What is a resistor and how to use resistors [complete tutorial]

In this tutorial, we shall discuss what is a resistor and how to use resistors.

In the previous tutorial, we discussed electronic circuit design for beginners, and one of the points noted was that, to design electronic circuit from scratch, one should know how the various electronic circuit components behave in a circuit.

When you open any electronic circuit, take the motor power supply for a small metal lathe shown below as an example,

power supply
Figure 1.0 Motor power supply for a small metal lathe

 you can see gamut of electronic components that are technically arranged and connected. It is the work of these electronic components or devices to manipulate the flow of electrons that emanate from the power source (battery or power supply) to achieve a desired goal.

It is human beings that designed and produced the motor power supply circuit above, you too as a human being can do that. All you need is to learn how to do it, and that is why I made this tutorial and others, and will keep making more to make sure you get all the information you need to become a professional circuit designer. So, make sure you check all other tutorials and read them.

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  5. What is electricity and electric charge

 Learning to design electronic circuit is a step-by-step process, there is no hard and fast rule to it, all you need is keep learning and keep practicing. Electronic circuit design is a fun-filled activity, the more you practice and design new things, the more you love what you do and enjoy what you do. So, don’t be intimidated by complex circuits when you see them, take it one step at a time and you will learn how to do it.

Every engineering field uses mathematics as a tool to get the work done, but in my tutorials, I will make sure I keep the mathematics to bare minimum. I will give you an intuitive understand of electronic circuit design.

So, in figure 1.0 above, we have a circuitry of a power supply that contains various electronic components. The designer of the circuit had already understood that electrons in a battery or electricity generated in power stations when stepped down to a reasonable quantity can be manipulated to achieve some purposes, this purpose might be to cause an alarm to ring or a wheel to turn. To cause electrons from a battery or power station to do that requires some materials to direct its move. These components are called electronic components.

 When someone who knows how these components behave put them together in the right way, their connection is called electronic circuit and when such is done and a power source connected, the purpose of the whole process will be realized, electrons will then flow to achieve the predetermined result.

One of these components that every designer should know how it behaves is called a “Resistor” a typical example of a resistor is shown below:

Figure 2.0 Resistor

And that is the component you can see many of them in the circuitry of the power supply in figure 1.0 above.

Application of resistors

Now, you have seen a typical resistor, but then in the circuit we are making reference to above, there are many of these resistors, what are they doing in the circuit? You need to be able to answer this question for you to understand how any circuit works. Every component in a circuit has a specific job. Again, we can have two or more of same component in a circuit doing entirely different things in the circuit but with the same principle.

Here is what I mean, in the circuit above, we have about 37 resistors used to design the circuit, but these resistors could be doing five or more different things but still with the principle of a resistor.

The work of a resistor in a circuit is to reduce current flow. This is the ultimate principle and function of a resistor, in line with this principle, a resistor can do the following:

  1. Reduce current flow
  2. Lower voltage levels within a circuit
  3. Set operating current and signal levels
  4. Set gain values in precision circuits
  5. Act as shunts in voltmeters and ammeters
  6. Act as damping elements in oscillators
  7. Act as line and bus terminations in digital electronic circuits
  8. Provide feedback networks for amplifiers

So, the resistors in the circuit above can be doing any of the following things listed above. However, they do so in an attempt to reduce the flow of current, sometimes, the resistor alone cannot achieve the functions listed above, they will need to work hand-in-hand with other components like the capacitor, transistor, inductor, diode, etc. and that is why we have other components.  

So, in an attempt to realize a function like generating a square wave  signal, we need a resistor to manage current flow to achieve that, but at the same time, a resistor cannot achieve the function alone, so we will need an integrated circuit like the 555 timer and a capacitor, and that is how we tend to have many components in a circuit. Apart from electronic components working as individual components in a circuit, they also work as a group in sections. See image below:

Power supply schematic
Figure 3.0 Power supply schematic of a switch mode power supply

The image above shows a SMPS, the part enclosed in red box is the Electromagnetic interference suppression section, it is made up of inductors and capacitors, here, one can see that two different components are used to achieve a particular purpose.

If you understand that you can dissect a complex circuit in sections, it becomes easy to analyze. And when you dissect a circuit in sections you can easily troubleshoot and repair any electronic device circuit.

Take for example we have the block diagram of a SMPS shown below:

Block diagram of an SMPS
Figure 4.0 Block diagram of an SMPS

Now, imagine you have dissected a circuit in this manner knowing the input and the output points of any block or section and what values of voltage and current these inputs and outputs offer, you can tell which section is faulty and then determine the components that caused the fault. This is an intuitive way of diagnosing and fixing electronic circuits.

Resistor measurement

This electronic component called resistor we are talking about is a component that is designed to have a specific amount of resistance. The resistance of a resistor can be measured with a test tool called ohmmeter; this ohmmeter is often embedded on a multitasking test tool called a Multimeter. The unit of resistance is the ohm abbreviated as Ω. However, resistors commonly make use of metric prefixes like kilo, and Mega, hence, we can have kilo-ohms, Mega-ohms. Kilo ohms means 1000 ohms and mega ohms means 1000,000 ohms.

