How to measure a capacitor's leakage current?

[YUV]

capacitor leakage current

I would like to measure some specific capacitor's parameters like leakage currents or an insulation resistance that might cause those currents. How to measure them properly?

 

[flatulent]

how to measure leakage current

Your basic problem will be the effects of the measuring instruments. In some cases the leakage currents will be so low that the input impedance of your instruments will draw more current. There are special instruments that have 10^12 ohms input impedance.

Here is one possible way to get approximate measurements.

1. Charge the capacitor to a known voltage and then let the capacitor stand with no connections to it.

2. Wait a certain amount of time and connect a volt meter to the capacitor and rapidly record the reading.

3. Assume that the decline in charge on the capacitor was caused by a constant resistance during the time interval between 1 and 2 above. Back out the discharge time constant and thus the shunt resistance.

 

[windman]

measuring capacitor leakage current

You can only use the so called electrometer to measure the leakage current of a capacitor due to its very weak can be less than 1pA and its very high output impedance at the mean time .

Normally this kind of instruments are expensive than the multimeters. If you want to get the results more actually. I recommend you to buy a electrometer, such as Keithley 6512 etc.

If you just want to know the rough level of the leakage,you may use a special operation amplifier with very high input impedance and very low input bias current like INA116P(producted by Burn-Brown/TI) to convert the weak leakage current to a low output impedance voltage,thus you can measure the voltage then caculate the leakage current.

Good luck!^_^

Added after 19 seconds:

You can only use the so called electrometer to measure the leakage current of a capacitor due to its very weak can be less than 1pA and its very high output impedance at the mean time .

Normally this kind of instruments are expensive than the multimeters. If you want to get the results more actually. I recommend you to buy a electrometer, such as Keithley 6512 etc.

If you just want to know the rough level of the leakage,you may use a special operation amplifier with very high input impedance and very low input bias current like INA116P(producted by Burn-Brown/TI) to convert the weak leakage current to a low output impedance voltage,thus you can measure the voltage then caculate the leakage current.

Good luck!^_^

 

[YUV]

capacitor leakage current measurement

Hi, windman. Thank you for your advice, but purchasing of the electrometer for measuring 1pA is too cool. It would be much cheaper to tie in parallel a thousand capacitors and use a microampermeter.

Thanks, flatulent. You are really gave me a valuable idea.
I have charged a group of capacitors (parallel connection) to -100V. I connected 10M-input oscilloscope probe in parallel and turned it in "single" mode. Then, I disconnected the input voltage. Oscilloscope snapped and saved the picture. Using time cursors, I measured the "delta" time between the beginning of discharge and the moment when oscilloscope trigged. I used the moment of trigging because the trigging voltage was exactly known.
Everything else looked like an easy calculation.
Basic formulaes were:

V1 = V0 * e ^ (-t1/tau),

where
V0 - starting voltage
V1 - trigging voltage
t1 - trigging time.

tau = RC,
where
C - capacity(the value was known)
R - the resistance that consist of unknown insulation resistance and known input resistance of ocilloscope.

At first, I calculated tau and then, R.

See a photo.
Also, I hope this experience will help somebody.
Regards, YUV.

P.S. flatulent, I added you "helped" score.

 

[windman]

measuring leakage current

if the insulation resistance is less than 100 times of the input resistance of the oscilloscope(<1Gohm), YUV's way would be a valuable solution.

 

[AndrzejM]

how to measure capacitors

YVU

Can you tell me what was the capacitor you have measured
(electrolitic, tanlal, polyester,other,......, what was its capacity and nominal working voltage, how many capacitors has been connected in parallel)
And finally:
What value of capacitor resistance you get as the result of your calculation.

I am asking these questions because , to say the true, I suspect that your measuring method is not a very precise, also I think, that your measurement does not comply to the "flatulent" idea.

 

[YUV]

how to measure a capacitor

Hello, AndrzejM
My "capacitor" is a plasma display panel and its parameters can't be disclosed.
I didn't need to make too precise measurement. I wanted to know just an order of value. I got about 100 MOhm resistance for a group of 64 row electrodes in parallel. That fact satisfies me completely.

 

[carrier]

how to measure capacitors

hi
this is full circuit of ESR tester.
enjoy

 

[AndrzejM]

esr tau measurement

Yesterday I was searching for a leakage,
I have done it as follow:

1. I took a 24 V voltage source ( 2x 12V accumulator)
2. Device ground connected to battery minus
3. 10 Mohm input resistance voltmeter between battery plus and the point for leakage measurement.
4. At first voltge reading was 24 V but it was quickly dropping down (in case of capacitance measurements, one can temporary short the voltmeter with a resistor to speed up the transient).
5. In circuit with good isolation the end voltage was about 0,5 V
6. In leakage circuit the voltage indicated by voltmeter was in 5V region.
7. The leakage resistance in point 5 is approximately:
[ (24V- U)/U]* 10 Mohm = 480 Mohm
8. The same for poin 6 gives 40 Mohm (still more than the max 20 Mohm available as direct measurement by "universal" meter )
**************
The method advantages:
- one can use simple universal meter (for limited range even the simplest one with 1 Mohm internal resistance).

Maximum measuring range is determind by minimum measurable voltage.
For the 3 1/2 digit meter with 10 Mohm resistance and 2000 mV range the readings above 100 - 20 mV can be accepted.
For 24V battery it gives a posibility to measure/estimate the resistances/leakages up to 2,5-10 * 10^9 ohm

If one need more?
There are higher resolution meters, the voltages higher than 24V
and the, mentioned in previous posts, electrometer as the final solution.

