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Electronics => Beginners => Topic started by: R502 on July 25, 2019, 09:30:16 am

Title: Purpose of 3rd band in resistor being black.
Post by: 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.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: R502 on July 25, 2019, 09:52:20 am
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 (https://www.eevblog.com/forum/beginners/resistor-confusion/msg752842/#msg752842)
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Berni on July 25, 2019, 10:05:34 am
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
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Siwastaja on July 25, 2019, 10:29:11 am
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!
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Brumby on July 25, 2019, 01:16:29 pm
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.

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.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Brumby on July 25, 2019, 01:30:18 pm
To put it another way, imagine you have a spreadsheet where you put each of the three numeric values into 3 cells and then write a formula to calculate the value.

eg
Cell A1 is:2
Cell B1 is:7
Cell C1 is:3
Formula is:=(A1*10+B1)*10^C1
Result is:27,000 \$\Omega\$     (red/purple/orange)

Now, using that SAME FORMULA you would use zero for the third value if you wanted to show 27  \$\Omega\$   (red/purple/black)

This is the point of having the black for the multiplier in such a case - it's so we always use the same formula in deciphering.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: GerryR on July 25, 2019, 01:51:06 pm
The way we learned it back in the old days:
        1st color = number represented (Blk=0, Brn=1, Red=2, Orn=3, etc.)
        2nd color = number represented
        3rd color = number of zeros

Therefore, red, red, blk = 22 with no zeros = 22 ohms
                 red, red, brn = 22 with one zero = 220 ohms,  etc, etc, etc.
(4th band for tolerance on 3 band value, silver, gold, etc.)

Also, not politically correct these days, but to remember the color sequence:

Bad      = Black
Boys     = Brown
Rape    = Red
Our      = Orange
Young   = Yellow
Girls     =Green
But      = Blue
Violet   = Violet
Gives    =Grey
Willingly  = White




Title: Re: Purpose of 3rd band in resistor being black.
Post by: MarkF on July 25, 2019, 03:12:04 pm
Quote from: GerryR on July 25, 2019, 01:51:06 pm
The way we learned it back in the old days:
        1st color = number represented (Blk=0, Brn=1, Red=2, Orn=3, etc.)
        2nd color = number represented
        3rd color = number of zeros

Therefore, red, red, blk = 22 with no zeros = 22 ohms
                 red, red, brn = 22 with one zero = 220 ohms,  etc, etc, etc.
(4th band for tolerance on 3 band value, silver, gold, etc.)

Also, not politically correct these days, but to remember the color sequence:

Bad      = Black
Boys     = Brown
Rape    = Red
Our      = Orange
Young   = Yellow
Girls     =Green
But      = Blue
Violet   = Violet
Gives    =Grey
Willingly  = White

To expand on the theme for resistance values less than 10 and value tolerance:

Bad        = Black    0
Boys       = Brown    1
Rape       = Red      2
Our        = Orange   3
Young      = Yellow   4
Girls      = Green    5
But        = Blue     6
Violet     = Violet   7
Gives      = Grey     8
Willingly  = White    9

Get        = Gold     0.1  or 5%
Some       = Silver   0.01 or 10%
Now        = None             20%
Title: Re: Purpose of 3rd band in resistor being black.
Post by: R502 on July 26, 2019, 02:31:09 am
Ok thank you guys for the replies. I understand it a little better now.

And that is not a very nice mnemonic.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: fourfathom on July 26, 2019, 02:51:37 am
Quote
And that is not a very nice mnemonic.

Here's a nicer one:

Better Be Right Or Your Great Big Venture Goes West
Title: Re: Purpose of 3rd band in resistor being black.
Post by: notsob on July 26, 2019, 03:25:36 am
you can also load electrodroid app to you android phone which has lot of basic electronic help
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Siwastaja on July 26, 2019, 06:07:54 am
You know, the someone who came up with the colors was a genius. They follow the spectrum of the visible light. If you take it further, even black and brown fit there in a intuitive manner; that's how black bodies look like when you lower the temperature further from the red-glowing.

The only thing left to remember is the order of grey and white appended at the end of the rainbow, they are kinda extras.

I find this much easier and more intuitive than any word play, but YMMV. Maybe it's because I'm messing up with light-related things so the spectrum is very deep in my mind.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: MarkF on July 26, 2019, 11:46:59 am
Quote from: R502 on July 26, 2019, 02:31:09 am
Ok thank you guys for the replies. I understand it a little better now.

And that is not a very nice mnemonic.

Here are a few others from Wikipedia:
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Brumby on July 26, 2019, 02:25:15 pm
I just remembered: Black - Brown - rainbow - Grey - White.  Starting at Black, ending with White, the Brown and Grey just naturally fit in.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Zero999 on July 26, 2019, 03:07:52 pm
Quote from: Brumby on July 26, 2019, 02:25:15 pm
I just remembered: Black - Brown - rainbow - Grey - White.  Starting at Black, ending with White, the Brown and Grey just naturally fit in.
Yes, that's how I remember it.

