The Classroom

# Resistors and How We Use Them

Daniel Koch

Issue 3, September 2017

This month in The Classroom, we’ll take a break from absolute theory and instead look at something students and makers alike need to understand: identifying and correctly using some basic electronic components.

One of the most common components any maker or student encounters will be resistors. Resistors come in a huge array of types and sizes, values, and power ratings.

In an average classroom kit or programmer’s board like Arduino or Raspberry Pi, the most commonly found components are the small signal resistors in the 0.25W-0.5W range.

What are resistors, and what does the other jargon (“metalanguage” for all the educators out there), actually mean?

Resistors are components designed to resist the flow of electricity, thereby reducing current flow, limiting voltage, or generating heat, often very precisely. Resistors are literally meant to get in the way, but only a little bit. The measurement of that resistance (i.e. the “value” of a resistor) is given in Ohms, which is symbolised using Ω. In Australia, the circuit symbol for a resistor is a rectangle with the value next to it or in it, always given in Ohms. Previously, the symbol for a resistor was a zig-zag line. You will often still find it in current literature too.

# Fixed Range of Values

Resistors are typically made in set values. You may have heard of the E12 series or the like. This means that for each power of 10, there are 12 values of resistor. So from the range 10Ω to 100Ω, there will be 12 values: 10Ω, 12Ω, 15Ω, 18Ω, 22Ω, 27Ω, 33Ω, 39Ω, 47Ω, 56Ω, 68Ω, 82Ω. For the next power of 10 – being 100Ω to 1000Ω – there will be another 10 values, basically the same numbers but ten times greater.

This is the system engineers developed so the percentage gap between values remains the same. E12 are for 20%, E24 for 10%, E48 for 5% and so on. The E series is defined by internationally standards, and not just for resistors.

However, some resistors, called ‘High Precision’, are made to a specific value for a specific use. In the world of RF, 50 Ohm resistors may be required. So, although most resistors conform to the E12, E24 or other series, occasionally you might find something different. It is unlikely to buy anything other than E24 values at an electronics shop.

# The Power

When you’re looking at a parts list, the next number you’ll typically see in a resistor name is a power value in watts. In real terms, this means the amount of power that a resistor can turn into heat before becoming physically damaged. Many of the resistors in educational kits, experimenter’s kits, and on Arduino and Raspberry Pi boards are 0.25W or 0.5W sized. You may have 1/8th Watt, (0.125W) in very small or even SMD resistors, but again won’t usually buy them for a project.

Interestingly, even though 0.5W dissipate twice the heat of the 0.25W, they are both the same size due to how they are made. 0.25W resistors are usually carbon film types. That is, they are a film of carbon around a ceramic former, with a metal cup and leg attached at each end. The resistor is covered in a protective coating of a colour that reflects the technology used. The coating carries information about the resistor via colour coded bands.

In contrast, 0.5W resistors are usually made with a metal film on the ceramic core, which can handle more heat for its size than carbon film. Metal film resistors are the most common today, after carbon film.

There are other sizes, and types of resistors around. Resistors rated at 1W or even 2W look a lot like a larger version of a 0.5W type, but can dissipate more heat. The 5W, 10W and 20W resistors, as found at your electronics store, are of a construction called wire wound, ceramic case. These are “Power Resistors” made with fine wire that is wound around a ceramic former, coated in ceramic compound, and embedded in a rectangular ceramic carrier, which both protects the resistance and dissipates heat.

Larger resistors are available, but they are usually hard to find and well out of the needs range of most of us. In fact, the elements in a Stove Hotplate or Electric Heater, Electric Hot Water System, or even an Electric Blanket, are resistors.

# Tolerance

The next parameter you will find in a resistor’s description is its tolerance. This is not always stated, but it is sometimes important, especially to the designer.

Whenever an engineer designs a part, as well as other dimensions, that part will have the tolerances specified. In many industries, this will be a range of acceptable sizes or other dimensions. Typically parts will be given a ‘Nominal’ value, perhaps in metres or millimetres, and a ‘Tolerance’ in millimetres or even micrometres.

For example in an Aluminium extrusion factory I used to work in, Aluminium tubes were cut to 3.6m +/- 2mm. I had to ensure that each each tube was between 3598mm and 3602mm.

