# Computing stuff tied to the physical world

## TK – Resistors

In Hardware on Apr 12, 2012 at 00:01

Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.

Yet another useful package from Conrad (NL #418714) – a set of 390 resistors from 10 Ω through 1 MΩ:

Resistors come in specific values and are based on a logarithmic range, i.e. you’ll see them organized as “E6″, “E12″, or “E24″, meaning that they are split up into 6, 12, or 24 values per decade, respectively.

Here’s some info about what’s in that above box:

This is actually mostly a subset of the E6 range (which is 10, 15, 22, 33, 47, 68) – see this Wikipedia article about preferred numbers for how and why things are organized that way.

The point is that you can never have enough resistors, which can probably be considered to be the most elementary components in electronics. Whether for limiting the current through a LED or creating a voltage divider, these things just tend to get used all over the place.

But what if you need a different value? Well, that’s often trivial: by using two resistors, either in series or in parallel, it’s often possible to get real close to the value you’re after.

The formula for two resistors in series is simply the sum of their values:

``````    Rseries = R1 + R2
``````

The formula for two resistors in parallel is slightly more complicated:

``````    Rparallel = (R1 x R2) / (R1 + R2)
``````

(this can easily be explained using Ohm’s law, I’ll be happy to write a post about this if you’re interested)

Here’s an online calculator which will find the proper values – although I recommend doing the math yourself, at least initially, because it will help you get a good grasp of how resistors work together.

1. If you’re doing purely digital stuff then you’ll quickly find there are some values which you use very very often.

For me that’s 330Ω (current limiting for LED and optoisolators driven from an ATmega) and 820Ω as a base current limiter for old faithful 2N3904 transistor used to switch larger loads than the ATmega can handle on its own.

Step out of digital world and you’ll be reaching for a whole range of them.

2. You can do a lot of analog electronics with just 1k, 10k, 100k values (maybe some trimpots too). 1M and up is more special-purpose, and prone to drift if the PCB isn’t clean. When I was learning, all my resistors had only 4 color bands (2 digits, multiplier, and tolerance) like the top example in your graphic. A 1k was always brown-black-red, plus some metallic band for tolerance off by itself on the other side. Now it seems like they typically have 5 bands (3 digits, multipler, tolerance) and without such a large gap to the tolerance band, which throws me off.

3. And a small trick in case you need exact values. Use a resistor with a just slightly lower value and use a sharp knife to scrape off a little bit of the resistor material until you have the exact value. Then use you’re wife’s favorite nail polish to seal it again and make it permanent. (Otherwise depending on the humidity the value will change over time.)