In The Three Numbers, Ohm’s law gave us 473 Ω for the LED’s resistor, but we used a 470 Ω one. Why not 473 Ω? And on a real bench, how do we even know a resistor’s value? There is no label beside it. The value is painted on the resistor as coloured stripes, and this is called the colour code.
A common resistor with wire legs, striped blue, grey, brown and gold.
Stripes and ends
- 1st and 2nd stripes
- Read them as the first two digits of the value. Here blue is 6 and grey is 8.
- 3rd stripe
- Write this many zeros after the two digits. Here brown is 1, so one zero, and the value is 680 Ω.
- tolerance stripe
- It shows how close the real resistance is to the value of the other stripes. This is called the tolerance, and we will come back to it later in this lesson. Hold the resistor with this stripe on the right, and read the other stripes from the left.
- ends
- It can go into a circuit either way round, because it has no + or − end.
The chart on the bench below gives the digit for each colour. Set the three wheels to match the first three stripes. There are three resistors to read, one after another.
The resistors are drawn much bigger than real ones, so their stripes are easy to see.
Some resistors are too small for stripes, like the tiny chip resistors inside phones and computers. Their value is printed on them as three digits instead. It works like the stripes, two digits followed by the number of zeros. So 331 is 33 followed by one zero, which is 330 Ω. Find the value of each chip below.
The small outline beside each chip shows its real size, about 2 mm long. If a value has a decimal point, an R is printed instead of the point, so 4R7 is 4.7 Ω.
On the bench below we have four cells, 6 V in total, and the ammeter again. The five resistors in the tray all have the same stripes, brown, black, red and gold, so all of them are marked 1 kΩ. Do you think all five will let the same current through?
The cells give a steady 6 V here, and the ammeter takes none of it.
As you checked, each one let a slightly different current through, so each one is a bit different from 1 kΩ. Resistors can never be made to exactly their marked value, and this is why they have a tolerance stripe. Gold means ±5%, which we read as plus or minus 5%. So a gold 1 kΩ resistor can be anywhere from 950 Ω to 1050 Ω. Each resistor’s real value is set when it is made, and heat and age change it only slightly.
Other colours mean other tolerances, and not every resistor has four stripes. Below are four 470 Ω resistors, each with a different tolerance. The ±1% one has five stripes, because it has three digits before the number of zeros.
Nobody makes a 473 Ω resistor, and there is no need to. A resistor with a gold stripe can be up to 5% off anyway, so values closer together than that would be of no use. Resistors are only made in certain values. These are called standard values, and for ±5% resistors they are about 10% apart. Here are the ±5% standard values from 240 Ω to 1 kΩ, each drawn with a band that shows its tolerance. The bench asks for three values, one after another. For each, which standard value would you pick?
The bands leave a few thin gaps, because the standard values are rounded numbers.
The nearest standard value was always close to the value we needed. So when Ohm’s law gives us a value, we pick the nearest standard value. If the current must never go above a certain value, we pick the next value up instead, because a bigger resistor lets less current through. For our LED, 15 mA was a target, not a limit, so 470 Ω is fine.
The bench only showed values from 240 Ω to 1 kΩ, but Ohm’s law can give us any value. The same two digits repeat ten times smaller and ten times bigger, so there are 47 Ω and 4.7 kΩ resistors as well. Between 100 Ω and 1 kΩ there are 24 standard values, and they are called the E24 series.
A ±1% resistor can only be up to 1% off, so its standard values can be much closer together. There are 96 of them between 100 Ω and 1 kΩ, called the E96 series. Many of them need three digits, like 249 Ω.
Which tolerance is good enough? It depends on the job. The LED circuit is on the bench below, with four trays of 470 Ω resistors, each tray with a different tolerance. Say our LED must get between 14 mA and 16 mA. With which tolerances does the current stay inside those limits for every resistor in the tray?
The 9 V battery has about 2 Ω inside it, and the LED has 1.9 V across it. Each tray’s three resistors are spread over its range in the same way, so the trays compare fairly.
The current changes by about the same percentage as the resistance, only the other way. For example, 5% more resistance gives about 5% less current. Our limits are about 7% above and below 15 mA, so a ±5% resistor keeps the current inside them and a ±10% one may not. If you are not sure, ±1% resistors cost about the same as ±5% ones today, and many builders use them everywhere.
Five questions, if you want to check yourself.
A resistor has the stripes brown, black, orange and gold. What is its value?
A chip resistor has 334 printed on it. What is its value?
A resistor is striped yellow, violet, brown and gold. Which of these could be its real value?
Ohm’s law gives us 250 Ω, and we use ±5% resistors. Which one do we pick?
A circuit needs a resistor that is never more than 3% from its marked value. Which tolerance do we choose?
How do we check what a real circuit is doing, part by part? We measure it. In The Meter we will learn how to use a multimeter, which measures resistance as well as voltage and current.
Lesson 5 done.
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