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I found a huge cap-like looking part in an old parts bin of mine. I tried making sense of its marking and I don't really understand what they mean. I googled for the part number and I found eBay listing and such, suggesting it's indeed a cap according to the eBay category, but even there I didn't find further information about the specs of this thing (somewhere it was suggested it's a 40uF 1250V cap but I don't see 40 anywhere written on the part).

Testing it with my component tester delivers very weird results: it's detected as a diode but also with 1.8mF capacitance...?! (usually the tester works nicely and rarely lies to me)

Can you help me identifying this?

Here is the picture: Picture

EDIT: Interestingly, while playing with it again, the tester detected it once as capacitor, with 1759uF and 7.9 ohms ESR. But I can't reproduce it, now it's back to the diode rubbish... Anyway, I don't get how to read these markings correctly.

EDIT2: Multimeter gives me 1755uF.

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3 Answers 3

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"1250 MFD" is 1250 µF — it's a capacitor.

"50 W.V." is 50 working volts.

Specifically, it's an electrolytic capacitor. If your tester is feeding it AC, it might identify it as a diode, because of the reverse leakage current.

When it does identify it as a capacitor, it isn't surprising that the value is off by quite a lot. Tolerances weren't tight to begin with, and this unit has some age to it, which probably means a lot of drift in the value.

You could try connecting it to a power supply of about 25V or so for a while to see if this re-forms the oxide layer. Pay attention to the correct polarity! If the electrolyte hasn't dried out completely, this might allow your tester to identify it more reliably.

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    \$\begingroup\$ remember to discharge it before connecting the tester \$\endgroup\$
    – Jasen Слава Україні
    Commented Jun 11, 2016 at 10:46
  • \$\begingroup\$ What does the acronym MFD stand for? Maybe "micro farad device" or something? \$\endgroup\$
    – Electrical Architect
    Commented Jun 11, 2016 at 10:56
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    \$\begingroup\$ Micro FaraD as I understand it. it dates from before µ was a common symbol in fonts. \$\endgroup\$
    – Jasen Слава Україні
    Commented Jun 11, 2016 at 11:56
  • \$\begingroup\$ looks like there's a date code there too 3-28-75, looking very "manual rubber stamp". so probably made in USA \$\endgroup\$
    – Jasen Слава Україні
    Commented Jun 11, 2016 at 12:02
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    \$\begingroup\$ A common tolerance on larger electrolytic capacitors is -20/+80%. The measured 1800 µF is well within the specs. \$\endgroup\$
    – pipe
    Commented Jun 11, 2016 at 13:51
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The Cornell Dubilier Electronics logo is marked below the 50 W.V.

enter image description here

That company was established in 1909: CDE History

The company is apparently now part of Kemet Corporation.

I suspect that the "R" in a circle to the right below "Computamite" indicates that word is a registered trademark. It probably designates "computer grade electrolytic." I don't think that term is recognized as a standard by any standard publishing organization, but it may have an informal "industry consensus," (some would say honorific) meaning.

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it looks like a capacitor to me. it says on the can 50V (working) 1250 Micro Farads

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