Welcome to the ultimate guide on everything about Resistors!
In this guide, I'm going to walk you through everything you need to know about resistors. We'll look at the basics all the way up to how to use them in circuits.
What Is A Resistor?
A common question is how do we define resistor? In its simplest form, the resistor definition is as follows:
resistor = electrical component that restricts the flow of electrons (current)
Although that sounds simple, you might be asking what does it actually mean. Let's go through a few key concepts so that you get a better understanding of it.
Resistivity and Conductivity
Remember that current is the flow of electrons through a circuit.
Any time there is a collection of electrons (Voltage) that is higher in one place than another place, those electrons will want to flow, depending on how much they are allowed to.
Resistivity and conductivity really come down to the properties of the materials we are using. If you are interested in how these work, there is a lot of information about the physics behind it.
What is Resistivity?
It is simply "how bad" a material is at passing current.
What this means is that the material doesn't easily allow electrons to jump from atom to atom which encourages current to flow.
What is Conductivity?
In order to define conductivity, we simply think about it as the inverse, and it is therefore a measure of "how good" a material is at passing current.
Electrons can easily jump from atom to atom in this material.
To give you a good example, metals like copper typically have a very low resistivity and high conductivity. That's what makes them such great conductors. They love to let electrons flow through them.
In contrast, insulators, like glass and air, have a high resistivity and low conductivity. They don't like electrons flowing through them.
The key take away is that these two factors are dependent on the material.
It turns out that resistance is a pretty basic concept.
If we take a certain material, say copper for example, and we know that the resistivity is very low and it has high conductance, then we can start playing with the length, width, and height of this piece of copper and we start changing its resistance.
This is exactly why bigger copper wires have less resistance when compared to smaller copper wires. The resistivity of copper is the same in both wires, but the dimensions of the two wires are different.
More material, or a greater length, width, and height allows more opportunities for electrons to flow through the material, which reduces the resistance.
The key takeaway is that resistance is based on the material we are using because of its resistivity, but it also depends on the length, width, and height of the material as well.
Now it's time for some resistor equations.
From Ohm's Law, we know that there is a nice relationship between resistance, voltage, and current:
Resistance = Voltage / Current
This equation is commonly written as R = V/I.
The units for resistance is the term Ohms, which has the symbol of Ω. What's really neat, is that from the equation above, we can see that 1 Ω is the resistance when 1 Amp of current flows and there is a voltage of 1 Volt.
Knowing this equation allows us to start manipulating how much current we allow to flow within a circuit.
It means that for any given Voltage, we can change the resistance in the circuit by the material we select and the dimensions of that material. The result is that we determine how much current will flow.
How cool is that?
What Does A Resistor Do?
Putting all of this theory together, you begin to see just how handy resistors are in controlling the amount of current flowing in a circuit.
A resistor simply resists the flow of electrons through it, based on its material properties as well as its physical dimensions.
It is therefore an intended device that we place in a circuit, with the knowledge of the physics, that allows us to control how much current is flowing.
That means that resistors have all sorts of useful purposes.
A by product of resisting current means that resistors will create a voltage drop across them. This is simply because of Ohms Law again.
If we look at the equation R = V/I, we can re-arrange it to be:
Voltage = Current * Resistance
This means that when we have a resistor with a known resistance, then the current flowing through that resistor will produce a Voltage across that resistor.
We have to always take this voltage drop into account in our circuits. Often, we even use this to our advantage to help manipulate the Voltage as well.
A resistor is often depicted as a two terminal device with either a jagged symbol, or a rectangular symbol, as seen below.
Potentiometers and Rheostats, as covered in the next section, have slightly different symbols.
What's more cooler than a resistor? The answer is a variable resistor.
For many circuits, a fixed resistor with an established resistance value works perfectly. Sometimes though, there might be specific needs on the circuit that require that the resistance changes to alter the behavior of the circuit.
The two main devices that can do this are rheostats and potentiometers.
A rheostat allows you to change the resistance with the intent to change the current. They are usually beefier so that they can handle higher currents. There are two connections so that you truly are just changing one resistor value.
Whereas a potentiometer allows you to change the resistance with the intent on changing the voltage. It has three connections which means it's a nifty little resistor divider where you can move the dividing point in the resistance with the middle connection.
One great example of this is the volume knob built into the cable of your headphones. Most have a little thumb wheel and that changes a resistance.
If we think about what is happening here, if you turn the volume knob down to lower the volume, then you are increasing the resistance in this resistor, which lowers the amount of current that makes it to the speakers in your headphones.
The opposite is true if you turn the volume knob up to increase the volume. This effectively lowers the resistance and allows more current to flow into the speakers, making them louder.
Some other examples include needing the ability to adjust resistance for calibration, which would not be used as frequently as our headphone example above.
Resistors comes in many different resistance values as well as physical packages. This allows a designer to have many options to choose from when selecting a resistor.
There are several main factors to consider when choosing a resistor. They include the following:
- resistance value
- resistor material
- package type
- maximum power dissipation
- physical dimensions
We will cover each of these in the next sections.
