Elektor_Mag_-_January-February_2021

38 January & February 2021 http://www.elektormagazine.com

Resistors
Resistors are classified as passive compo-
nents because (in contrast to active compo-
nents) they cannot amplify signals. Passive
components include resistors, capacitors
and inductors; active components include
transistors, triacs and, yes, diodes are also
considered to be active components. This isn’t
correct, strictly speaking (a diode, after all,
does not amplify), but because we tradition-
ally pile all the semiconductors into one great
heap, diodes are also called ‘active’.

Putting this aside, let’s first take a good look at

what resistors actually look like. In Figure 1 we

have portrayed the most common examples.

At the bottom you will see a metal-film resistor

and a carbon-film resistor above that, both

with a power rating of 0.25 W. The power

rating indicates the maximum power that

a resistor can convert into heat without the

resistor failing. This is something to keep an

eye on!

In addition to the maximum power rating we

also need to consider maximum voltage rating.

If this is exceeded, the resistor can break-over

and it will likely fail. With the common-or-gar-

den resistors that are typically used in hobby

projects, the maximum voltage is generally

rated at 250 V, so we don’t have to worry too

much about this in practice.

Back to Figure 1: in the middle on the left

are two resistors with a higher power rating

(0.5 W and 1 W) and, right at the top, a cement

version with a resistance of 39 Ω that can

handle 11 W. The power resistor in the middle

on the right is a type that can be bolted to a

heatsink that can handle up to 25 W.

Normal resistors for regular use are small (the

bottom two in Figure 1) — so small in fact that

it is impossible to print their resistance value

on them in a legible form. This is why the value

is indicated with coloured bands. These days

there are much smaller components available,

known as SMDs or Surface Mount Devices

but, because this series of articles is aimed

at the electronics beginner and SMDs are

not really suitable for an initial introduction,

we will not go into these. We will stick with

trusty ‘leaded’, easily-handled components.

The colour code

There are resistors with both four and five

coloured bands. The first two or three bands

indicate the value of the resistor. This has to

be multiplied by a (power of 10) factor that

is indicated by the penultimate band. The

last band indicates the tolerance. Tolerance

— what’s that? When we take a batch of

1000 Ω resistors and measure them with a

very accurate ohmmeter, we will find that no

two resistors have the exact same value. For

example, one will be 1001.3 Ω, and another

998.6 Ω, an so on. This is because of unavoid-

able small variations during manufacturing.

The tolerance indicates the limits of the range

of values that the resistor can have, with the

actual value lying somewhere in between.

For a 1000-Ω resistor with a tolerance of 5%

(this is a ‘normal’ carbon-film resistor) the

actual value is somewhere between 950 Ω

and 1050 Ω; with a 1% version (a metal-film

resistor) the actual value is between 990 Ω

and 1010 Ω. There is also a chance that outliers

exist with values outside the tolerance range,

but this does not happen very often.

By the way, for most applications the 5%

carbon-film resistors are good enough. Only

when it is really necessary to be very accurate

(such as filter circuits) and/or when the circuit

must be as low-noise as possible (a high-end

audio amplifier) do we use the, more expen-

sive, metal-film resistors.

Table 1 lists the colour codes for resistors. A

1% metal-film resistor with a value of 1 kΩ has,

according to Table 1, the colour code brown,

black, black, brown, brown. Unfortunately,

in this case it is not immediately obvious in

which direction the colour code should be

read — in reverse it could be a resistor of

110 Ω. However, don’t give up too soon —

practice makes perfect and in the event of

doubt an ohmmeter can be the final arbiter!

Another disadvantage of the coloured

bands is that the colours red and orange

Starting Out

in Electronics (6)

Easier Than Imagined!

By Eric Bogers (Elektor Netherlands)

As promised in the previous

episode of this series, we

will now occupy ourselves

with ‘tangible’ components

— with resistors, to be more

specific. A great deal more

can be said about them

than that which you may

initially appreciate: there

are resistors, resistors, and

resistors...

basic course

Figure 1. A few resistors.