Capacitor leakage testing question - Page 1
Forum section: electronics / general

author: powersupply
author_rank: Contributor
timestamp: October 06, 2025, 10:00:20 pm
content: I bought 8 capacitors shown in photo. Philips 1500uf 450v from eBay and want to determine leakage. Here is the procedure:1. Charge capacitor to 60v. And verify charge.2. Hookup cap to Keithley 2010 (7.5 digits, it’s my treasured instrument) on dc volts measurement. Intended to monitor voltage change over time.3. Capacitor loses 1 volt every 5 secs. It concerned me that the meter may be corrupting my measurement.4. Had bright idea to put 100Gohm resistor from plus side of cap to plus input of meter to eliminate meter itself dropping charge on the capacitor.5 Keithley went berserk, voltage jumps all over the place.6 Tried 1Gohm in the path and the meter did same thing.7. Tried to check input resistance of the meter, none of my flukes could do it, I forgot to use my 121Gw, I’ll try it.8. Just to verify I put a 1k ohm in the path and the drop is the same as without any resistor, so procedure itself is valid. BUT obviously I am missing something.My Question;Why is putting a 100G or 1G resistor in the capacitor to Keithley path causing issues reading the voltage?(I know the 100G is good, I verified it with a Keithley 480 picoammeter using another method.)Thank you,Billy

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 06, 2025, 10:55:43 pm
content: G ohms will be floating lead unstable.Another approach is to connect the meter to 59v so the meter drain is much less.Some meters also increase impedance on low ranges, as no divider is needed.

author: Hydron
author_rank: Super Contributor
timestamp: October 06, 2025, 11:44:53 pm
content: Charge up the cap, leave it disconnected a set period of time, then quickly measure the voltage before the 10M input R of the meter drains it. You can work out leakage current based on the initial vs delayed voltage measurements and time between them.Or just get an electrometer or SMU with tera-ohm input Z and directly measure it

author: gbaddeley
author_rank: Regular Contributor
timestamp: October 07, 2025, 05:21:22 am
content: Did you first ‘reform’ the caps?Eg Charge up from 400V (or less) power supply via a 10k 10w resistor. Measure volts across resistor using a DMM and time how long it takes to drop below 10V (=1mA). Discharge cap using the 10k and repeat. Times to drop should get longer.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 07, 2025, 10:17:29 am
content: A 1F capacitor looses 1V/s at 1ASo a 1500uF losing 1V every 5 seconds is a current of 30uAAnd that would be 200 kOhm at 60V.(Did I calculate that correctly, I'm quite rusty)But as others have already written, Just leave the cap alone after charging and measure briefly after some interval.It's quite normal for electrolytic capacitors to leak, especially when they have not been used for a while. If you leave it charged for a few days to reform it, then leakage may get a lot less. Leakage can be so low that those big capacitors can hold a charge for days.On long unused caps, leakage may be so high that it heats up and destroys the capacitor without proper reforming first.I've got a few of those cheap chinese frequency inverters, and the manuals says that if it is stored for a long time, I should take it out of storage once a year and apply mains voltage for a few days.

author: Hydron
author_rank: Super Contributor
timestamp: October 07, 2025, 10:41:56 am
content: He's testing at >20V, so no high-Z mode on the meter, hence my comment about an electrometer or decent SMU being the way to continuously measure the voltage without disturbing it.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 07, 2025, 11:26:20 am
content: Two other questions:Why is this in the metrology section?It's much more appropriate for the beginners section.Why is someone with barely basic electronic knowledge using a keithley 2010?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2025, 01:11:02 pm
content: Measuring the leakage is tricky because there is also dielectric absorbtion, that can look very similar to leakage.To really see the leakage one would have to also check the DA. Without leakage it would be symmetric for the step from 0 to voltage and the opposite direction. Leakage would only effect the case with voltage, not so much the step back to zero.At least one would need to wait quite some time before the voltage drop is more from leakage than DA.