Although resistors have physical dimensions and tolerances, the important tolerance is of the actual resistance. Unavoidable differences exist in any batch of material, so although a precisely calculated and applied layer of metal film is deposited on the ceramic, and then laser cut, or trimmed, the exact resistance will be between the two values of tolerance, the upper tolerance, and the lower tolerance.

Electronic component manufacturers work in percentages for their tolerances. Commonly, metal film resistors have a tolerance of +/- 1%, while carbon film resistors often have a value of +/- 5%.

This means that a given metal film resistor may be lower or higher than its stated value, by an amount of up to one per cent of that value. In real terms, a 100Ω resistor may be up to 1%, or 1Ω, higher or lower, making it potentially anywhere between 99Ω and 101Ω.

Tolerance values are written on resistors with a tolerance of better than 20%, as the fourth or fifth band on a resistor, depending on the type. Colour banding is discussed below. Some applications allow a wide range of tolerances without a significant issue, but others require a high level of accuracy, i.e. a low tolerance for error. Electronic Instrumentation for example becomes less accurate with badly chosen values of resistance.

# Knowing Versus Finding

The resistance, power rating, and tolerance of a resistor are certainly the three main pieces of information needed for most of us to choose and use a resistor. Although not discussed here, some resistors also show a temperature band, although not normally required for hobbyists.

So, how does one find this information on the actual resistor? In the case of power dissipation, this is basically indicated by size, but the background colour defines the type of technology in the resistor. Carbon Compound Resistors had a brown body, but were rated at several power levels and poor tolerances. Carbon Film have a cream body and also come in different power levels, Metal Film are often blue, and so on.

Knowing the technology therefore helps find the power dissipation levels, but, if you are purchasing new components, then that information will be included in the packaging or the catalogue, or the manufacturers data sheet. Often, a project or kit comes with metal film resistors, and when one is lost or damaged in assembly, a common mistake is to replace it with the same value carbon film type. Remember carbon film (cream) are half the power rating of Metal Film (blue).

Resistors of 5W and above actually have the dissipation stamped on them, although size is a fair guide too. You will quickly come to recognise what is normally a 5W, 10W or 20W Ceramic Block Wire Wound Resistor.

There are three common systems that manufacturers use to detail the value and tolerance information for resistors. As stated above, at the 5W mark and above, resistors are large enough and made of a suitable material and shape to have the information stamped or printed on them. The value may be written as a number, such as 100Ω, but this is less common. Instead, a decimal multiplier system is usually used. Two are common: the first is the same decimal system we are familiar with from weight, volume, and mass measurement. That is, below 1000, resistance is simply printed straight on the resistor with the letter “R” after it. This is because the symbol for Ω often looks far too similar to a “0”.

For numbers between 1,000Ω and 1,000,000Ω, the decimal multiplier “K” is used. This is also often abbreviated in speech as just “K”, for Kilo-ohms, which is often called “Kilohms” with a single “Oh” sound. Grammar fanatics like the primary teacher in me struggle with this, but as it is a piece of jargon or metalanguage, absolute grammar rules do not apply.

For numbers one million Ohms or more, the letter “M” is used, for “Mega-ohm”, which is often pronounced “megohm” and simply abbreviated to “Meg”. For many of us, these letters are familiar from day-to-day life, being standard Metric terms.

As an exercise, teachers should collect a number of 5W to 20W resistors, or find some in an old TV or such, and compare the markings. Decimal points get easily lost or confused with dirt and stray marks. For this reason, a value of, say, 4.7Ω will often be stamped as 4R7. Likewise, 4700 will be stamped as 4K7. Including the multiplier as the decimal point creates clarity and removes doubt, and reduces the number of characters to be printed, but can confuse new users.

Tolerance is stamped with a single letter code, and unfortunately, while there are industry standards, in my experience, manufacturers may apply their own rules.

Generally, you will find they are labelled “K” for 10%, “J” for 5%, “G” for 2% and “F” for 1%. There is a problem here though, as K is used as both the decimal 1000 multiplier, and the common tolerance of 10%. So care is needed here, although thankfully, the tolerance letter is usually all by itself, away from other letters and numbers, and nearer the edge of the component.

# Two of Three

The second textual system is also used on larger format resistors, and usually on surface mount devices. This system consists of three numbers and a letter. The letter is the same tolerance indicator as above, making that one fairly easy, as long as you do not mistake the ‘K’ for kilohms.