The biggest factor is the resistance value. The units of the resistance is in Ohms. It determines the behavior of the circuit by affecting the current.
Typically, the average resistor has an resistance accuracy of 5%. This means that if you choose a 1,000 Ohm (also written as 1k Ohm) resistor, then the value could really be any where between 950 Ohms and 1,050 Ohms.
If the design requires a much more precise value, then there are options to get resistors with higher accuracy. The price naturally goes up because it takes more process to create higher precision resistors.
Most sensitive circuits often use 1% precision resistors, but depending on the design, resistors with 0.01% and even 0.001% accuracy are available.
The goal of good design is to get only the precision that is needed for the circuit in order to save cost.
There are many different types of ways to make a resistor. Manufacturers will often use different materials and methods.
Some common examples are carbon film, ceramic, metal film, metal foil, thick film, thin film, and wirewound.
Metal film is the common type for through hole resistors, while thick and thin film are common for surface mount resistors.
There are three main categories for packaging.
Through Hole: resistors with two leads that can be put through holes on a circuit board
Chassis Mount: resistors in a metal or ceramic case that can be mounted to a structure, usually to help dissipate heat
The following picture shows through hole and chassis mount together to show the size differences.
Surface Mount: smaller rectangular shaped resistor that can be soldered to the surface of a circuit board
To show just how small surface mount resistors can be, see the image below where there are several (see R36) on a circuit board next to an IC chip.
Maximum Power Dissipation
It is important to determine the maximum amount of current that the resistor will pass. That way, you can calculate the maximum power (Watts) that the resistor will dissipate in the form of heat.
There is a simple equation for the power dissipated by a resistor. It is:
Power = (Current)² * Resistance
If you work back from Ohm's Law, you can also derive the power equation as follows:
Power = (Voltage)² / Resistance
For most cases, you will simply be able to select a resistor with a higher power than what it needs in the circuit.
Most rules of thumb suggest you use at least a safety factor of 2 or more, which means you pick a resistor with at least double the maximum power dissipation than what it will experience in the circuit.
However, this isn't always 100% safe. Many circuits don't have much power dissipated in its resistors, but some do.
In order to really do it right, you need to consider the environment that the circuit will be in on top of the power dissipated in the resistor under max conditions. The key is making sure the heat can get off of the resistor before the resistor burns up.
There is a lot of advanced thermal analysis that might need to take place that is beyond the scope of this article.
You can imagine how a circuit might fail if it is in your car on a hot day compared to if it is in a refrigerator. As the designer, it's your job to do the right thermal calculations to figure out what is required and choose the parts accordingly.
The physical dimensions is simply the length, width, and height of the resistor.
Surface mount resistors are the smallest, then through hole, and then chassis mount resistors in terms of smallest to biggest.
For through hole and chassis mount, the size is usually dictated by the desired max power it can dissipate.
There are many different sizes available for surface mount resistors. The package codes are either in the Imperial or Metric systems.
Here are the most popular Imperial code options and their dimensions:
Package Code (Imperial)
Resistor Color Codes
Through hole resistors often have a banded color code scheme to show the resistance value. Some through hole resistors don't have this color banding, but instead have a number code.
Here is a great tutorial in the following video on how to determine what a resistor value is from the color codes.
Surface Mount Values
Surface mount resistors will have a number code on them that signifies their value. Check out how to determine the value in the following video.
Standard Resistor Values
There is a standard out there that has been established to determine resistor values. The standard is known as the IEC 60063:1963.
It's a nifty system that has an increasing resistance value of 20% to step up to the next standard value. This was smartly done so that 10% accuracy resistor values would overlap each other.
To understand this, let's take an example of a 10 Ohm resistor. If we take the next step up by 20%, we get a 12 Ohm resistor.
When we look at the fact that these could be 10% accuracy values, then the max 10 Ohm resistor might be 11 Ohms, and the min 12 Ohm resistor could be 10.8 Ohms.
That way there is overlap in the resistance values between the two steps if you take into account the 10% accuracy.
A 10k resistor, which means 10,000 Ohms of resistance, is one of the most commonly used.
Often, a 10k ohm resistor is handy for pull up and pull down situations, which we will discuss later.
Another popular value is a 100 ohm resistor. They are useful for limiting current.
More examples include a 1 ohm resistor, 220 ohm resistor, and 1k resistor. They all have different uses depending on the circuit.
As the designer, you will need to calculate which resistor values you need for your circuits and then see what standard values are available and select the closest ones.
Now we get to the fun part, where we get to see how resistors are used in circuits. Let's first talk about what happens when we put resistors in series and parallel.
Resistors In Parallel
The easiest way to think about this situation is the fact that you are creating multiple paths for current to flow through.
The equivalent resistance equation for resistors in parallel is:
Let's look at the example circuit of parallel resistors in the following diagram and calculate the new resistance.
Here we simply do the math of: 1/100 + 1/200 + 1/300 = 0.01833, then take the reciprocal of that, so 1/0.01833 = 54.5 Ohms.
As you can see, by adding resistors in parallel, we effectively reduced the overall resistance because now more current paths were created.
