Author Topic: Testing Capacitors  (Read 3487 times)
HPSM250R2
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Testing Capacitors « on: October 10, 2020, 11:36:27 PM » Author: HPSM250R2
A while back I remember someone mentioning how I could test the capacitors in my old fixtures. I can't remember who but it may have been in the comments section of a photo. Anyways I am wanting to know what instrument I would need to buy, and how I would test the capacitors in an old fixture. I had the capacitors in an old Westinghouse OV-25 bulge and leak when I plugged it in to test it years ago and I don't want that to happen again. They got really hot too in a matter of maybe a minute. I have another OV-25 I haven't tested yet and I have had it for a while but I will not test it until I am sure the capacitors are good and will not leak, bulge, or overheat. This will be good information to know for other fixtures as well though.

Thanks everyone.

-Ryan
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Ash
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Re: Testing Capacitors « Reply #1 on: October 11, 2020, 12:09:01 AM » Author: Ash
Multimeter with capacitor test function
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joseph_125
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Re: Testing Capacitors « Reply #2 on: October 11, 2020, 01:50:36 AM » Author: joseph_125
Yeah that's what I use to test capacitors too.

Granted I tend to replace all pre 1980 capacitors regardless if they test ok or not due to PCB concerns especially when keeping them indoors. I try to get oil filled replacements instead of dry capacitors but the modern capacitors tend to be physically smaller than the old PCB ones they replaced.
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Medved
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Re: Testing Capacitors « Reply #3 on: October 11, 2020, 03:20:32 AM » Author: Medved
Depends on which degradation mechanism is the most likely. It does not make sense to test for just a capacitance alone, when humidity induced leakage is the most likely component killer (a wax paper will very likely retain its correct capacitance, but it will liak so it will fry the output tube or transformer in an old radio). Or testing of leakage on a component integrating discharge bleeder resistor (and panicking the CWA capacitor is leaky).

First is universal: Visual inspection, looking for swelling, leakage,... Suspiciously looking component is likely defective and need not be tested further (be aware many liquids used in capacitors are toxic either normally, or when they degrade due to e.g. arcing, so do not mess with it unnecessarily).

Leakage due to humidity ingress ("wax paper" capacitors in old electronic): Measure it as a resistance in the 10Mohm..1GOhm ranges using elevated voltages (like 30V or above; for these defects it does not need to reach the operating voltage at all, but few 100's mV of a regular ohm meter is not enough - water polarization barrier gives false pass, you need a voltage to be way above that effect, but dont need kVs and handling potentially deadly charge/energy levels or violent failure events during testing)

Capacitance reduction due to too much breakdowm/self recovery cycles (most common in lighting gear or AC motor circuits):
Connect in series with some current limitting device (an incandescent,...) to an AC supply in series with an A-meter, measure the current and voltage across the capacitor, calculate the reactance (X = V / I) and from that the capacitance (2 * pi * f * C = 1 / X). Then it is a good idea to measure the real losses (using a kill-a-watt or similar), it should be below 1% of the reactive power (normally the 1% limit reading is the limitation of such instruments, not the capacitor losses itself).

Elevated ESR (electrolytics in general, both in electronic, as well as motor start,...):
You need to measure the impedance at high frequency (or steep rectangular current steps). You need a dedicated tester for it or a set of more advanced electronic test gear (waveform generator + digital storage oscilloscope able to read out the waveforms + accurate resistor + some spreadsheet calculator SW to do the captured signal processing to extract the ESR). There are abput $10 testers available on the known sites.

Electrolytic leakages/reforming: Needed when the device was sitting without voltage for way too long. Use limited current DC source to charge it to its rated voltage (or at least 30% above normal opersting voltage) and monitor current. At least after few minutes it has to drop to uA levels (depends on the capacitance and voltage rating, consult the datasheet). At the end of the test, safely discharge it (into some resistor, never hard short) and leave the discharge connected for few hours (overnight,...) to bleed off the residual polarization charge (after long time charge to the full voltage and only only brief discharge to 0V, they temd to build up significant part of the original voltage after just few minutes just sitting on the desk, so always treat capacitors like they could be still charged). But keep the capacitor enclosed in something able to resist its potential explosion.
Of course, visual inspection and an ESR test should follow.
« Last Edit: October 11, 2020, 03:37:38 AM by Medved » Logged

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HPSM250R2
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Re: Testing Capacitors « Reply #4 on: October 11, 2020, 10:45:54 PM » Author: HPSM250R2
Thanks everyone for all the helpful information for testing my old capacitors. I will use the information in this topic as a guide when I am ready. I'd really prefer to leave the old capacitors in my lights if possible to keep as much original as possible. Replacing them would likely guarantee no issues but then I need to try to find one that is the same physical size. Then there's the fact that the new one will stand out and the fixture will not be original anymore.
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Medved
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Re: Testing Capacitors « Reply #5 on: October 12, 2020, 12:28:11 AM » Author: Medved
Well, at least inspect them visually, once you are already inside. Because some failure modes may damage the other parts...
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lightinglover8902
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Re: Testing Capacitors « Reply #6 on: October 12, 2020, 11:23:32 AM » Author: lightinglover8902
Basically just use a multimeter on capacitance and it will tell you if the capacitor is bad or not.
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Re: Testing Capacitors « Reply #7 on: October 12, 2020, 02:32:13 PM » Author: BT25
I have an OV-25 that had old rusty PCB capacitors in it. I bought a multi-meter that had a micro farad setting so that I could measure the capacitance. The capacitors looked really bad and the identification markings were gone.
Like Ryan said above, I don't want PCB capacitors indoors where they can leak or smoke when they fail. I've had this happen before, and the smell is hard to forget.
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HPSM250R2
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Re: Testing Capacitors « Reply #8 on: October 12, 2020, 02:41:35 PM » Author: HPSM250R2
I have an OV-25 that had old rusty PCB capacitors in it. I bought a multi-meter that had a micro farad setting so that I could measure the capacitance. The capacitors looked really bad and the identification markings were gone.
Like Ryan said above, I don't want PCB capacitors indoors where they can leak or smoke when they fail. I've had this happen before, and the smell is hard to forget.