The three numbers are not as complicated as they look. What you are looking at is two significant figures and an exponential number, or multiplier. Think of it as a simplified scientific number. The multiplier is literally the number of zeros that follow after the two significant figures.

For example, a marking of ‘152’ is the same as 1500 Ohms, where 1 and 5 are the significant figures, which are 2 place values (or 2 zero's) away from the decimal point. Likewise, 473 is actually 47,000Ω or 47kΩ. In all cases, the number will be in ohms when expanded with all the zeros. There are some challenges with this system for smaller values. In fact, I have only seen it used recently on variable resistors, but more about these soon.

# It’s All About The Colour

So this is all well and good for the larger resistors, which are big enough to physically print numbers on, and for SMDs that use very good printing, but what about the simple resistors that most of us use? Although some readers may measure every resistor on their multi-meter, most readers will be familiar with the little coloured bands seen on these resistors, and some can read them.

These coloured bands painted around the case may look random, but there is a reason to them, and a formula to read them. What you should have is a total of four or five bands, (occasionally six) with one being separated from the others by a larger gap. This is not always the case however, so be careful! If all is going according to plan and you do have three or four bands, then a gap, then one more, well your life is easier! Read it from one end, and if it doesn’t make sense, and doesn’t appear to be a standard value, try reading it from the other end.

The numbers grouped together are either two or three significant figures, then a multiplier, and each coloured band represents a number [1].

The first two in a three-band group indicate the value, and the third is the number of decimal places. In a four-band group, you have three significant figures and one multiplier. The multiplier works out to literally be the number of zeros you add after you figure out the significant figures as in our second method above.

The band all on its own indicates tolerance, which is also identified by its colour, which can be helpful if the spacing isn’t great.

So for a five-band resistor with colour coded bands of brown, green, black, red and brown, has significant figure values of 1, 5, and 0, and a multiplier of 2. There is also a tolerance band representing 1%. Therefore, the resistor will be 1, 5, 0, and two 0s, equating to 15 000Ω 1% or 15k 1%.

A four-band resistor will only be 5% tolerance at best. So the same value, 15k will be 1,5,3, will have bands of brown green, orange, gold. There may not be a gap before the tolerance band, but as the tolerance will only be 5% (gold) or 10% (silver) they will be easily recognised as the tolerance band.

Values of resistor below 10 Ohms are uncommon but still required to be marked. For example a 1.2 Ohm resistor is 12 with a multiplier of 1/10. The appropriate multiplier code for that is a gold band. Similarly a 1/100 multiplier is represented by a silver band.

Colour banding is the most common type of resistor marking that most of us will come across.

# Changing Resistance

There is a lot of information here; but wait, there’s more! It’s also worth discussing a different type of resistor: the variable resistor.

Many circuits use adjustable resistors, or variable resistors, to control volume, speed, brightness, and on/off thresholds. Variable resistors are typically rotary types that revolve about 270 degrees, but some are sliding types as used on Audio mixing desks. Yet others are multiple turn rotary or screw types. As with fixed resistors, the common variable resistors are only able to dissipate a little heat, so they are often used to control a driving circuit.

In their most common form, variable resistors feature a carbon film deposited on a resin bonded paper base, which is held in a pressed steel frame, with a rotor shaft through the middle of a mounting boss. The rotor is connected to a wiper, which touches and slides along the carbon film. The ends of the film are connected to the outer pins of the case, while the wiper is connected to a third contact.

Current flows in one pin into the carbon track and out the wiper. At the initial position there will be very little resistance and possibly a direct contact with the pin, but as the wiper begins to move around the carbon track, current must flow through more and more carbon in order to reach the wiper. At around 270° of travel, the wiper reaches the other end of the track. The current must pass through almost all of the carbon in order to reach the wiper. In this way, the resistance between the wiper [2] and the pin at the other end varies from ~zero to the nominal resistance of the track.

There are variations on this theme. Some variable resistors slide like the volume control faders on a sound desk in a music space. Others are built for operation through 90°, such as those in radio control situations. Still others look like the regular 270° round variable resistors but are wire-wound for increased power handling.

Yet another variation is the trimpot. These are smaller variable resistors, which are adjusted by a screwdriver or dedicated tool. They are used for “set and forget” applications, like adjusting upper and lower limits of a larger range. Trimpots come in a variety of forms from open frames with carbon film to 25-turn precision wire-wound varieties.