author: mawyatt
author_rank: Super Contributor
timestamp: October 07, 2025, 01:52:43 pm
content: Use methodHydronmentioned in post # 2. Charge cap up and measure V, disconnect meter, do not keep meter connected to cap this causes cap to discharge thru meter impedance. Wait a few minutes quickly connect meter and measure V, then disconnect, repeat as necessary to create a discharge profile. From the profile you can begin to access the cap leakage and other effects.For example, we charged a Rubycon 1500uF 16V cap to 16 volts. Made periodic measurements and after a couple days the voltage dropped to ~13V, after a week 12V and so on. Leakage can becrudelyestimated as I = C dv/dt, where dv is delta voltage change and dt is measurement period.Best

author: EC8010
author_rank: Frequent Contributor
timestamp: October 07, 2025, 03:24:28 pm
content: Those capacitors are probably quite old and need reforming before you will get any sensible answers for leakage. They need to be gently taken up to working voltage and held there for 45 minutes before leakage will settle. Frankly, I'd be very nervous about such a large capacitance charged to 450V. Fully charged, one of those capacitors stores 150 joules; that's a lot of energy. There is a very real danger of explosion if they don't take to reforming. If I had to do it, I'd probably test one in 50V steps, monitoring charging current all the time and ensuring it doesn't go drastically high and if I got to 450V without anything untoward, I'd look at the others. I'd probably put the capacitor in a sturdy plastic bucket, add a lid, and invoke a 10mA current limit on the power supply.

author: Vtile
author_rank: Super Contributor
timestamp: October 10, 2025, 06:58:52 pm
content: Another method is measure directly the series dc leakage. For that you make a series connection with ammeter and capacitor. With this way you can observe mere mortal level metrology phenomenoms even with modest instruments, like 10Mohm input impedance (nominal) voltage meters. Then using a ohm law to turn the seen voltage over the meter to actual current .. or with also applied voltage division the ohmic or impedance equivalent of the DUT. Eg. A common power cords are interesting as one can see how much they actually do leak and especially when one slowly turn the voltage source adjustment knob.

author: free_electron
author_rank: Super Contributor
timestamp: October 10, 2025, 07:15:32 pm
content: Bob pease had an application note on how to do that. He used a very simple sample and hold circuit. took a sample once every few seconds. he used a 555 and some TTL counters. It could also do soakage.The idea was that the sample and hold does not load the capacitor until a sample is taken. A tiny amount of charge is transferred from the capacitor under test to the sample capacitor. the instrument that sits after forms no load to the cap under test.Those sample/hold amplifiers are cheap.

author: RolandK
author_rank: Regular Contributor
timestamp: October 11, 2025, 10:16:31 pm
content: Measure the leakage at the interesting voltage or better at rated voltage. Use a variable power supply, a current limiting resistor (which can handle the supply voltage alone), the unknown capacitor and an amperemeter in series.Slowly turn up the voltage. The amperemeter shows the capacitor charging current. This should slowly drop to very low values. At higher voltages the current drop slows down and remains at an increasing higher value. After a minute your capacitor is formated by this procedure and should stay at rated voltage with very low leakage current (e.g. some µA, idealy much below datasheet value) and is now ready to be used.Electrolytic Capacitors after long time in storage being exposed to their working voltage may have a very high leakage current at the beginning which heats them up to explode or develop a shortSame for equipment beeing long in storage - if they have no smps or a voltage monitor you can bring them up slowly with a variac. Otherwise just switch them on for a short time (1s) and then off for a minute. By the time slowly increase the on-time and decrease the off-time.This is one of the simple basic measurements. The charge time (T = R * C) gives you a hint about the capacity, but better measure C and ESR with a 4-wire LCR Meter which can measure at 100 Hz or 120 Hz(eg. DER-EE 5000)For low voltage capacitors you may use a ohmmeter - it has the voltage source, the resistor and the meter build in. Only it's working voltage is some volts. I use them for supply capacitors in devices. There is some load resistance in the circuit arround 10k. The measured value should increase to the value of the circuits load. If the speed is to high for the rated valuethe capacitor is open. If the resistance ramains in the ohms value there is a short somewhere.

author: ivo
author_rank: Regular Contributor
timestamp: October 12, 2025, 12:36:06 pm
content: Keithley 2010 can only directly measure the voltage drop if the voltage is under 10V, because in that way you can set it to its high impedance mode (>10 GOhm). If the voltage is above 10V, then it puts its 10M voltage divider in parallel to the source, and this 10M resistance will always be 'artificially' draining the capacitor.Instead, you can connect the Keithley current jacks in series with the -cap to ground. After the cap has been fully charged for a while. It should hopefully measure down to 100nA easily. Or, you can connect it in voltmeter mode, in series again. But set the input impedance to 10M on purpose. That way, any leakage current flowing through the cap will flow across the 10M resistor and generate a voltage for it to display. Simply use ohms law to discover the current leakage running through the cap. With a 10e6 multiplier, you can easily measure nanoamps and smaller.