Is this a valid method for checking capacitor leakage? - Page 1
Forum section: electronics / testgear

author: watchmaker
author_rank: Unknown
timestamp: October 10, 2024, 10:02:05 pm
content: Leaving aside the obvious safety issues (if this is valid I will make a safe enclosure), is this a valid approach to leakage testing?After looking at several designs/kits, I realized I had this transistor withstand tester that does in fact put out over 1000 volts DC. The display voltage agrees with my DMMs.The cap shown is a 10uF 200 volt electrolytic. I started with lower voltage caps of course.If this is deemed valid, then it avoids a host of construction issues and the tester itself cost about $20 on Ali. Just need a box some jacks, discharge and test switches.

author: IanB
author_rank: Super Contributor
timestamp: October 10, 2024, 10:06:48 pm
content: You put an ammeter in series with the capacitor, and then charge it up on DC to the working voltage, and when it stops charging, measure the residual current?I think that is a valid method for the DC leakage current.

author: watchmaker
author_rank: Unknown
timestamp: October 10, 2024, 10:14:05 pm
content: Yes, that is exactly what I did.THANKS!! I will take some shots as I put the thing into an enclosure.

author: IanB
author_rank: Super Contributor
timestamp: October 10, 2024, 10:23:01 pm
content: You might have to be a little careful about the inrush current if the capacitor starts out empty. Might be appropriate to put a 100 ohm resistor or so in series with the capacitor? It depends on the DC supply and what it is expecting as a load.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 10, 2024, 10:52:47 pm
content: If you're using high voltage (and 200V is high voltage) you need to have some way to discharge the capacitor to a safe voltage before you touch it. It could be a SPDT switch that goes between 'charge' and 'discharge' with a voltmeter to measure the voltage on the capacitor.

author: watchmaker
author_rank: Unknown
timestamp: October 10, 2024, 11:12:11 pm
content: Quote from: edpalmer42 on October 10, 2024, 10:52:47 pmYes, the discharge switch should be emphasized. I did say in my original post that I would install a discharge switch. But I like your emphasis in case others want to follow this path.Until last night and 3 hours of youtube/reading, I did not realize the importance of leakage current; or that capacitors are a primary failure point in electronics. (As someone taught on tubes, I always thought it was those damned transistors).This makes the variety of caps less bewildering to me. It is much more than value, tolerance, temp coeff and ESR. What a fascinating puzzle for such a "simple" device.

author: IanB
author_rank: Super Contributor
timestamp: October 10, 2024, 11:23:16 pm
content: The leakage current is not always "leakage" either.Electrolytic capacitors are what they say, they have electrolyte inside them. The dielectric is actually an insulating layer formed on the surface of the aluminum foil by electrolysis.Old capacitors that have not been used for a long time may need "forming" to rebuild the insulating layer. This forming process will consume a certain amount of current while it is occurring.Furthermore, until the forming is complete, the capacitor will not withstand the full working voltage, and may fail if you increase the voltage prematurely.Therefore, old electrolytics need to be handled with care, and if they need reforming, this should be done in the appropriate way before trying to test them.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 11, 2024, 01:48:16 am
content: Another trap with capacitors is dielectric absorption.Imagine measuring the energy you put into a capacitor to charge it and then measuring the energy that comes out when you discharge it."Wait a minute! I put more energy in than I got out! Where did the missing energy go??"That's dielectric absorption. The real kicker is that the missing energy will slowly come out and recharge the capacitor. When you touch the terminals or connect it to a test meter, you can get a shock or blow up your meter. You sometimes see high voltage capacitors stored with the terminals shorted to prevent this.