The one our electronics teacher used to teach us way Bye by Rosie, off you go, but via Great Western.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Berni on July 26, 2019, 07:20:55 pm
Yeah i always used the reasoning from the rainbow with the black and brown sort of going into infrared as that's pretty much what it would look like it you swept the wavelength down so deep.The white and gray are so rare to see on resistors i never bothered.

Colorful ribbon cable also follows the same color sequence:
(https://i5.walmartimages.com/asr/8f1cd771-761d-4447-b101-d6445382784c_1.09960930fa38749ba45fcaac972a3ede.jpeg)
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Stray Electron on July 27, 2019, 02:08:40 am
Quote from: R502 on July 26, 2019, 02:31:09 am
Ok thank you guys for the replies. I understand it a little better now.

And that is not a very nice mnemonic.

  Then you would have HATED the original. Substitute the word "Black" for "Bad"
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Brumby on July 27, 2019, 02:15:57 am
Having learned the colour code (so many years ago I dare not even think how many) I was intrigued when the company I worked for at one time did a re-vamp of their filing system.  All new fittings and folders - with a colour-coded labelling system.

The alphabetic colour assignments were curious, however I didn't have any previous scheme to compare against, so that was just a learning curve.  However, I was extremely annoyed to find the creators of this system did not use the numeric:colour coding system that had been in place for more than 50 years - but created their own.  It had no logical structure.

If they had used proper colour coding, I would have been able to pick out a file from the other side of the room on day 1.  I am thankful I only had to deal with it on an infrequent basis.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Brumby on July 27, 2019, 02:18:16 am
PS.  The choice of blue bodies for metal film resistors was, I think, the worst decision possible.  Makes picking the colours very difficult at times.

Whoever agreed to that needs to be shot.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: T3sl4co1l on July 27, 2019, 08:10:59 am
Also, going the other direction: what if 5-band resistors dropped the black multiplier?  Then a 475 ohm resistor would look like 4.7 megs!

Likewise, dropping the multiplier on a 4-band marking would make e.g. 47 ohm 5% look like 4.7 ohm 20%.

Tim
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Zero999 on July 27, 2019, 09:24:21 am
Quote from: Stray Electron on July 27, 2019, 02:08:40 am
Quote from: R502 on July 26, 2019, 02:31:09 am
Ok thank you guys for the replies. I understand it a little better now.

And that is not a very nice mnemonic.

  Then you would have HATED the original. Substitute the word "Black" for "Bad"
The one I learned also substituted boys for bastards.

Generally more politically incorrect and offensive things are easier to remember, probably because they invoke a strong emotional response, whether it be disgust, humour or a bit of both.

Quote from: Brumby on July 27, 2019, 02:18:16 am
PS.  The choice of blue bodies for metal film resistors was, I think, the worst decision possible.  Makes picking the colours very difficult at times.

Whoever agreed to that needs to be shot.
I agree. I'm slightly red-green colour blind. I have deuteranomaly. This doesn't mean I can't tell the difference between red and green at all. I have both red and green cones on my retina. It's just the green cones have a slightly lower peak frequency response, than normal, so it's a little more tricky for me. Large blocks of solid colours are easy for me. I can distinguish between brown, red, orange, yellow and green fairly easily, in a well lit area. I struggle with darker shades of green, red and brown, subtle differences between shades of orange and green, poor illumination and small objects. I remember confusing the yellow stripe on a blue resistor for green. It was only when a college pointed it out to me and I looked at it through a magnifying glass, did it become obvious to me it was yellow.

On the flip side, there is some evidence to suggest those with deuteranomaly may be better at distinguishing between blue and green, than those with normal vision, since their green cones are shifted towards longer wavelengths, blue will stimulate then less, than those with normal colour vision.
Title: Re: Purpose of 3rd band in resistor being black.
Post by: Berni on July 27, 2019, 09:47:14 am
Im not color blind but i still find reading the colors on those blue metal film resistor pretty much hopeless. Quite often the color bands have a bit too little pigment in them so the blue background ends up mixing into the colors, making anything towards red just look like different shades of brown. Something that is very unhelpful given that most bands are yellow or below.

Then again even your usual tan colored carbon resistors are not perfect. Ones that have been running hot for a long time tend to darken the colors so orange can look like red, or red can look like brown. Also i noticed that some resistors use a pretty crappy orange color that looks too similar to red. Sure its easy to tell on a 3.3K where you get Orange Orange Red, but grabbing a 33K where its Orange Orange Orange can look a lot like a 2.2K where its Red Red Red
Title: Re: Purpose of 3rd band in resistor being black.
Post by: GerryR on July 27, 2019, 11:53:34 am
A 5X eye-loop makes everything a lot clearer, even against the light blue background.  Also, I always double-check with a meter if taking resistors out of a "pile" and not from the boxes they came, and are filed, in; keeps the smoke inside more often this way.  ;)