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I actually did these tests way back in early 2018
with the intention of writing a more in-depth article. Though I thought
I would throw this information out there meanwhile. This page is purely
information to illustrate how capacitors can vary tolerances and values
on the test. It is not a how-to guide on how to test capacitors!
Test were done with 3 capacitors removed from STF/E power supplies. There were actually a lot more capacitors tested during these experiments. But I basically cherry picked the ones below to illustrate what happens. One of the problems with testing capacitors in such a way is we do not actually know the tolerance or ESR rating to start with. You can probably assume 20% tolerance. But I have seen capacitors up to 50% in the past. I was doing some other tests under load originally but tests were not exactly portraying a fault condition. Because the capacitors which were measuring higher value were actually regulating better because of the inherent higher value. But of course the capacitor being so far out of tolerance indicates it is failing. So I did not include tests or take it any further because I really needed a better is test setup and never got around to designing one. The first set of images (taken on carpet) were the "cold" tests. The images taken on my workbench were after they were gently warmed in my ultrasonic cleaner heater tank. This was to simulate them running for a long period of time. |
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CASE 1 - 1000uF JAMICON 1,000uF capacitor @ 20% tolerance , 800uF - 1200uF This one seems to be within 20% tolerance but the actual ESR value varies somewhat. |
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| CASE 2 - 2200uF WANGS
2,200uF capacitor @ 20% tolerance , 1760- 2640uF ESR remains consistent but the value changes a fair amount. |
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| CASE 3 - 4700uF WANGS
4,700uF capacitor @ 20% tolerance , 3760 - 5640uF ESR actually gets lower as it heats up and capacitance increases. |
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| CASE 4 - 1500uF PANASONIC FR - 20% 0.02R
1,500uF capacitor @ 20% tolerance , 1200 - 1800uF As expected Panasonic is well within tolerance hot or cold. ESR does drop down to below what the meter can detect. |
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In most cases you could assume the capacitors are all intolerance and ESR is "good" but unfortunately things are not as black and white. The Panasonic as a base reference is what we are likely to expect from a good capacitor. It varies just only 78uF from cold to hot. With case 2, value is clearly out of tolerance when cold or hot which is approximately 42%! Coming from a company where we have serviced endless thousands of power supplies, I can confirm that when the value starts getting higher the capacitor has reached its end of life. The power supplies used to power fruit machines where they would be generally pushing several amps or higher. We could always visually tell when the capacitors were failing because the lamps started flickering. Generally the capacitors would measure a lot higher value than would ordinarily be expected. Unfortunately this is not a surefire method of capacitor failure is one of the main ones. Capacitors could still measure perfectly well and still cause lamps flickering. One thing to watch out for is the apparent "good" ESR value. Generally when the capacitor starts reading higher value than it should be, the internal dielectric is starting to break down and fail which is bringing the internal foil layers closer together increasing the value. Because of this failure the ESR value will generally get lower because it is approaching a short circuit type of failure.This is NOT a "good ESR" value or capacitor! In some cases we just used to measure capacitors on basic multimeter resistance test. Sometimes the resistance would keep on climbing and then hit a point and simply stop. For example a faulty capacitor as it charged could reach 500ohms and go no higher indicating the capacitor has failed. Again this is not a surefire method because this failure may only happen when the capacitor has been in operation for some time. Measuring "cold" clearly can give a different results than when it is warmed up. In general using a oldskool AVO analogue meter is generally better for testing capacitors because it has a much lower impedance than digital mean the meters which have very high impedances so do not "load" the capacitor the same as oldskool meters do. So while these ESR meters can give you a indication, and obviously if the capacitor is measuring way higher than it is supposed to be, then sure, that capacitor is bad. BUT, you basically cannot trust them to give you the real picture. You are personally introducing a discrepancy if you are just testing the capacitors "cold". Also if you are not measuring the resistance with a analogue meter, then you are skipping a series of tests completely. So again while these meters can give you a indication, you really need to actually know how to use them properly, and understand their limitations and limitations of the tests you are actually doing. Going back to the ESR value again. These old capacitors seem to be like the ESR value of modern day capacitors. I would assume this is unlikely that 30-year-old technology would equal today's technology. But without finding the datasheets for the original capacitors this is very hard to prove. For example the original capacitors could may well have been 0.10R or even 0.20R etc. as a dielectric fails the resistance progressively goes down until it reaches 0.02R. So you could mistakenly think that this capacitor is really good as it is on par with modern capacitors. But that is simply not true! The reverse problem is that a capacitor could actually read 0.10R and you could then think is faulty. But again, without the datasheet, this may or may not be the proper value for that particular capacitor. I think even the modern high-voltage caps have around 0.5R resistance in general for example. Of course if the ESR is like 0.5R or higher for values like 4,700uF then you can pretty much assume that capacitor is faulty. But overall the ESR value as tested is somewhat meaningless unless you have some sort of baseline comparison to compare against. The only indication you could actually go on is the measured capacitance value. But again because of tolerances, it is very difficult to go on that value alone. Of course if the capacitor is higher than 20% value, then the likelihood is that capacitor is failing. The Panasonic capacitor varies about 5% in capacitance value from cold to hot. Obviously this has to be a "good" figure for a modern new capacitor. While the capacitors tested are generally varying 10% from hot to cold. You could use this as a indication, but without the datasheet for any sort of method of comparison then there is not really any reliable way to test them. What about a "double failure" mode ? maybe a 4,700uF capacitor has decreased in value to 3,000uF due to the electrolyte drying up. BUT, because of the dielectric failing it then increases the value back to 4,700uF. So you measure your capacitor and it measures exactly the value expect so all is good right ? Similar with ESR values. capacitor on test measures as good as one of recent years so all is good again right ? what about hot or cold measurements ? visual inspections ? So everyone who reaches for a ESR / Capacitor tester really needs to understand all the implications of what they are actually doing. Without a proper datasheet or any sort of frame of reference to what values you should be expecting from the capacitor, you're basically guessing at what the results are supposed to be. In which case, you might as well not bother measuring it in the first place.
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