

However, they are far from it.  Electrolytic's are the worst of the lot.

If you have a look at a capacitor datasheet, you'll find a graph of impedance vs frequency (well you will for ceramics anyway, they probably don't even bother for electros.)  This relates the the ESR, ESL and capacitance.

Then there's the high temperature dependance electros have (i.e. it'll change by a percent or so just from the heat of holding it tightly.)

Just to throw yet another cat in the bag, electros are quite non-linear when you impress a reasonable voltage across them.

I would suggest that none of the meters are "wrong", as in being broken, it is entire due to the measurement methodology and which of the capacitors problems show up.

So in summary, I don't believe what you have is a broken meter or a waste of $400.  But, if I can be so bold as to suggest, if you want to get the most out of your $400 investment you'll need to do quite a bit of reading into the non-ideal nature of the different capacitor/inductor types and how that relates to the circuits you're going to using them in, (e.g. a SMPS designer looks at caps in a completely different perspective to how a HIFI amp designer does.)

[amspire]

The LCR meter does at least measure the series capacitance and resistance at a known frequency using a very low distortion sinewave test signal. That is an accurate result and it does give you something firm to work with.

The capacitance range on a multimeter is saying that when the capacitor is connected to the multimeter's sawtooth wave oscillator, the frequency of the oscillations is that same as you would expect from an ideal 328uF cap. That is not telling you much at all, unless you know that you have a low loss type of capacitor. The test waveform is not a sinewave. The multimeter doesn't say what frequency it is testing at. It doesn't indicate how much error is the result of the capacitors internal resistance. If you know you have a lossy capacitor, then the multimeter does not really give you any firm results at all.

The reason all the multimeters were out in the same way is probably they all use the same kind of oscillator circuit, so they get the same kind of errors with a lossy capacitor. It is an indirect test of capacitance.

This is not a problem in most cases as you hardly ever need to know the value of an electrolytic cap to better then 20% and at least the multimeters appear to do better then that.

[T4P]

That's the only reason they have the capacitance , it's just basic capacitance measuring .
They caps are measured on a normal DMM by measuring the time delay of the cap .

[Aido4ever]

Thank you guys for your help, I've learned allot from you..

Since i got this LCR meter i went deeply to know more about capacitors... I've learned about quality factors, dissipation factors and such.. and it change my all understanding of a capacitors...

I know that capacitors change there capacitance according to the frequency in the cirquit.

But what is the factory-capacitance-marking and how come the less-accurate-meters are more accurate if comparing to this marking?

From what i know the factory calculate the capacitance according to a formula which take into account the area of the plates and the dielectric constant am i right?

[Mechatrommer]

I know that capacitors change there capacitance according to the frequency in the cirquit.

capacitance should change little based on frequency. its the function of temperature and other nature, but not frequency. whats changed based on frequency is impedance and phase angle. pls correct my knowledge.

[rf-loop]

I know that capacitors change there capacitance according to the frequency in the cirquit.

capacitance should change little based on frequency. its the function of temperature and other nature, but not frequency. whats changed based on frequency is impedance and phase angle. pls correct my knowledge.

Capacitance is not pure ideal capacitance! Measuring method, including frequency may affect result, even highly.

Quoted from Nichicon
"The capacitance of aluminum electrolytic capacitors
changes with temperature and frequency of measurement,
so the standard has been set to a frequency of 120Hz and.. "

Normal practice for big AlElco is 120Hz frequency for measuring "capacitance" but also this is compromize.
Different capacitor and different inductor need different measuring methid including also understanding what is frequancy meaning. (just becouse there are not ideal components in real world.)

Also it need select if use "parallel" or "series" method.

Real LCR parameter measuring equipments have many times big manuals and applicatuon notes how to do measurements. Also it is good to know how manufacturers specify component values.

Typically higher freq give too low cap values. (but there is also exptions and possible sometimes measurement may give very confusing results if frequency is near resonant freq.)
Some good LCR meter (example many Tonghui professional models)  can of course change test frequency but user is responsible for make selections!

It is good to know fundamentals and principles and practices how to measure. The manufacturer may also be assumed that the measuring device used by a trained person who has sufficient knowledge and skills of the measurement technique. In this case, the manufacturer does not need to include in the basic textbook. But today it is easy, nearly all can easy find in internet.

Here some basics

Nichicon

Cornell Dubilier
Here read carefully also about "Self-resonant Frequency" !
Quoted from Dubilier:
"
Self-resonant Frequency
The self-resonant frequency is the frequency at which the
capacitive reactance (1/(2?fC)) equals the inductive reactance
(2?fL). Because the capacitive reactance is 180
degrees out of phase with the inductive reactance, the two
reactances subtract out, and the remaining impedance is
purely resistive and is equal to the ESR at that frequency
Above self resonance the device is inductive. In aluminum
electrolytic capacitors the self-resonant frequency typically
occurs at less than 100 kHz. The self-resonant frequency is
equal to 1/[2?(LC)1?2 ]. It occurs at a frequency higher than
expected based on 120 Hz capacitance because capacitance
decreases with increasing frequency, and it can decrease
with increasing temperature from capacitance increase."

[Aido4ever]

Thank you.

It's true, i'm new to LCR and i'm reading and trying to learn how to propper take messures with it.

For capacitors it's easy, i read the capacitor marking and test to compare. I've also found out that big capacitors (nF and above) i need to messure in low freq while pF need to be tested in High frequency (please correct me if i'm wrong).

My main problem is with testing Inductance... since inductance don't have writing on it which says how much Henry it's should be and running through the frequencies gives different results so it's hard to guess which one is the "real" one....

If you know of a good articals that i need to read , please feel free to direct me to one.

[Mechatrommer]

thanks rf-loop for the detailed explanation. it showed to me like the fundamental teaching in school is flawed? about Zcap = (-1/wC)j, if C=f(w) "function of frequency", then Zcap = (-1/wf(w))j? hmm, maybe. otoh, i know there's nothing pure about any component, there's no single R, or single L or single C. every component have all RLC and will be prominent at high frequency. so your link have some point on testing component at not so high frequency, so the other "stray" elements will not mess into measurement.

[amspire]

Here is a graph of the capacitance of some of the Genrad 1409 series Standard Capacitors from IET Labs.

They are specified as 0.02% at 1Khz. These are very high quality hermetically sealed silver mica capacitors.

They are definitely not constant with frequency at any point of the frequency spectrum.

Richard.