Cap testing question

Yes, caps that read good value can still be bad. Most electrolytics that ar e bad will show high ESR readings (bad) before they show low capacitance. ESR meters are cheap enough that anyone who screws with electronics should have one. Despite the advice of others, I always remove the capacitor to t est ESR with an "in-circuit" ESR tester.

Also, a good percentage of wonky electrolytics will show signs of venting a t the top of the cap, if not an actual electrolyte leakage, then a discerni ble bulge. If you see any bulged caps, change them.

Most small disc and film caps will not become temperature sensitive. For a device to be warm up sensitive like your device it's usually a weak electr olytic , so concentrate there.

If there's only a few electrolytics, change them all with good quality (I l ike Panasonic) caps and they'll probably run.

Bad caps are certainly one possibility, but there are other things to check for as well.

There could be a cold solder joint that makes contact when a lead expands with heat.

A connector may have a bit of corrosion that slips away when the metal contacts slide past each other under heating. Try disconnecting and reseating all connectors.

Intermittent issues like these can be a pain to diagnose...

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I agree with changing all the electrolytics as there are only a couple of them. Even though the value may check out ok , there could be other problems with them such as the ESR value.

I would guess the .068K would be a .068 UF at 250 volts. They seldom go bag. If only a few of the 47 uf you might as well change them while you are at it.

While you could have other problems, the heating up seems to indicate the classic bad capacitors in the power supply.

I would first put the caps back in the circuit and see if it still behaves the same way. Your unsoldering and re-soldering may have fixed things due to a cold solder joint or other possible connection problems.

Mpfff.... Some basics on electrolytic caps:

a) an electrolytic cap, unless otherwise marked can test +100%/-20% for value and still be in spec. And unless you have the ability to test them at full operating voltage, the information from a typical DVM is incomplete. b) an ESR meter is your friend. As noted elsewhere, testing is best done out of circuit. c) switching power supplies are very hard on caps.

Replace them anyway, but also as noted, look for other heat sensitive failures also!

Peter Wieck Melrose Park, PA

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Those are the classic symptoms of a defective electrolytic capacitor. You can't test an electrolytic with just a capacitance meter. you need an ESR (equivalent series resistance) meter. Often, I see them with the right capacitance, but with a high ESR. It's a common problem: Notice how some of them are bulging at the top or bottom. As others have mentioned, it probably best to do an pre-emptive replacement than to try an save a few pennies seperating the good from the bad caps. In general, if one electrolytic is bad, they're all bad.

After some provocation by Phil Allison, I ran a temperature test on various electrolytic capacitors: Simple setup: Test caps: Spreadsheet showing results of temp test: Notice how the ESR improves (goes down) with increasing temperature. That's why your power supply screws up at room temperature, but works when it's warmed up.

There are a number of different ESR meters available. If you plan on doing such repairs in the future, you really should buy one. If you're not sure which one is suitable, I don't have any experience with this one, but it sure is cheap:

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Jeff Liebermann     jeffl@cruzio.com 
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Santa Cruz CA 95060 http://802.11junk.com 
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Note that does NOT apply to the big huge ones. Like the 470/200 main raw B+ filter for example, or those 10,000 at 80 volts in an amplifier.

When it has that plastic sheath over it, the bulging top does NOT indicate it is bad.

A guy I used to work with was told that and changed a like 470/200 and that did not fix the set, of course because it was not bad and he was not a rea l tech, he just did what he was told. Well days later I get to work on it b ecause nobody else could fix it. The cap was in a bag with the screws and r eplaced parts. The cap was still charged !

That doesn't mean it could not have high ESR, but it was definitely not the cause of the problem. There are people who like to change ALL the caps in old hifi amps, I think they're wrong. While I will agree with changing the corresponding cap in the other channel when you find a bad one, or even wea k one (significant ESR compared to Xc) it does not good to just change them all arbitrarily.

It depends on where they are in the circuit, if used for bypass, what kind of current drain ? If used for coupling, what impedences ? It matters.

For a few dollars more you can get them already put together. I bought a similar one a few years back. Not sure how well it does the ESR but usually does a good job of testing many components.

** Or several OR a dry solder joint or many of them.

** You went to lot of trouble to see the obvious.

All I did was try a few 450V electros, heat them with a hot air gun until they were darn uncomfortable to hold and note that in every case the ESR reading had plumeted by 5 to 10 times.

Then I found some graphs on-line that backed up the finding.

** I use the same one as in your pics - the Bob Parker design. He lives not far from me and we have conversed about that an many other electronics matters.

Have you tried your meter to test the ESR of cells ?

Easy to tell if a Lithium memory cell is good or not - also good on NiCds, NiMH and alkalines.

Also reads low value resistors, even if there is an inductor or transformer winding across one.

.... Phil

ISTR once coming across a failed zener in a smps causing this works when warm syptom

I've seen a dot on ICs signifying PbF

lots of esr meters to choose from : to build or to buy

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Jean-Yves.

e it is bad.

Ummmm..... Not on my watch! I do keep an ESR meter, and I also keep a cap-t ester that can test at actual operating voltages and in AC or DC. ANY physi cal indications of ANY nature other than friction or impact will get a cap changed. And every electrolytic in an item that affects the perceived probl em will get tested with that ESR meter. When in doubt, change it out.

The problem with swelling is that it may not be affecting the cap *today*. And the actual stresses on the cap may be well below its rating. But when ( not if) it fails, it will be at the most awkward possible moment and take m uch with it. Murphy's Law. Caps, even very good ones, are much cheaper than the time it takes to replace them. Just keep that in mind.

Peter Wieck Melrose Park, PA

I keep the "Blue" meter. Easy to use and as tested against others, quite accurate.

