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I'm having trouble to identify this (presumably broken) part for replacement:

Unknown resistor/capacitor

It was used in a broken LED power supply, with 220-240 V AC on the input and 120-200V DC on the output. I suspect this part is broken, because there is always 230 V across it. One side of it is attached directly to the mains input. Its resistance appears to be infinitely high (measured resistance up to 20M, max. range of my Ohmmeter).

It looks like a resistor, but wikipedia tells me it could be a capacitor as well (source: http://en.wikipedia.org/wiki/Electronic_color_code#Capacitor_color-coding)

I have tried to figure out the value and I came out at a 1000V - 0.05 Farads capacitor, but I find this hard to believe. Can anyone confirm? Thanks!

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2 Answers 2

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I'll be an outlier here and guess that it's a (blown) 0.5A "pico fuse" style fuse, and I'd be pretty sure if it's the first thing connected to the mains wire.

Break it apart you'll quickly see what its made of (and can easily rule out inductor, if there's no copper wire inside). A fuse and a resistor may not be easy to distinguish.

If it is a fuse, and if it's blown, there's a significant chance other stuff is blowed up good. Poke around and check the power semiconductors and diodes.

Edit: For those who have little experience with Asian manufacturing, here's an example of the type of fuse, mostly supplied by Chinese manufacturers:

enter image description here

The 'cement' finish in the OP's photo (rather than the smooth lacquer you'd expect on an inductor or resistor) is another not-so-subtle clue as to the functionality.

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    \$\begingroup\$ Indeed, it appears to be a fuse! I replaced it with a temporary 800mA fuse (smallest one I had laying around) and tried my luck... The LED's are working :) Circuit draws around 120 mA so I guess nothing's wrong with the other circuitry in the power supply. Thanks a lot everyone for your answers! \$\endgroup\$
    – Stephen B
    Aug 14, 2014 at 17:11
  • \$\begingroup\$ That is NOT a fuse. Fuses are marking with numbers and letters rather than color codes. Pico fuses are here: littelfuse.com/products/fuses/axial-radial-thru-hole-fuses/… Note that there are no color codes on the. Putting a fuse in in place of the inductor might work (obviously does work) but what ever the inductor was supposed to be doing isn't being done. \$\endgroup\$
    – JRE
    Aug 14, 2014 at 19:13
  • \$\begingroup\$ @JRE You are NOT correct. Such fuses do exist and photos can be found on the net. I carefully referred to the style of fuse not the exact type and manufacturer for that exact reason. Jeesh... \$\endgroup\$
    – Spehro Pefhany
    Aug 14, 2014 at 19:24
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    \$\begingroup\$ I can confirm it isn't an inductor, I scraped off the color coding and found no copper wire inside, but rather a gray kind of material, looks as if it's solid but it probably isn't. \$\endgroup\$
    – Stephen B
    Aug 14, 2014 at 21:17
  • \$\begingroup\$ @StephenB Thanks for the feedback, it's always nice to get confirmation. \$\endgroup\$
    – Spehro Pefhany
    Aug 14, 2014 at 21:36
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With the silver and gold bands being where they are, it appears to be a four-band color coded resistor.

With the first band being green, the second band brown or blue, the third band silver and the fourth band gold, it would be either a 0.51 or 0.56 ohm +/- 5% resistor.

Judging from the body's rough finish I'd guess that it's a power resistor, and the fifth band - while not standard - is the manufacturer's mark denoting either power or voltage rating.

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  • \$\begingroup\$ Hmmm... I'm currently more convinced that this is an inductor, because I don't really know what the function would be of a resistor directly connected to the mains input. I think it is an inductor to be used as a filter. But I could be wrong! \$\endgroup\$
    – Stephen B
    Aug 14, 2014 at 13:33
  • \$\begingroup\$ The definitive way to tell would be for you to a little reverse engineering and post what the front end of the supply looks like \$\endgroup\$
    – EM Fields
    Aug 14, 2014 at 13:51
  • \$\begingroup\$ I don't think it is a resistor. Check the chart you referenced. Only bands 4 and 5 (multiplier and tolerance) can be silver or gold - and the part shown has silver for band 3. \$\endgroup\$
    – JRE
    Aug 14, 2014 at 14:16
  • \$\begingroup\$ Check the four-banded resistor on top of the chart and you'll see that the entry into the column for the third significant digit doesn't exist. \$\endgroup\$
    – EM Fields
    Aug 14, 2014 at 14:24

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