author: Paul T
author_rank: Regular Contributor
timestamp: February 25, 2025, 12:35:52 pm
content: Here's something I made to do the job - when a standard power supply isn't enough voltage. Otherwise, I follow the procedure with a typical power supply.The attached pictures show a $5 project - no need to build a fancy unit (unless you want to). I also skipped any capacitor discharge mode (I have another dedicated tool handy for that). However, I left room at the top left in case I want to add it later.At the 9:00 position, I supply a DC voltage to the jacks. The voltage is increased by a "Nixie Booster", available at the 6:00 position jacks. I connect a voltmeter to those jacks and read volts directly. I can also just make use of this unit as a DC-DC booster. I de-soldered the 10K pot from the board and attached a 50K to the case, larger value gives a lower minimum voltage.There is a 1K resistor across the leads at the 3:00 position. This is a current-limiting resistor in series with the capacitor. I connect a second voltmeter (in mV range) here for monitoring the leakage current (as micro amperes). Unlike an ammeter, you won't blow a fuse measuring voltage correctly. You should have the resistor anyway, so why not use it to measure?And of course, I mount the capacitor at the 12:00 position. I start the voltage low, and creep up to 95% of the rated voltage over a couple minutes. It takes at least half hour to reach the leakage current and "fully" reform an electrolytic capacitor, so I only invest the time before using a capacitor in a circuit. Otherwise, I can assess older capacitors for just a minute or two for a quick check.There are three different SMPS nixie modules I tried on AliExpress, they all work fine. You don't need much current at all. Each version has different input and output voltages and different features. Increasing the value of the potentiometer enables lower voltages. Because a reforming/leakage test happens over a long duration, I attach a heat sink to the module and leave off the lid to avoid building up too much heat.MC34063MAX1771UC3845AThe box was 99 cents from a commercial-chain craft store.In general, ceramic, paper, and mica capacitors shouldn't have any leakage current. Electrolytics and Tantalums have a leakage current that varies based on their capacitance and voltage. There are standard methods, such as IEC 60384-4 or whatever the capacitor spec sheet says. Notably, non-polarized electrolytic capacitors should be tested in BOTH directions.Bonus! I've attached a few charts I use as reference. The color chart I made myself.

author: jonpaul
author_rank: Super Contributor
timestamp: February 25, 2025, 01:01:03 pm
content: Hello there Watchmaker!We used a 0-400V valve lab PSU with meteres for decades.Similar to the Heathkit, and RCA Vacuum Tube Handbook classic designs.Slowly increase PS V from zero, observe current, when cap is fully charged allow to sit a while at the rated voltage, to see if leakage current stabalized.For lytics, we use a higher current range and increase very slowly in stages to charge and reform the dielectric.Photo is my 1960 DIY 0-400V 250 mA metered PSU.Your feedback appreciated.Have a great day!JonPS: is that Dick in Netherlands?

author: watchmaker
author_rank: Unknown
timestamp: February 25, 2025, 01:16:04 pm
content: Jonpaul,That is the same procedure I use. "Dewey" is Welsh I believe.Paul T, that is cute. BTW, thanks for the charts.Given the high voltages at DC, I like having no current path unless the uA meter is connected and then the 3-position test/off/discharge switch. After charging, I can switch to the "off" position and time the discharge using the built in voltmeter.I do have to put the meter in its "short" position before discharging though.

author: Paul T
author_rank: Regular Contributor
timestamp: February 25, 2025, 05:50:13 pm
content: Quote from: watchmaker on February 25, 2025, 01:16:04 pmIf you like those you might also like a similarly colored ESR chart for general Electrolytic caps.Good luck finding a 100,000 uF capacitor at 630V

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 25, 2025, 10:35:40 pm
content: So, what's the test frequency for that ESR chart?

author: Paul T
author_rank: Regular Contributor
timestamp: February 25, 2025, 11:28:52 pm
content: That ESR chart is derived from common curves used as guidelines. They don't usually publish the frequency.However, I measure it at 100 or 120 Hz.

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 26, 2025, 01:18:48 am
content: Quote from: Conrad Hoffman on February 25, 2025, 10:35:40 pmYou can often find test frequencies in datasheets. I've seen 100kHz as pretty common for ESR testing.

author: jheissjr
author_rank: Contributor
timestamp: February 26, 2025, 01:32:46 am
content: Quote from: IanB on October 10, 2024, 11:23:16 pmWhat method do you use for reforming?

author: garrettm
author_rank: Frequent Contributor
timestamp: February 26, 2025, 07:29:56 am
content: Reforming can be done with a 100k to 1M resistor in series with the capacitor. I'd suggest 80% max rated voltage.When the voltage across the capacitor stabilizes, the oxide film is probably thick enough to prevent failure without the series resistor. R_series and R_leakage form a voltage divider with C_cap in parallel with R_leakage. So the voltage across the cap is primarily determined by R_leakage. As the oxide layer thickens R_leak increases. Once R_leak >= R_series, the voltage should be stable.