Peter Wieck Melrose Park, PA

Sure it does. If the problem is with one of these caps, changing them arbitrarily FIXES the problem.

?Penny wise, pound foolish.? Yes, you?re right, finding the ONE cap that is the cause is technically proficient. But just to say ?I found the culprit!? does not help the customer (or yourself, if it?s you). The unit will be back on your bench very soon if you don?t replace all electros now, if you find one or more below spec.

Bob Parker / Dick Smith / Silicon Chip meter!!!

Bob is the original designer of this meter. Last I looked Bob was still giving help to builders of his kit.

Cap-B was one that had failed with high ESR. There was plenty of data and anecdotal evidence on how a normal capacitor would act. I wanted to see how a defective capacitor acted. I also wanted to see how the ESR meter functioned with small value and size (0.22uF 50v) caps, which was Cap-D. The change in ESR with temp was far less radical than the others. I also found as similar problem with high capacitance values (1800uF

6.3v) which was Cap-A, where the lower limit capability of the ESR meter (about 0.03 ohms) also caused the graph to flat line. I didn't mention this in your previous discussion on the matter, but I was trying to determine if it made sense to use a fixed 100KHz sine wave signal, and whether higher or lower frequencies might help test a wider range of capacitance values. I have a pathological aversion to accepting the obvious, and will take the time to test the obvious, which invariably produce surprises.

I wasn't interested in high voltage electrolytics. There are usually two of those in a typical ATX power supply. They never seem to fail. It's the low voltage electrolytics, that inhabit the output filters and regulator circuitry that fail and were of interest to me. I probably could have done it using your method, but I thought a hot water bath was more interesting and accurate.

Yep. It's an impressive instrument, quite accurate, but a little tricky to build (Dick Smith kit version). Methinks the later "blue" variety might be better.

One of the common modifications to the original ESR meter is to add two back to back power diodes across the input to protect the meter from residual voltages. The problem with doing that is that the diodes short out the battery. Since I expected to be testing far more capacitors than batteries, this made sense.

I also have a Capacitor Wizard ESR meter that I picked up cheap at a flea market: The CapSRV model has the same diode protection circuit making it useless for battery measurements. There's nothing particularly wonderful about this meter except that it's very fast and convenient. One problem is that I seem to have misplaced it and can't seem to find it. Argh.

I also have a Peak Atlas ESR70 "purple" ESR meter. It has a built in discharge resistor to help deal with charged capacitors and therefore is useless for testing battery ESR.

Bottom line is that I don't use any of my ESR meters to test batteries. I probably should build or buy something specifically for the purpose to see what I'm missing. All of my battery testing involve small rechargeable batteries, a discharge tester, and computized graphs such as: However, I'm getting into larger cells and sealed packs, where an ESR tester is quicker and probably more useful.

Yep. I've used that to determine the value of a charred resistor. Usually, it's a wire wound resistor with a break somewhere along the winding. I measure from one lead to the middle of the winding. One end is usually open, while the other end reads half the resistor value. However, testing electrolytics is my main use for an ESR meter.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

I agree. The K probably indicates 10% tolerance.

Regardless, it is either .068 Microfarads or 68K (68,000) Picofarads, which is the same thing!

Fred

Good advice. But there is one more small-ish pitfall for the unwary. If a power supply refuses to start when cold, it's usually an electrolytic cap. But in many cases it's not one of the big caps but the small one that is located in the start-up / auxiliary section.

Mains power supplies usually have an auxiliary section that is used to power the internal needs of the primary side switching circuitry. This section is powered by a small auxiliary winding on the transformer in steady-state. But because it needs to start up first, and because at first the main transformer is not yet operating, it is precharged with a high value resistor (or sometimes a string of 2 or 3 resistors in series) directly from the rectified mains. There is a small (47 uF or similar, 25 to 35 V typically) electrolytic. At first it gets precharged through the resistor up to the point where the controller chip can start up. Once the controller starts, it would rapidly drain the small cap, so the main switching circuitry has to come on line fast in order to top up the the auxiliary supply cap and keep it from discharging.

Now this small cap only provides little reserve. It's usually sized very spartanically and it lasts for just one try and only if the start-up timing of the switching circuitry is not delayed for any reason. If it drains before the switcher can fully start, the switcher will stop and wait for another retry at a later time.

Often this small cap is considered "non-critical" and carelessly placed by the power supply designer into whatever corner had whatever little free space. This may happen to literally be a "corner" of a heatsink!

When this cap dries out either from overheating of just being selected from the cheapest manufacturer, it will rise in ESR and thus no longer provide the peak current that the controller needs in order to start. This either delays the start to the point where the cap is drained or causes it to outright abort prematurely because the auxiliary supply rail tanks right at the first few switching cycles.

When warmer it has lower resistance, thus it can provide higher peak currents and have better chances of starting the supply successfully. Also if the supply is working continuously for many years and never switched off, the condition of this capacitor is of no consequence during operation. Even if somebody just cut it out of a working power supply, there are usually some small ceramics on the rail and the auxiliary transformer winding will keep them topped up, so that the supply would quite likely keep on working. But if the supply ever gets switched off, even for a very short time, it will never start up again.

If you see this type of behavior, find and replace the cap in the auxiliary section. You can normally identify it as being physically small, a low voltage type (from 25 to 50 V) and being connected to a sizeable precharge resistor that gets supplied from the high voltage DC rail.

If you don't have an exact replacement, it's normally OK to use the next higher capacitance as long as the voltage rating fits. If you find that yours sits in the corner of a heatsink, use a low ESR high temperature rated replacement (no matter what the original has been) and some reasonable reserve capacitance-wise won't hurt either.

Dimitrij