Components, Part 2: Resistors and Their Variants

The resistor is probably the most common electronic component. I’d even venture to say it’s also the oldest component we still use. So I’ll start my detailed look at different electronic components with it.

Let me say right away that I’ve decided not only to talk about different electronic components, but also to suggest ways to use them. We’ll look at some simple but useful circuits as examples. Each topic will have its own article, and together they’ll make up a sort of course for beginners just starting out in the world of electronics.

A resistor, also called a resistance, is used to introduce a specified resistance into an electrical circuit.
Resistors set the operating conditions of other components, limit current (for example, through LEDs), and help measure current by measuring the voltage drop. They’re also used as loads and even in ADCs and DACs, but we’ll cover all that later. For now, let’s get to the subject of this article.

There are many different types of resistors, for example:
Varistor — resistance depends on the applied voltage
Thermistor — resistance depends on temperature
Photoresistor — resistance depends on the light level
Strain gauge — resistance depends on the amount of deformation
Magnetoresistor — resistance depends on the strength of the magnetic field.

In a sense, even an ordinary light bulb is a resistor, but its resistance depends strongly on the current flowing through it: as the current increases, the filament heats up and its resistance rises. This and the types listed above are examples of nonlinear resistance.

But in the ideal case, an ordinary resistor is perfectly linear, meaning its parameters don’t depend on anything. Of course, in reality there will be some influence, since nothing in the world is perfect. We’ll talk about that too, but next time.
That’s why Ohm’s law for instantaneous current and voltage values is generally considered valid only in resistive circuits.

On circuit diagrams, a fixed resistor (one with constant resistance) is shown in one of two ways:
1. As a rectangle (European style)
2. As a zigzag line (American style).

The unit of resistance is the ohm. Multiples are used: 1,000 Ω = 1 kΩ, 1,000 kΩ = 1 MΩ. Note that a value of 1 milliohm is written as mΩ, which looks a lot like MΩ, so don’t mix them up.
Electronic devices most often use resistors with values from 1 Ω to 1 MΩ, although many values above and below this range exist and are used much less often.


In the American style, the power rating is usually shown next to the resistance value; in the domestic style, it’s indicated by stripes.
Though these days, nobody really marks them that way anymore; they just use a rectangle.


Over the years, resistors have been made in all sorts of forms, and there are still an enormous number of types in production. But in most devices, you’ll probably see something like the one in this photo.


But resistors like these exist too. They’re made from a piece of high-resistance wire, such as nichrome, manganin, or constantan.
These resistors are often used as shunts or together with power transistors, for example in power amplifiers.


Sometimes an ordinary resistor is used as a former for winding wire, but you need to be careful here. The same approach is sometimes used when making an inductor. The difference is that an inductor usually has more turns and is wound with copper wire. Copper isn’t used for resistors because its resistance changes significantly with temperature.
The resistor is on the left, the inductor on the right.


More common resistors are cylindrical, with different diameters and lengths, and leads coming out of each end. The body can sometimes be square or rectangular, but that’s much less common.
There are also resistors with more than two leads, such as shunts for measuring current. There are also resistor networks containing several resistors, for example in a multimeter’s voltage divider.


There can be a catch here too. For example, in the photo the old varistor is on the left and the relatively new resistor is on the right. They’re the same size and differ only in color and markings.
The varistor in the photo is very old, but that doesn’t really change the point: manufacturers sometimes use the same body for different components to save money.


But probably the most common type is the low-power resistor. These are the ones you see on many circuit boards.


In recent years, leadless resistors have become increasingly common. The photo shows 2512, 1206, 0805, and 0604 resistors. The 1206 and 0805 sizes are the most convenient for hand assembly; 2512 resistors are more expensive and used less often, while 0604 resistors are too small.


Most resistors can be divided into two construction types.
Wirewound.


These use wire made from a high-resistance alloy. You can also make out a few turns of wire on the resistor on the right.


And metal-film resistors. These have a thin film deposited on a ceramic substrate.


If you remove the protective coating, you can see the turns in the conductive film that form the resistor.
There are conductive end contacts at either end, with the leads welded to them.
Sometimes the film pattern is visible even through the paint, as on the resistor on the right.


As I mentioned above, a ceramic substrate is most commonly used because it is inexpensive, can withstand high temperatures, and is stable, since its dimensions change very little with temperature.
The substrate is clearly visible on high-power resistors.


Besides their main parameter, resistance, resistors are also rated by power.
For example, the photo shows resistors rated at 0.062, 0.25, 0.5, 1, 2, 5, 8, 10, and 50 W.
This rating indicates the power the resistor can dissipate while keeping its parameters within the tolerance marked on it.
Since this rating is specified for an ambient temperature of 20 degrees, and temperatures inside a device are usually higher, keep that in mind and try not to run resistors right up against their maximum ratings.


As technology advances, resistors are gradually getting smaller. More stable materials are used, whose parameters change less with heat, so resistors with the same power rating can come in different sizes.
The photo shows pairs of 0.062 W, 0.25 W, and 0.5 W resistors.
The newer ones are on the left, the older ones on the right. The size difference is noticeable.


But there are precision resistors that can be much larger than ordinary ones. I’ll talk about precision resistors and how they’re used next time. For now, here’s a photo: three 0.5 W resistors are side by side on the left, and a 0.25 W resistor is on the right.


That’s all for today. If you have any questions, I’ll be happy to answer them. And as a bonus, here’s a video on the subject.