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Monday, 3 September 2012

Learning to Read Resistors

I decided recently to make an effort to learn a bit of electronics, using a kindly donated Arduino Uno and a few electronics components that we had around the house. The choice of resistors this gave me was seriously limited, so I popped over to Maplins and bought a pack of twenty different resistor types. To my dismay, when I got them home, I realised that the only indications of their strength were the colour bar codes painted on their sides.

No doubt if I continue with my current interest I will grow to be able to read these codes as if they were English text. At the moment though it's rather harder to decipher them. A quick bit of googling found sites like this one that provide handy converters; however, to use these tools you have to be able to read the colours off the resistors in the first place, and there are a couple of issues here that I would like to raise.

Firstly, it is by no means obvious in which direction to read the bars. Supposedly there should be a bigger gap between the last bar and its immediate neighbour than between the first bar and the second. In my experience so far this difference is usually not noticeable, and possibly non-existent. Secondly, the choice of colours, while naturally restricted to what the limits of human eyesight have provided, does allow for an incy-wincy bit of ambiguity. Telling the difference between white, grey and silver, when only one of them is present on the resistor, is no laughing matter. Gold and orange are another two colours that some might think are rather similar.

These codes have been around for a long time, so I'm probably going to look back on this post with amusement at some point (oh, please, please be sooner rather than later), but for now I find the best way of decoding them is to measure the resistance roughly using my multimeter (had one for years, absolutely no recollection of why I needed it), and then matching that value with the colours (bright light and a magnifying glass very useful at this point).

It took a while, but this is my list of resistor strengths from Maplin's "Lucky Bag" of resistors:

5.62Ω, 24Ω, 43Ω, 78.9Ω, 113Ω, 287Ω, 309Ω, 422Ω, 1.18KΩ, 1.62KΩ, 3.6KΩ, 5.6KΩ, 10KΩ, 15.4KΩ, 33KΩ, 97.6KΩ, 115KΩ, 261KΩ, 619KΩ, and 2.4MΩ.

There aren't many values there that I would have expected. Whether there's a reason for such obscure numbers, or whether that's why it's called a 'lucky' bag is currently an open issue as far as I'm concerned, and clearly further research is called for.

Working out the last few values was considerably simpler than the first ones I did, as I started to remember which colour matches which digit. There are acronyms to help, and googling "resistor colour code acronyms" throws up several. I was appalled to see how many (including the top result) involve sexual violence towards women, with a couple of people mentioning that that was the acronym they were taught at school!

A few decades ago, in a male-dominated field, this might have been acceptable (though still very wrong), but surely in the 21st century people would think twice before posting this crap onto the web as a learning aid?

For the record, the one I picked to learn is

Bad Beer Rots Our Young Guts But Vodka Goes Well

which also incorporates some health advice. Not sure about the vodka message though.

1 comment:

  1. I suspect you have the E12 range of values. These resistors have a tolerance of 10%, and the values are chosen to cover nearly every possible value within 10%. 100R covers 90-110, 120R covers 108-132, 150R covers 135-165 etc.
    You can get more precise series of resistors with more values