Resistors In Series
In this situation, it is convenient to realize that the same current will flow through each of the resistors. With that reasoning, it makes sense that we simply have to add up the resistors for a total resistance.
The equivalent resistance equation for resistors in series is:
Here is an example circuit in the following diagram.
This one is simple. The new resistance = 100 + 200 + 300 = 600 Ohms.
This is a generic term that people usually lump many different uses of resistors into.
The most appropriate use of the term is in simple circuit analysis where you have a voltage source and you need a resistance to demonstrate some electronic fundamentals.
In industry, it is often used as a reference to a resistance that will be connected to something at a future point in time, and you may not know the value of the resistance.
For example, if you design a power supply that outputs 10 Volts and then someone needs to use that circuit to power something, they will hook a load to it.
That load will have a resistance. It is a convenient way for the designer to understand what the circuit should be able to do.
Many people use the term to describe any resistor that has current going through it. However, any circuit that is turned on most likely has resistors with current flowing through them.
Other examples of the term is when people want a resistor to create a certain desired voltage drop across a resistor. They create a load by inserting a resistor that drops the voltage so that other components in the circuit have less voltage and less current to deal with.
There are no right or wrong uses of the term any more. It's become more electronics slang than anything else.
Power resistors are the name that people usually call resistors that are intended to dissipate power and that will have a higher maximum power rating.
Typically resistors with a max power rating of 1/2 Watt and above start falling into this group.
Usually this involves resistors in chassis mount packaging. This package allows you to bolt the resistor to a panel to help get the heat off of the resistor by conduction and into the surrounding environment.
You can often use air to help transfer heat from the resistor to the environment as well. A nice example of this is the common hair dryer.
In other designs, there is often a lot of heat dissipated as a byproduct of the electronics and it needs to be removed.
Pull Up Resistor
Pull up resistors come in handy when you are using an input digital pin to a device and you don't want the signal line "floating".
This simply means that something is not actively driving that input pin. The voltage level can drift up and down and cross over the logic levels of the device pin, causing false triggers.
If you have a resistor between that pin and your voltage supply rail, the input pin will be pulled up to a high and will not be able to move around the voltage range, preventing issues.
These resistors don't interfere with the normal functioning of the pin when it is being driven. Usually values of 10k or 5k ohms are used, but you can also find the most appropriate value in the data sheet of the chip you are using.
Another place where these resistors are used is for open drain or open collector transistor logic. Sometimes these devices are a little too weak to reach certain voltage levels and the pull up allows a safe way of taking those signals all the way high.
Pull Down Resistor
Pull down resistors are similar in that you can place the resistor between an input digital pin to a device and connect the other end to ground.
This ensures that an un-driven state by default is low because it forces the voltage to ground.
These are less common that pull ups, but can still be useful, especially if you are dealing with a need for defaults to be low, or ground.
Current Limiting Resistor
A current limiting resistor is exactly what its name describes. Any time you need to limit the current going to another component, you can do so by inserting a resistor in series with that component.
As we saw from our equations, this newly inserted resistor adds to the resistance of the existing component, and therefore reduces current flowing through both of them.
LED Load Resistor
A resistor that limits the current through an LED is a common requirement when you have LEDs in your circuit. Usually you have a few different supply voltage rails to choose from, like 5 Volts, 3.3 Volts, 2.4 Volts, etc.
An LED will have a certain forward voltage (Vf) and a specific current that it requires to be turned on and giving off light.
Therefore, in order to give the LED its needed requirements, but also use a pre-existing supply voltage in your circuit, you need a resistor in series with the LED. The resistor will drop the excess voltage between your voltage source and LED.
Check out the circuit below which shows a typical resistor and LED.
Here we have an average green LED with a forward voltage of 2 Volts and a forward current of 20 milli-Amps.
How do we figure out what resistor R1 we need? Well, we know that our supply voltage is at 3.3 Volts. We also know that the green LED needs 2 Volts and 0.02 Amps. So we can do a little math.
If we take the 3.3 Volts minus the 2.0 Volts that we want on the LED, we get 1.3 Volts left that we need the resistor to consume.
Next, we use V = I * R, and solve for R. This equation becomes R = V / I. We plug in our known values and solve for R = 1.3 Volts / 0.02 Amps = 65 Ohms.
Bingo, we now know that a 65 Ohm resistor will limit the current in the LED to 0.02 Amps and will drop 1.3 Volts. That way the LED will have the remaining 2.0 Volts and also the 0.02 Amps of current.
Pretty neat stuff.
A shunt resistor is a very low resistance component that has very high precision. It is often used to measure current.
This is done by putting this resistor in series with the circuit whose current is being measured.
There will be a voltage drop across this precision resistor, and then you can measure the voltage drop to determine the precise current. The voltage drop will be low because the resistance is low.
Often times, the designer should measure the voltage with dedicated signal lines across the resistor for maximum accuracy. These differential lines can be fed into a dedicated chip or an op amp circuit and then digitized.
Got any great resistor tips? Let me know what you think in the comments below.