# Society of Robots - Robot Forum

## Electronics => Electronics => Topic started by: Redcap on March 08, 2011, 04:57:00 AM

Title: [Solved] Help working out Resistor Values (Colorblind)
Post by: Redcap on March 08, 2011, 04:57:00 AM
Hey all,

I've found some resistors laying around, but the value has rubbed off the container I had them in.

Because I'm colorblind, I find it very difficult to (correctly) tell what color the bands are in order to find out the values.

Is there any way I can find the value without having to rely on the color bands..? :(

Redcap
Title: Re: Help working out Resistor Values (Colorblind)
Post by: Redcap on March 08, 2011, 05:14:10 AM
I know I can use a multimeter to find out how much resistance it has, but how do I know if this is within the 10% tolerance or if the resistor is damaged?
Title: Re: Help working out Resistor Values (Colorblind)
Post by: knossos on March 08, 2011, 06:24:59 AM
You are probably aware of the fact that resistors come in different preffered number ranges (e.g. E6 series, E12, E24, etc.).  Commonly, the E6 series is used for 20% tolerances, E12 for 10%, and E24 for 5%.  Most color blind people can still typically see the differences on the tolerance bands (None, Gold, or Silver).  If they are 10% tolerance resistors that would limit the values to the E12 range: 10 12 15 18 22 27 33 39 47 56 68 82.  With only 12 possible combinations per decade, your best guess on the color values (even if color blind) combined with a multimeter reading is likely to give you a pretty good idea of the correct value, particularly if you have several of the same value resistors.

For example, you have a resistor with no tolerance band making it a 20% tolerance resistor and placing it in the E6 series (10 15 22 33 47 68).

The first two bands could be red or orange, but you can't tell.  RED-RED would be 22 which is part of the E6 series,  RED-ORANGE or ORANGE-RED would be 23 or 32 respectively, which aren't part of the E6 series so we can discount those possibilities.  ORANGE-ORANGE would be 33 which is also part of the E6 series.

So given 22 or 33 as the possible resistor value.  With a 20% tolerance that would give us a range of 17.6 to 26.4 for RED-RED or 26.4 to 39.6 for ORANGE-ORANGE.

Lastly we look at our multiplier band and compare with a multimeter reading.  For example if its yellow, you would have a multiplier of 10^4.

Your multimeter reading is 36.2Kohms, which places it solidly in the ORANGE-ORANGE range.  After measuring a couple more resistors they all fall within the ORANGE-ORANGE range so you can be confident that they are all ORANGE-ORANGE-YELLOW resistors and are all within the 20% tolerance.
Title: Re: Help working out Resistor Values (Colorblind)
Post by: Admin on March 08, 2011, 08:39:54 AM
I know I can use a multimeter to find out how much resistance it has, but how do I know if this is within the 10% tolerance or if the resistor is damaged?
You only need to know tolerances for manufacturing purposes, meaning you can be sure that the batch will fit within a certain range.

The multimeter will give you an exact value, making the tolerance a useless value for a small project.

That said, if you measure a few thousand resistors and write down the value of each, you'll find them within X% off from a specific value, giving you the tolerance.
Title: Re: Help working out Resistor Values (Colorblind)
Post by: Redcap on March 09, 2011, 04:40:39 AM
Thank you both very much for your replies :)

knossos, thanks for taking the time to write that reply. You've made a lot of things "click" and have given me a very solid starting point for future reference.

-Re-reading what I wrote in my question, I've worded it badly, but both have given answers I was needing, cheers :)