Forms of resistors

Resistors come in form of:

  1. Fixed resistors
  2. Variable resistors

Fixed resistors have specific resistance values, while Variable resistors have varying resistance values that change as a result of mechanical impact on the resistor by either turning a screw or knob, loosing a clamp and sliding a wiper physically up and down, or as a result of changing temperature in the case of a thermistor or as a result of changing brightness as in the case of a light dependent resistor, or as a result of changing voltage as in the case of a metal oxide varistor or voltage dependent resistor.

different kinds of resistors (image courtesy of electronic clinic)
Figure 5.0 different kinds of resistors (image courtesy of electronic clinic)

The symbol of a resistor in a circuit is as shown below

resistor symbols
Figure 6.0 resistor symbols

How a resistor works – What is a resistor

Current simply is charges in motion. when these charges move, they do so with a force, this force that constitutes electrical energy is what is called voltage or current pressure. I have written intuitively on the concept of current and voltage, you can click here to read the tutorial.
When you connect a voltage source to a resistor, say a battery.  This voltage source delivers current to the resistor, the electrons that carry the said charges interact with the electrons of the resistor, this interaction is an obstructive one, the electrons in the resistor obstruct the free flow of charges from the battery. It should be noted that these charges from the positive terminal of the battery are flowing to return to ground which is the negative terminal of the battery, watch the video below to learn more about circuit grounds.

Video: how to design electronic circuit from scratch

The moving charges from the power source interacting with the electrons of the resistor will lose some energy (voltage) as they move across the resistor and this process will result to the following:

  1. Reduction in the current passing through the circuit because of reduced energy of charge flow or voltage
  2. Energy (voltage) drop on the said resistor:  Since the whole electron-charge interaction is happening in the resistor, the initial energy which the charges had, that had been lost due to this interaction will be dropped on the resistor.
  3. This voltage drop on the resistor will in turn be converted to heat.

 Resistor networks – What is a resistor

To apply a resistor to circuit design we can have make the following connections:

  1. Voltage divider: this is one of the basic resistor circuit connections. It is a basic way of sharing voltage in a circuit. Say you have a voltage source having a value of 6V and you want to share the voltage such that it will be divided into three places in the circuit, you can use a voltage divider to achieve that. See image below  
voltage divider
Figure 7.0 voltage divider

In the figure above, we have a voltage source from a battery connected to 3 resistors, each of these resistors will tend to restrict current flow in the circuit from the positive terminal of the battery to the negative. The amount of resistance offered is proportional to the resistance value of the resistors. At the same time, the amount of voltage that drops across the resistor will depend on the resistance value of the resistor. hence, large resistance value brings about large current flow resistance and large voltage drop. Therefore, more voltage will drop on the 3k resistor followed by the 2k and then the 1k.

To see exactly how the voltage is shared, we connect DC voltmeters across the resistors to measure the voltage drop. The circuit is simulated with Proteus and the result is shown below:

voltage divider measurement
Figure 8.0 voltage divider measurement

Note that the voltmeters are connected in parallel with the resistors. You measure voltage in parallel. Read more about that here.

Mathematically, we can calculate the voltage drop on resistor R1, R2 and R3 in the circuit above using the for formulae. See below:

voltage divider calculation formula
voltage divider calculation

You can see that the simulated results conformed with calculated results

2. Current divider: when current flows from a power source and meets a node or a junction that creates branches in a circuit, the current will divide and enter those branches, the amount of current that enters a branch is determined by the electrical resistance of that branch. Thus, we use resistors to divide current in a circuit.

current divider
Figure 9.0 current divider

When the circuit is simulated with a 6A current source and ammeters connected in series in each branch, we obtained the result below.

current divider measurement
Figure 10.0 current divider measurement

We can as well obtain this result from calculation just like we did for voltage above. The method is simple for two resistors, but for more than two resistors, However, the method will be a bit tricky. I will write about that in a different tutorial.

For just two resistors, the formula is as shown below.

current divider calculation
current divider calculation

The simulation for two resistors shows that 4A flows in the branch with 1k resistor and 2A flows in the branch with 2A resistor. This conforms with the rule that more current will flow in the least resistive part in a circuit. Watch the video below.

Video: current flow simulation

The formulae and calculations are shown below

3. Current limiter: the main duty of a resistor is to limit current flow in a circuit, this is the simplest action a resistor can perform in a circuit. In any circuit that has an LED as an indicator, a resistor is always connected in series with the LED, this is to limit the amount of current that enters the LED to avoid damaging it.
The circuit diagram below shows resistor connection for limiting current flow entering an LED. To make sure the right amount of current is entering the LED, you have to calculate that.

current limiting resistor circuit
Figure 12.0 current limiting resistor circuit

Types of fixed resistors – what is a resistor

There are different types of fixed resistors, the difference is in the materials used in making them. Below are most of the various fixed resistor types we have:

Carbon film: It is made from the deposition of a carbon film onto small amount of ceramic. This resistor is one of the most popular resistors used today. They are very reliable, easily soldered, noise and heat stable with a typical power rating of ½ to 2 watts. Resistances range from 10Ω to 1M Ω, with tolerance around 5%.

