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How can I find out the value of this burnt-out resistor from a power supply?

burnt-out resistor

I've tried a few online resistor calculators and every single of them gives different values ranging from 0.1 ohms to one gigaohm. I've tried substituting gray for silver, reading the bands from both directions and leaving out the last band. My multimeter gives readings over 2 MOhms.

Edit: The resistor was on the hot primary side of the power supply, residing between the transformer and some SMDs on an otherwise through-hole design. A 0.1 ohm shunt resistor for current sensing is a likely candidate. Looks like many if not most online resistor calculators freak out when they see a gold or silver band where they would expect a normal color.

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  • \$\begingroup\$ I wouldn't trust what the DMM says, even as a ballpark. \$\endgroup\$ Feb 1, 2014 at 12:54
  • \$\begingroup\$ I would read it as either brown-black-silver-gold (0.1 ohm, 5%) or possibly red-black-silver-gold (0.2 ohm, 5%). Probably a current-sensing resistor of some sort. Can you reverse-engineer the board it came out of at all to get an idea of how it was used? \$\endgroup\$
    – Dave Tweed
    Feb 1, 2014 at 13:12
  • \$\begingroup\$ The bad news is that this resistor was connected to something that didn't turn off because it's fried, most likely. \$\endgroup\$ Feb 1, 2014 at 13:18
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    \$\begingroup\$ Speculation: The silver and gold bands indicate a fractional Ohm value, so it was possibly a current sensing shunt resistor. Since we are all lazy, current shunts are most often selected as 0.01, 0.1, 1 etc, to make the calculation simple. Thus, my contention is that this was a 0.1 Ohm resistor. \$\endgroup\$ Feb 1, 2014 at 13:20

2 Answers 2

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Looks like brown black silver gold black.

According to this web page, if the fourth band is gold or silver, the 5th band is temperature coefficient.

That makes this 0.1 ohm, 5%, 250 ppm/K, which makes perfect sense for something like an overcurrent sense shunt.

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There are several difficulties with reading resistor colour codes.

  • Sometimes colors can be difficult to distinguish. Even more so after the resistor has overheated which may shift the colours.
  • There are multiple different "5 band" codes, 3 digit + multiplier + tolerance, 2 digit + multiplier + tolerance + tempco and apparently some resistors where the fifth band indicates that the resistor is "non-inductive".
  • While in principle there should be a wider space between bands to avoid backwards reading this is often not the case in practice.
  • You are trying to identify a burned-up resistor, this may well change the colors.

There are however also things we can use to see if a particular interpretation is plausible.

  • resistors come in standard values, defined by the E series, most notablly E24 and E96*. E96 values will need three value digits.
  • gold and silver can only be multiplier or tolerance, they can't be value bands or tempco.

We see in your picture two bands that clearly look metalic, gold and silver. This quickly narrows down the possibilities.

  • Reading from left to right is out, that would make the second value band gold, which is not allowed.
  • Reading from right to left with 3 value bands is out. That would make the third value band silver which is not allowed.

So the only remaining option is reading from right to left with two value bands, multiplier, tolerance and the final band being tempco, non-inductive indication or something else special.

Brown, black, silver would be 0.1 ohms. 0.1 ohms is a standard value and is not unreasonable in a power supply. Unfortunately red, black, silver is 0.2 ohms which is also a standard value and overheating could easily turn a red into a reddish brown.

* E6, and E12 are subseries of E24. E48 is a subseries of E96. There is also E192 but that is getting rather esoteric.

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