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testing capacitors

stormin norman

Dec 1, 2011
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I recently bought a Southwire testor so I had something to check small capacitors with. On some units I get a OL reading...the manual does not address this reading. Anyone know what/why this is?
 

Fish4Fun

So long, and Thanks for all the Fish!
Aug 27, 2013
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I would assume "OL" = "Outside Limit", "Over Limit".... or something similar...ie the capacitor being tested is not within the selected range of the meter. Typical capacitance meters have at least two "ranges", much like an ohm meter // voltage meter // current meter....if for example it only has two "ranges", Hi/Lo and you attempt to test a 10,000uF capacitor with the range set to "Lo", then the display will report that the capacitor in question is "Outside the Limits" of the range....At least that is the way MY meter works, lol.

Fish
 

stormin norman

Dec 1, 2011
41
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I would assume "OL" = "Outside Limit", "Over Limit".... or something similar...ie the capacitor being tested is not within the selected range of the meter. Typical capacitance meters have at least two "ranges", much like an ohm meter // voltage meter // current meter....if for example it only has two "ranges", Hi/Lo and you attempt to test a 10,000uF capacitor with the range set to "Lo", then the display will report that the capacitor in question is "Outside the Limits" of the range....At least that is the way MY meter works, lol.

Fish
All right...so after digging out the manual that came with this I read that OL means overload/overrange. Which really kinda sux as the caps I am testing are only a 470uF 35V. These I get the OL display on. Did I buy the 'wrong' testor if I can't even read these 'small' caps?
 

Colin Mitchell

Aug 31, 2014
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In electronics terms these are not "small" capacitors. We normally think 100p is small.
Provide the meter catalogue number and the range you are testing the capacitor on.
 

stormin norman

Dec 1, 2011
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In electronics terms these are not "small" capacitors. We normally think 100p is small.
Provide the meter catalogue number and the range you are testing the capacitor on.
I am using a Southwire 10040N Autoranging Multimeter. In the capacitance setting the meter will auto adjust between nF and uF. There is no referance given to actual range it will measure within.
edit here: I just opened email and the customer service dept at Southwire tells me the following.
The 10040N is capable of testing capacitors to 100µF. Check to see if the capacitor is bigger than 100µF. Another possibility is the capacitor is 100µF or less and is open, which will also cause the meter to display OL because it’s not seeing anything.
 

Colin Mitchell

Aug 31, 2014
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A very poor multimeter. The actual max capacitance is not specified but it has a reference to 100u.
You can buy the meter on Amazon:
1 new from $32.95 1 used from $45.00 5 broken from $65.00

I would buy all the broken meters.


The only way to measure electrolytics above 100u is to place two 100u in series and record the reading.
If it read 50u, you can ASSUME both are 100u.
Now put the 470u in series with the 100u and go to the web and look up the value of 100u and 470u in series to produce the result.

I bought a cap tester on eBay from Thailand for $18.00 and it goes up to 1,000u.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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An easy way to "guesstimate" the value of large capacitors is to charge them to constant voltage, remove the charging supply, and connect a fixed resistor across the capacitor. The time required for the capacitor to discharge through the resistor to 37% of the charging voltage is equal to one "time constant" which is the product of the discharge resistance in ohms and the capacitance in farads.

For example: charge a 500 μF capacitor to 10 V and connect a 100 kΩ resistor across it. The product of 500 x 10^-6 F and 100 x 10^3 Ω is 50,000 x 10^-3 seconds or 50 seconds, at which time the voltage will now be 3.7 V. If you leave your POS multimeter connected while discharging through the 100 kΩ resistor, it adds a parallel load of 7.8 MegΩ, which is insignificant compared to your ability to watch the meter and operate a stop watch at the same time. If you can measure the time to within a few fifths of a second, you can calculate the capacitance in farads by dividing the time in seconds by the resistance value in ohms.

You can actually leave the resistor and voltmeter connected to the capacitor while charging the capacitor. Just remove the power supply connection to the capacitor and start your stop watch at the same time. When the voltmeter indicates 37% of the initial charging voltage stop your stop watch. That's one time constant. Divide by the resistance in ohms and you have the capacitance in farads, accurate to at least one significant figure, maybe two.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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... Did I buy the 'wrong' testor if I can't even read these 'small' caps?
Well, you get what you pay for, and you bought a meter that only reads up to 100 μF.

The actual capacitance of "large" capacitors is usually not important. What is important is ESR (effective series resistance), leakage current at a specified operating voltage, the operating or working voltage, and the maximum ripple current (if used as a power supply filter or to couple a significant amount of power). Almost all large electrolytics, when new, will measure more than their stated capacitance. Check catalog and/or spec sheets for allowable tolerance. Don't be surprised to find -5% to +20% as typical tolerance.
 
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