Carbon Film resistor
Figure 13.0 Carbon Film resistor

Carbon composition resistor: This type of resistor is made by mixing carbon powder with glue-like binder. They can show predictable performance, low inductance and low capacitance. Their power ratings range from 1/8 to 2 watts. Resistance range from 1Ω to 100MΩ with tolerances around ±5%.

carbon composition resistor
Figure 14.0 carbon composition resistor

Metal oxide film: this general purpose is made with a ceramic core, coated with metal oxide film. At high-temperature operations they are electrically and mechanically stable and readable. They are resistant to solvents, heat, flames and humidity. Their resistances range from 1Ω to 200kΩ with typical tolerances of ±5%.

metal oxide film resistor
Figure 15.0 metal oxide film resistor

Precision metal film: they are very accurate and very low-noise resistors. they are made with ceramic substrate coated with a metal film, all enclosed in an epoxy shell. These precision resistors are used for making precision devices and instruments like measuring instruments, audio and video devices, etc. Their resistances range from 10Ω to 2MΩ with rating from 1/8 to ½ watts and tolerances of ±1%.

precision metal film resistor
Figure 16.0 precision metal film resistor

High power wire wound:  these are used for high-power operations. They include: vitreous enamel coated, cement and aluminum housed wire-wound resistors. They are the most durable with high heat dissipation and high temperature stability. Their resistances range from 0.1Ω to 150KΩ, with power ratings form around 2 to 500 watts or more.

Square ceramic resistor
Figure 17.0 Square ceramic resistor
wire-wound resistor
Figure 18.0 Wire-wound resistor
typical wire wound
Figure 19.0 Typical wire wound

Variable resistors

Sometimes, we need to vary the resistance of a resistor in order to create voltage or current division in a circuit. This requires varying the resistance of the resistors in use. Changing the fixed resistor every now and then to bring about this division doesn’t look smart; hence, we use a variable resistor to achieve this smartly. With a mechanical variable resistor, you can change the resistance of the resistor by turning a knob. Below are few variable resistors we have.

variable resistors
variable resistors

Common variable resistors include: rheostats, potentiometer and trimmers. A potentiometer can be converted to a rheostat by joining one of its end legs with the middle leg. Or by using the middle leg of the potentiometer and any of the two end legs.

potentiometer circuit diagram
Figure 21.0 Potentiometer circuit diagram
rheostat circuit diagram
Figure 22.0 Rheostat circuit diagram

Rheostat is used to vary current, while potentiometer is used to vary voltage.
Potentiometers are used mainly for low-power level dc electricity, why rheostats are used for high-power alternating current. Trimmers, on the other hand are miniature potentiometers that have turning life-span. They are used for fine-tuning circuits.

Potentiometer Taper – what is a resistor

When buying a potentiometer to be used for signal amplitude control, be sure to know which one you need for the design. potentiometer tapering is what you must know to make that decision. We have linear tapering and non-linear tapering. I will make a tutorial on them in the future.

Photoresistors and Thermistors – what is a resistor

 A photoresistor or Light Dependent Resistor (LDR), is a type of resistor that changes resistance as light is applied or removed from it. Typically, the resistance of an LDR reduces as light intensity increases and the resistance increases as light intensity reduces. They come in different sizes and shapes. An LDR is otherwise called light sensor, it is used in designing light activated switches like street lights.

light dependent resistors
Figure 23.0 Light dependent resistors

On the other hand, a thermistor is resistor whose resistance varies with variation in applied heat. A thermistor can be either NTC or PTC, NTC means negative temperature coefficient, while PTC means positive temperature coefficient. NTC means that the resistance goes negative or reduces as temperature increases, and PTC means the resistance goes positive or increases as temperature increases. We commonly see NTC thermistors at the AC input side of a switch mode power supply (SMPS).

The work of this NTC in most switch mode power supply circuits is to reduce the inrush current that enters the circuit once it is powered ON.  See the circuit diagram below.

power supply
Figure 24.0 SMPS input stage circuit showing an NTC
Figure 25 SMPS input stage circuitry showing an NTC
Figure 25 SMPS input stage, circuitry showing an NTC
an NTC- Negative temperature coefficient resistor
Figure 26.0 NTC- Negative temperature coefficient resistor

Advancing your Learning in electronics

So far, we have discussed reasonably well the concept of resistor, in the next tutorial I will discuss resistor colour codes.

To keep learning, you can check out the tutorials below:

What is a capacitor
How to simulate AC voltage step-down transformer on Proteus
Arduino tutorials for beginners
How to make a robotic arm with arduino
Arduino tutorials

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