# What’s in a Name?

Many people think a variable resistor is called a “potentiometer” but this is not strictly true as the term potentiometer actually refers to a voltage divider. Originally the Potentiometer was a piece of Science Lab equipment to measure voltage in an experiment. A voltage is applied to one end, current flows through the whole resistance and exits somewhere into the circuit at the other end. The variable wiper taps off the desired level of that signal, as a part voltage of the total, for the circuit to use [3]. It is the application of a variable resistor that determines its name; however, as most often we are connecting them as potentiometers (or “pots” for short), most of us know them by this name.

A variable resistor may also be used as a rheostat. In the rheostat, a voltage is applied at one end, but the other end is either not connected, or connected to the wiper. The resistance itself is the variable value, causing variation in the current passing through it, or the voltage across the rheostat.

In the case of a potentiometer, having three terminals means that the current entering one terminal is divided between the other two terminals. The voltage on the wiper is always a value between the values at either end pin. Most designs are calculated to have a current passing through the carbon track, that is ten to twenty times the current at the wiper terminal, so the wiper current makes a negligible difference to the voltage at the wiper position.

The circuit symbols for some variable resistors are shown here too. You’ll see that the chief difference is the screw-adjust type versus the fully variable, hand-adjusted type.

# What Does It All Mean?

Now that you can identify most resistors in both physical form and circuit diagrams, it’s time to learn a bit about how to use them. As alluded to in the beginning of the article, resistors exist to oppose the flow of electricity. Those who have read previous editions may remember resistance as being like a smaller section of pipe that slows water down; resistors are basically just that. You might want to do this for timing reasons, or to reduce the current flowing through a component to a safe level.

All resistors follow Ohm’s Law, which states that the voltage being dropped across the resistor (the potential difference existing across it) is equal to the resistance of the component (in ohms) multiplied by the current flowing through it (in amps). It is important to convert milliamps (mA) to amps (A) before doing this though, as many people struggle otherwise.

"In any given circuit, where the voltage remains constant, the current in such a circuit is directly proportional to the resistance of that circuit."

The easiest way to remember the formula is to draw it as a triangle [4] with V at the "Very top", and R on the "Right". That leaves only one place for I, which is the letter we use to represent current.

As a practical example, let’s take the common LED. LEDs are made to emit light (we will cover PN junctions, diodes, and LEDs in future editions), and as such, they are not manufactured to be rugged. So an LED that draws 30mA at 2.3V voltage drop would quickly burn out if connected to 5V. In fact, as an LED has no current limiting properties, it would burn out even if running on 2.3V with no resistor.

For this reason, we need a resistance in series with the LED as a current limiter. This resistance is correctly called a ‘Ballast Resistor’. To calculate the ballast resistor value, we need to use Ohm’s Law. If the supply is taken as 5V, and the LED terminal voltage when lit is specified as 2.3V, the voltage difference across the resister needs to be 5 – 2.3 = 2.7V. The specified LED current is given as 30mA, which is 0.03A. Remember: We must always work in amps, volts, and ohms, never their multiplied or divided versions such as “milli”.

Using Ohm’s Law to calculate the resistance or our resistor, we have a voltage drop (desired) of 2.7V (V at the Very top of the triangle), and the current (I - below left) 0.03A, i.e. underneath the 1.8V, which looks like this:

2.7 / 0.03 = 90 - This gives a result of 90Ω. The nearest standard E12 value is going to be 82Ω or 100Ω, which are both about the same percentage difference, so we would normally use the higher value and accept a little lower light output.

Let’s try again for the same LED, but running this time on a 12V supply. Now we need to drop (12V - 2.3V = 9.7V) at the same 0.03A.

9.7 / 0.03 = 323.3 - In this case, the resistance is calculated as 323.3, which is again a non-standard value; so to run our 2.3V, 30mA LED on 12V, go to the next higher standard value of resistor, which would be 330R.

There are many other reasons for using a resistor, although unfortunately many are far beyond the scope of this article. Most are very situational, such as timer circuits where the resistor controls the charge rate of a capacitor to reach a threshold voltage to set a flip-flop; however, that is worthy of a whole other article!

Hopefully you have the knowledge now to identify the type, tolerance, and value of the resistors you are working with, and a basic understanding of how to safely use LEDs in your circuits. Remember, the DIYODE classroom only exists as long as you continue to let us know what you want to learn about, so please keep those topic suggestions coming in.