More than one color to resistor number, what to do?
December 23, 2012 12:40 PM   Subscribe

Resistor color coding (the electronic color coding) has more than one color combination to resistor value. How do you know which one to use?

I'm a lackluster soldering noob who's trying to get used to basic electronics, and thus have started to rewrite this calculator as a learning exercise.

This ran into the issue that you can have a resistor of (brown, black, brown) which has the same value, 100 ohms, as a resistor of (black, brown, red).

What's the rule for determining a 'canonical' color code - should you maximize the exponent? Or minimize it? I'm finding it hard to find the answer from Wikipedia and such. Thanks!
posted by tmcw to Technology (4 answers total) 2 users marked this as a favorite
I'm sure there's a scheme for it, but I can usually guess the orientation based on spacing between bands and overall position of the markings on the resistor body. It's a bit fuzzy, sorry.
posted by krilli at 12:48 PM on December 23, 2012

If you look at the resistor coding scheme you'll see the first band determines the first significant digit. Leading zeros are never significant, so your code of (black (0), brown (1), red (2)), ie (0*10 + 1) x 102 is not legal since you are not permitted to code a leading zero.
posted by RichardP at 12:49 PM on December 23, 2012 [9 favorites]

What RichardP said. As far as I know, leading with a zero (black) band is only legitimate in the case of zero-ohm resistors.

The reason you're being confused by the resistor calculator is because it (apparently) does no checking to ensure that the band colors you're entering would be encountered in real life.
posted by neckro23 at 3:46 PM on December 23, 2012 [1 favorite]

As RichardP says, you want to minimize the exponent, so that the leading digit is not zero. Also (as krilli said) the tolerance band is usually set off a bit from the other bands.

In practice though, you never see 20% resistors any more (manufacturing techniques have improved), and 5% and 10% resistors use a tolerance band color (gold or silver) that isn't used for the value marking, so it's only the high-precision resistors that are ambiguous.
posted by hattifattener at 7:37 PM on December 23, 2012

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