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Purpose of 3rd band in resistor being black.

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R502:
I'm just learning about electronics. What is the purpose of the 3rd band being black? Is it simply because there must be a 3rd band? I don't understand the purpose of multiplying by 1. Because it will always be itself, won't it? Just something that I was curious about. Maybe I'm just being dumb.

R502:
I found this forum when searching for an answer. This post from this thread.

"Divide by 1? That's rather useless. But I do understand why they do it, as I'd rather multiply by 1 which seems equally useless... but isn't..."

https://www.eevblog.com/forum/beginners/resistor-confusion/msg752842/#msg752842

Berni:
Because having resistors with 2 color bands would only make things more confusing.

This 3rd band is also consistent with how troughhole ceramic capacitors, tantalum capacitors,SMD resistors...etc are marked with numbers.

1 KOhm = 102 = Brown(1) Black(0) Red(2)

Also say we used your 2 band code what happens when you get a resistor with colors Brown Green? Depending on how you turn the resistor you can get:
Brown(1) Green(5) = 15 Ohm
Green(5) Brown(1) = 51 Ohm
Both of these are values that commonly exist as they are within the E24 ladder

Siwastaja:
The color codes are of fairly little importance, and are heavily overemphasized in most beginner teaching material. This is legacy from the seventies.

They are often hard to decipher since resistors that add fourth, fifth or even a sixth color band exist.

Then, in some cases, it may be hard to figure out the order you need to read them in. When the tolerances were 10% or 5% typically in the 1970's, the silver and gold stripes were easy to recognize, but today, many resistors are of 1% tolerance, hence brown for tolerance, but brown is also a very common color for the first value stripe (any value starting with 1).

It tends to be quicker to just measure the resistance using a multimeter. (Remember: for high-value resistors, don't hold both terminals by your hands at the same time.)

In practice, you buy resistors by values, then organize them in bins, boxes, drawers, plastic bags, whatever floats your boat - and you have marked the part number you have, so you can refer to the full specsheet later for important information like power handling, not specified by the color codes!

Brumby:

--- Quote from: R502 on July 25, 2019, 09:30:16 am ---I'm just learning about electronics. What is the purpose of the 3rd band being black? Is it simply because there must be a 3rd band? I don't understand the purpose of multiplying by 1. Because it will always be itself, won't it? Just something that I was curious about. Maybe I'm just being dumb.

--- End quote ---

The purpose of the 3rd band being black is exactly the same purpose for the 3rd band being red or orange or green.  It is the multiplier - being a power of 10.

All you need to remember is:
 1. The numeric value associated with each colour: Black = 0, Brown = 1, Red = 2, etc...
and
 2. The positional significance
       - for a 4 band resistor, position 1 is the first digit, position 2 is the second digit, position 3 is the multiplier (power of 10) and the fourth position is tolerance.
       - for a 5 band resistor, there are 3 digits, a multiplier and a tolerance

By labeling components following such a scheme, there are no "exceptions" - you just have to get used to the idea that when the multiplier comes into play - which it does - a multiplier of 1 is JUST AS VALID as a multiplier of 100 or 1,000.

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