Yes I had purchased a used OV-25 a few years back and when I tested it the first time, after maybe a minute of it firing up I started hearing this hissing sound and that's when I noticed the capacitors had bulged and were leaking all the PCB fluid out. After unplugging the cord is when I eventually touched the capacitors and they were so hot nearly burned me. After maybe only running maybe a minute they got that hot, that quick. I was in the garage with the door open but I did come into direct contact with the PCB oil. I took the fixture out to the driveway, leaned it up against the house and used the hose to wash out the oil. Before I was told shortly after about the oil containing PCB's, I was standing in the driveway barefoot. With the water/oil mixture from me spraying out the fixture running down the driveway right under my feet. So to this day I'm afraid of firing up any old fixtures I get.
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Re: Testing Capacitors « Reply #9 on: October 12, 2020, 09:10:33 PM » Author: wide-lite 1000
I think I'll lean on the side of replacement for all old capacitors where possible . I actually tested the capacitor in my Norelco 180w SOX Streetlight and it tested fine . HOWEVER , I let the light run for a hour or so as it hadn't been run for a while. WELL , all of the sudden , the lamp went out , I went over to see what was going wrong and as soon as I went to unplug it I heard a hissing sound and the room filled with a HUGE cloud of smoke !! :o The capacitor that had tested good a few months ago went POOF !!!  Besides , I've heard stories of people frying rare lamps or ballasts because of a no good capacitor .
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HPSM250R2
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Re: Testing Capacitors « Reply #10 on: October 12, 2020, 09:13:45 PM » Author: HPSM250R2
I think I'll lean on the side of replacement for all old capacitors where possible . I actually tested the capacitor in my Norelco 180w SOX Streetlight and it tested fine . HOWEVER , I let the light run for a hour or so as it hadn't been run for a while. WELL , all of the sudden , the lamp went out , I went over to see what was going wrong and as soon as I went to unplug it I heard a hissing sound and the room filled with a HUGE cloud of smoke !! :o The capacitor that had tested good a few months ago went POOF !!!  Besides , I've heard stories of people frying rare lamps or ballasts because of a no good capacitor .

Ah ok. Well there it is then. I guess replacing old capacitors is the best option. If I must keep a fixture all original then I can either just not fire it up, or remote ballast.
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wide-lite 1000
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Re: Testing Capacitors « Reply #11 on: October 12, 2020, 09:23:33 PM » Author: wide-lite 1000
Honestly , most people won't know the difference or really care if the cap has been replaced anyways . I consider it a safety item , like changing the tires on a car that has been sitting for a long time .
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HPSM250R2
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Re: Testing Capacitors « Reply #12 on: October 12, 2020, 09:28:56 PM » Author: HPSM250R2
Honestly , most people won't know the difference or really care if the cap has been replaced anyways . I consider it a safety item , like changing the tires on a car that has been sitting for a long time .

Yeah but for me, I'd like to keep the capacitors original. The ones in my OV-25 have a reddish look to them. Not sure if that's age or how they are supposed to look. If I replace them they'll probably be shiny and silver and look out of place. However I only have one OV-25 that's all original at the moment. If I had multiple maybe I'd replace capacitors. But only having one, I want to keep it original. It is missing one of the 4 slipfitter bolts though. And I'm not sure where to find new ones. The bolts I seem to find at hardware stores are all silver in color. These bolts are a yellow/gold color. I'd love to find replacements for those at least so I can have all 4 bolts in the same condition.
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wide-lite 1000
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Re: Testing Capacitors « Reply #13 on: October 12, 2020, 09:38:36 PM » Author: wide-lite 1000
Well , if they test fine leave them . Just keep a close eye on them Just in case . FYI capacitors should get warm while in use . I have one of those laser thermometers, it's great for instant temp checks of almost anything .

 One more thing , when you fire up the OV-25 , also check the amp draw once the lamp is fully run up . When I had the bad capacitor in the 700w MV OV-50 , the amp draw was WAAAAAY higher than it should have been due to the defective cap .
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Re: Testing Capacitors « Reply #14 on: October 12, 2020, 09:44:22 PM » Author: HPSM250R2
Well , if they test fine leave them . Just keep a close eye on them Just in case . FYI capacitors should get warm while in use . I have one of those laser thermometers, it's great for instant temp checks of almost anything .

 One more thing , when you fire up the OV-25 , also check the amp draw once the lamp is fully run up . When I had the bad capacitor in the 700w MV OV-50 , the amp draw was WAAAAAY higher than it should have been due to the defective cap .

I don't even have any instruments for testing amp draw, capacitance, and things like that. I'd have to buy one. First I'd have to know what I'm looking for when I go to the store lol.

With this being my only original one ill likely just leave it be for now.

Do you know where I can get the type of bolts that were original to the fixture? Not sure what type they are but they're not silver in color. Kind of yellow/ gold.
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