Yet another low voltage capacitor leakage tester - Page 1
Forum section: electronics / projects

author: MiroS
author_rank: Regular Contributor
timestamp: June 28, 2018, 10:02:49 am
content: Built for fun and for checking if low voltage test can substitute high voltage test. Tested to work for power supply 9-30V, simple but effective. Btw, low voltage can not substutue high voltage test, no way, but for a quick&dirty test it will work.For compnents like on schema it will be sensitive to ca. 100nA leakage , it can be more sensitive by substituting 10M by like 50M or higher , or less sensitive if 10M will be substituted by 1M or even less.

author: Gyro
author_rank: Super Contributor
timestamp: June 28, 2018, 02:53:51 pm
content: Pity the poor LED at 30V.

author: MiroS
author_rank: Regular Contributor
timestamp: June 29, 2018, 09:04:47 am
content: No warries, maths and physics still works , at least on EarthTake 80 mA LED or replacy 270 with 470 if you think that Led will be to bright when cap is shorted.

author: xani
author_rank: Frequent Contributor
timestamp: June 29, 2018, 10:52:09 am
content: Quote from: MiroS on June 29, 2018, 09:04:47 amThen maybe do actual math, because you will be cooking your 0.5W resistors on 30V (aside from a fact LED could be used to light up the bench, not just indicate

author: MiroS
author_rank: Regular Contributor
timestamp: June 29, 2018, 12:02:59 pm
content: Oh well, iftest will last a few seconds so no warries too. Anyway it is quite simle to adjust it to any need or splt cap test voltage from current amplifier suply or ...

author: MiroS
author_rank: Regular Contributor
timestamp: July 03, 2018, 02:45:52 pm
content: I have replaced 2N222A with BC547C, this is now limiting voltage to 30V, but sensivity went down by factor of 10. This maybe a little to sensitive for old caps, but for my primaryuse it is quite OK .LED limiting resistor incresed to 1k from 270 due to the need for a longer tests for old caps. Old caps are simply formating during the test therefre leakage is reduced during the test , for some sort it is15nA for every 10 minutes. If you want to see the end result of test oor formating - it can really take quite a lot of time therefore 1k is really needed for caps if tested with 30V.All packed in a small back box , big red LED looks like eye of bull. You can see light even if LED current is approx. 300uA so it is not limiting leak detection. You can find this even to sensitive.All is working very well. It is powered from power supply, that way I can adjust voltage for capacitor limit.Tested to work and confirmed by mesurements for several caps; from aliexpress , Panasonic , Nichicons from tusted source, from quite fresh delivery (order from May, 2018) and 30 y. old caps. Tested with bigger caps like 1000uF or more and small like 10p. I have found quite a few ready to go to trashIf I would measure this with metter this would take ages. I think it is quite useful tool.

author: JS
author_rank: Frequent Contributor
timestamp: July 03, 2018, 04:22:09 pm
content: why not connect the cap to 30V with a ~10M input DMM to ground and measure voltage?Use some resistors to charge/discharge the cap. If you want build it in to a box with the resistors, a switch and terminals, the DMM should be always close to it anyway...JS

author: MiroS
author_rank: Regular Contributor
timestamp: July 03, 2018, 04:49:33 pm
content: >why not connect the cap to 30V with a ~10M input DMM to ground and measure voltage?Yes you can, but1. With 10M charging time for bigger caps will take ages , tester has charge mode with smaller resistor and meseasrument is done with 10M2. If you will take resistor from ground for second or two voltage on cap will go down and cap will be charged via this 10M resistor, it will tak some more time again3. If cap is leaky or in worst case shorted your multimeter may blow expensive fuse, so you have to set current limit in power supply, something to remember4. If your metter can go so down to 100nA range or so, regula mutimeters simply can not. I have old analog metter which can measure down to 20pA, but it also a pain :- Leakage current is not constant , it goes down and with analog metter I have to look at arrow till it will get lower range , switch range again, again ... down till current will be about stable value or below certain threshold- Most of regular DMMs are usless for leakage below 100nA range, you will simply see random value or zero5. If you have a bag of 100 capacitors , I think after second cap test you will drop any further testing with DMM. DMM and resistor is simply less efficient solution.

author: 001
author_rank: Super Contributor
timestamp: July 03, 2018, 09:09:27 pm
content: Interesting job for hobby time!But I hate any "low voltage leakage testers" for last 40 yearsIn fakt, You can`t test caps without REAL working voltageThis rule is written by blood, sorry

author: Kalvin
author_rank: Super Contributor
timestamp: July 04, 2018, 08:31:21 am
content: I created similar design, but I added current limiting circuitry to limit the LED current to 10 mA (for example), and adjustable test voltage. The circuit provides also a rudimentary voltage output for the measured leakage current in addition to the LED indicator which can be handy if adding a circuitry for a LED bar indicator. The transistors are 2N3904 or similar. The Q1 needs to be changed to 2N2222 or similar if the LED current is more than 10 mA and test voltage is around 30V as the maximum power dissipation of 2N3904 is only 600mW (30V * 10mA = 300mW).R1 limits the maximum LED current. R3 adjusts the minimum leakage current detected ie. sensitivity. D2 provides a discharge current path. The charging/discharge resistor R5 should be connected to a SPDT push button which will keep the capacitor discharged until the push button is being pressed. Alternatively a DPDT push button can be used to connect/disconnect+discharge the capacitor under test. R2+Q4+10Kohm potentiometer are optional, but they provide a simple way to adjust the test voltage if one wants to use three 9V batteries in series, for example.I have only simulated this design, so all bets are off how this will work in practice. I also simulated a temperature sweep 0C - 60C and the performance was good enough for practical use.Edit: Added R7 to limit the LED current at startup.

author: MiroS
author_rank: Regular Contributor
timestamp: July 04, 2018, 08:03:22 pm
content: Quote from: 001 on July 03, 2018, 09:09:27 pmI agree , this is also my experience, but if capacitor will fail with low voltage than for sure will fail with high voltage.I have 500V / 1A power supply for ultimate tests , but I never attach it to cap without paying A lot of attention. Even with current limit set to 10 mA this is 5W of power , this can send bad cap on the Earth orbit. I remeber one detonated 30 cm from my face ...

author: bitseeker
author_rank: Super Contributor
timestamp: July 05, 2018, 07:02:52 am
content: Quote from: MiroS on July 04, 2018, 08:03:22 pmAgreed, if it fails the low-voltage test, then toss it. The danger to the casual hobbyist, though, is that the reverse is not necessarily true (and many could incorrectly assume that it is). If a high-voltage capacitor passes the low-voltage leakage test, it does not mean that the capacitor is necessarily good. At most, it means that itmightoperate underspecificconditions. Install it in a high-voltage circuit and it could still go into orbit.Although the high-voltage testers are generally pricey on eBay these days, they're still worth getting if you're working on high-voltage circuits with high-voltage caps. There are also threads here about building your own if current market prices are out of budget.(This post is meant as general information, not addressed to anyone in particular.)