author: FixyMixy
author_rank: Contributor
timestamp: February 26, 2025, 10:30:05 am
content: There is ONLY ONE valid method: check the capacitor datasheet, and make a test specifically for the capacitor you're testing.BTW: DON'T test non-electrolytic capacitors with that high voltage source. Well, you can, but only once per each capacitor

author: David Hess
author_rank: Super Contributor
timestamp: February 26, 2025, 10:57:54 am
content: That is how I do it. Shown below are 3 x 50 year old 50 volt 4.7 microfarad solid tantalum capacitors being tested for leakage. I started at the surge voltage of 65 volts and am up to 70 volts. I had to wire two of my Tektronix PS503A power supplies in series to get above 40 volts. The capacitors came out of the older power supply on the right when I refurbished and improved both power supplies a couple days ago.70 microamps of the leakage shown come from the 10 megohms of input resistance of the voltmeter on the left, leaving about 12 microamps of leakage into 3 capacitors. The power supplies are set to a low current limit in case one of the capacitors shorts out.

author: Paul T
author_rank: Regular Contributor
timestamp: February 26, 2025, 11:44:36 am
content: Quote from: KungFuJosh on February 26, 2025, 01:18:48 am100 kHz, or 100 Hz?I have one more handy reference to share regarding selection of a discharge resistor. Of course the wires and capacitor itself should be able to handle the resulting current and voltage.Which reminds me, I need to go back to my "cute" test box, and ensure the HV wires stay spaced apart appropriately.

author: David Hess
author_rank: Super Contributor
timestamp: February 26, 2025, 02:53:15 pm
content: Quote from: Paul T on February 26, 2025, 11:44:36 am100 kHz is included when the aluminum electrolytic capacitor is intended for switching regulator applications.

author: Paul T
author_rank: Regular Contributor
timestamp: February 26, 2025, 03:45:59 pm
content: Quote from: David Hess on February 26, 2025, 02:53:15 pmThanks for clearing that up. Another case where you need to consider the application and also the specific capacitor datasheet.I do not know where the general tables come from, probably what is used for general testing devices - 1 kHz ??Found this article which covers both ESR and leakage testing:https://community.element14.com/challenges-projects/element14-presents/workbenchwednesdays/b/blog/posts/measuring-an-aluminum-electrolytic-capacitor-s-esrIf anyone knows what frequency common reference tables are using, please speak up!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 26, 2025, 06:15:03 pm
content: Most people shouldn't be thinking about ESR at all. If you're designing switching power supplies, yes, otherwise use dissipation factor. Given the value, loss and frequency in any unit system, you can calculate one from the other. The problem with ESR is that you need a table to determine if it's good, and it varies wildly with frequency. If you're working with very high frequency switching power supplies, the capacitor value isn't very relevant as the reactance at the frequency of interest is going to be low. You care about the ESR because the ESR is where the power is dissipated- the loss term. You just scroll down the manufacturer's listing until you get to an acceptable ESR, pretty much ignoring the capacitance value. ESR is traditionally measured at 100 kHz. It's silly to measure it at lower frequencies. You can probably tell I'm not a big fan of ESR meters.At the frequencies we usually care about, the reactance of the cap may be significant. Dissipation factor is the ratio between reactance and resistance. All you need to know is if the dissipation factor is on the order of 0.1 for an electrolytic cap, it's probably good. It's even reasonably stable with frequency, though not entirely. Cap manufacturers will always give you the dissipation factor. They will also tell you the cap has reached the end of life when the dissipation factor doubles over its initial value.People seem to go through all sorts of gyrations to measure cap parameters. It's almost trivial to build (or buy) a simple bridge that gives you a very accurate measurement of dissipation factor, from which you can calculate ESR if you can't help yourself.My experience is if a cap (I'm talking entirely about aluminum electrolytics here) measures good for value, loss and DC leakage, it isn't going to fail any time soon, regardless of how old it is.Last note- measuring the low frequency loss (in the form of dissipation factor or ESR) of very large caps is difficult because it's going to be low. You need a 4-wire measurement system.Back to the OP's question, any of those power supply and current meter solutions is fine. I like to include a protective series resistor and it's important that the supply voltage be less than the cap rating, lest the voltage drift up as the cap leakage goes down. I also prefer to use some throw-away current sensing resistor and a voltmeter so if something shorts, like the DUT, a meter fuse or shunt doesn't get blown out. The protective resistor and the current sensing resistor can be one and the same. FWIW, the most sensitive leakage scale on a General Radio 1617-A is 60 uA. I find most small caps will drop down to 5-20 uA, so this is a good datapoint for selecting a current sensing resistor and meter range, though you also have to accomodate the possible hundreds of mA for a cap needing reforming.