 

[windman]

1pa current measurement

If anybody have interested in low level measurements ,I would like to discuss with you. I measured once less than 1pA current(typecally fA level).

Added after 54 seconds:

If anybody have interested in low level measurements ,I would like to discuss with you. I measured once less than 1pA current(typecally fA level).

 

[AndrzejM]

leakage current capacitor

If anybody have interested in low level measurements ,I would like to discuss with you. I measured once less than 1pA current(typecally fA level).
/

I am interested, please describe the method.
May be you can open a new topic?

 

[power-twq]

measure leakage current capacitor

apply a higher voltage that the capacitor can withstand

and measure current use a multimeter.

best regards

I would like to measure some specific capacitor's parameters like leakage currents or an insulation resistance that might cause those currents. How to measure them properly?

 

[objcom]

Using a reference capacitor and a test capacitor it should be possible to do a quick test using the following method.

Charge both capacitors by connecting a 9 volt battery to each in turn. Note which capacitor terminal is positive and which is negative. The reference capacitors specification should match that of the capacitor / capacitors to be tested.

Connect the capacitor negative terminals together. Select a suitable range and connect the ammeter between the capacitor positive terminals.

With identical good capacitors the ammeter should balance at zero. This may take a short time. However, if the test capacitor is faulty it will steal charge from the reference capacitor which will show as a current flow on the ammeter.

 

[Kripton2035]

here is he beginning of an article from silicon chip magazine about it.
Silicon Chip Online - Capacitor Leakage Adaptor For DMMs

 

[Prof78]

Though this is an old post there seems to be renewed interest in measurement of small currents.
Many approaches to measuring leakage currents have been provided in previous replies but direct measurement of the current has advantages. One of the most effective ways of measuring small currents is the transimpedance amplifier, with the basic circuit shown (Fig.1). For this the output is simply:

v(out) = i . RF or v(out) / i = RF (1)

Thus for any given current it is necessary to select a suitable feedback resistor RF. The ‘gain’ of the amplifier is thus RF and the unit is ohm.
However there are a number of important additional matters to be considered. For very low currents, of the order of say pico or femtoamp, the resistor then has to be very high: for a current of 1pA and a full scale output of say 10V then RF =10Gohm (10^10), and for 1fA, RF =10Tohm (10^13). Though such resistors are available they are generally rather expensive. The amplifier A must have a bias current low relative to the current i to be measured. There is necessarily an input capacity CI (deliberate or stray, Fig.2) which will tend to cause dynamic instability. To counteract this it is necessary to add a suitable feedback capacity CF (and there is in addition the self capacity of RF). CI should be kept to a minimum to minimize noise [3, 4]. The combination RF of and CF leads to a system bandwidth of:

f = 1/(2pi . RF . CF) Hz (2)

and though CF may be small RF can be large so the bandwidth will be low. It can be shown that adding the components R1 and C1 (Fig.3) [ref 1, 2] such that:

R1.C1= RF.CF (3)

will provide some compensation for the restriction of the bandwidth. The system will again be tending to instability, but splitting R1 as shown in Fig.4 (R2<< R1), but still with:

(RF+ R2)C1= RF.CF (4)

provides a damping term in the transfer function that allows adjustment of stability. The bandwidth is now:

f = 1 / (2 pi . R2 . C1) Hz (5)

a considerable improvement over (2). Connecting the component under test (DUT), say CT together with a bias voltage VT, as shown in Fig.5, now allows measurement of the leakage current. The voltage VT should be increased slowly so that the CT charging current does not overload the system.
Since high value resistors can be (very) expensive it is worth considering an alternative approach which eliminates this component. If RF is removed and CF increased then the input current will charge CF and for a fixed i the output v(out) will be a ramp. CF must be a good quality capacitor with low leakage. If Q is the charge on CF and t is time, then:

CF = Q / v(out) and since Q = i . t
= (i . t) / v(out)

or v(out) = (i . t)/CF and hence
dv(out)/dt = i / CF or i = CF. dv(out) / dt (6)

so measuring the rate of output change dv(out)/dt and knowing CF allows the determination of i. CF needs to chosen to suit the magnitude of i and some means of resetting by discharging CF provided.
Though in the past it has been necessary to use special opamps in TO5 cans (that use high resistivity glass seals and hence are rather expensive) it is now possible to use some low cost CMOS type opamps in high resistivity DIL epoxy packages that can provide very low bias currents e.g. LMC6041/61/81. The LMC6041datasheet quotes a bias current of about 2fA, though it has a very restricted bandwidth. In mounting the opamp the input pin should be bent out (or the IC mounted upside-down with pins in the air) and not connected to anything other than the input and the feedback resistor, thus avoiding any additional leakage. Remember that in using very high value resistors there are no insulators, only other high resistances.
Some additional information is available in references [3] and [4].

[1] Pelchowitch I., Zaalberg van Zelst J.J. (1952): A Wide-Band Electrometer Amplifier;
Rev. Sci. Instrum. 23, 73-75.
[2] Hamilton S (2007) An Analog Electronics Companion (Cambridge University Press
ISBN 0780521687805), section 5.12, p488.
[3] Burr-Brown: Photodiode monitoring with op amps; Burr-Brown Application Bulletin
AB-075. See Analog, Embedded Processing, Semiconductor Company, Texas Instruments and search for sboa035.
[4] Burr-Brown: Noise analysis of FET transimpedance amplifiers; Burr-Brown
Application Bulletin AB-076. See Analog, Embedded Processing, Semiconductor Company, Texas Instruments and search for sboa060.