Why use resistors in your circuit board?
A resistor is one of the most common components in any electrical circuit. This may have left you wondering whether you can do without it.
Resistors are core components in regulating current flow; without them, our circuits would not operate as smoothly as they do.
So what specific roles do they play in circuitry? Let us explain it in detail below.
What is a Resistor?
One of the basics of electricity is the Ohm’s law which states that R = V/I whereby R is the circuit’s resistance, V is the potential difference between one point and another, and I is the electric current.
Therefore, the resistor is the passive electronic component responsible for introducing the element of resistance in current flow.
We give resistance in ohms (Ω), named after Georg Simon Ohm. One unit of ohm represents the resistance of an ampere of current flow when the potential difference (voltage drop) is equivalent to one Volt.
Resistor Symbol and Definition
Electronic resistor symbols.
In summary, we have defined a resistor as any component in an electric circuit that controls the current flow rate per unit voltage.
As per the table below, we represent resistors in two primary forms depending on the applied naming technique.
| Naming Convention | Symbol |
| US American National Standards Institute Standard (ANSI Standard) | |
| International IEC symbol |
Overview of Resistor Types and Materials
We use two main classification criteria for resistors:
- According to their functional types,
- According to the materials used to make them.
Resistors Functional Types
| Resistor Type | Examples |
| Fixed resistors | Surface Mount Resistors, Chassis Mount Resistors, and Through-Hole Resistors. |
| Variable resistors | Trimpot, Potentiometer and Rheostats. |
Still, we have another classification on the functional types whereby we base the resistance value on different physical quantities.
These resistors have a unique standard symbol primarily because they serve a specific role from the general purpose resistors.
Check out the illustrations below.
| Resistor Type | Physical Quantity | Symbol |
| Strain gauges | Mechanical Load | |
| Thermistors (Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC)) types. | Temperature | |
| Magneto resistors | Magnetic field | |
| Varistors | Voltage |
Resistors Types According to Material
Lastly, we can classify resistors according to their manufacturing procedure and the make-up material.
Examples include the following:
- Metal oxide film resistors
- Carbon composition resistors
- Foil resistors
- Metal film resistors
- Wire wound resistors
Notably, each of the materials and resistor technology is specific to the respective application of each of the resistors.
| Resistor Type | Make-up Material | Resistor Application |
| Carbon Composition Resistor | A mixture of non-conductive ceramic and fine carbon particles | It is a low-precision type of resistor useful in high energy pulses applications. |
| Wire Wound Resistors | A metal alloy wire wound around a non-conductive core. An example of such an alloy is nickel-chrome. | They are useful in high-power and high-precision applications. Other remarkable features include the capacity to reach an extremely low resistance value, high durability and accuracy. Downside: When functioning at high frequencies, they are prone to parasitic reactance. |
| Carbon Film Resistors | A thin carbon film layer wound around a non-conductive rod. | They have better tolerances than their carbon composition counterparts. |
| Metal Foil Resistors | A special alloy cold-rolled film on a ceramic core substrate material. | Foil resistors are renowned for their extremely high precision. |
| Metal and Metal Oxide Film Resistors | A resistive film wound around a cylindrical non-conducting material | These can withstand temperature variations and are highly durable and stable. |
Resistor Characteristics
Several resistors.
The primary consideration when dealing with resistors is the electrical resistance value.
Nonetheless, another important characteristic is the resistor tolerance which we give as a percentage of the overall resistance value.
For instance, you will likely find the resistor tolerance expressed in the following form:
±6%.
Moreover, manufacturers will also provide other ratings that affect the resistor value, such as the following:
- Temperature coefficient
- Maximum voltage
- Operating temperatures
- Maximum Power Rating
- Maximum rating
- Electric Noise (for resistors in audio applications)
- Pulse Stability
- Failure Rate or Mechanical Strength for components that operate in harsh conditions like military work.
Resistor color code
A resistor color decoding diagram.
When dealing with axial leaded resistors, we use their color code to identify their resistance value and tolerance.
You have probably noted that most resistors we use in DIY projects are multicolored.
So how do you decode the colored stripes representations on resistors?
Search for a resistor color code calculator online and calculate the resistance value.
However, note that when dealing with SMD resistors, we use numerical rather than color codes to identify their rating.
This is primarily because of their miniature size, making it hard or nearly impossible to engrave color bands.
Also, SMD resistors come in a standardized package, meaning that their physical sizes are the same, with their only differentiating aspect being the number code.
What does a resistor do?
Different types of resistors.
A resistor is a material made of elements capable of controlling the flow of electric charges.
Thus, they determine what current volume flows from one point of the circuit to the other.
Shielding Circuits Against Voltage Spikes
Some electrical components, such as LED lights, are prone to damage resulting from fluctuations in voltage, especially when there’s a spike.
A resistor comes in handy in curtailing this issue by ensuring that only the necessary current flows to the load.
Components such as circuit breakers and fuses complement the operation of resistors.
Enabling Proper Voltage Flow
A resistor creates a temporary voltage drop in electronic circuits, especially when some circuit parts require a lower current than others.
They are, therefore, regulators of voltages, which explains why we connect a resistor next to every auxiliary component of a circuit.
So how does a resistor achieve this? It simply curtails the speed of the electrons’ motion which subsequently controls the current flowing to a specific load such as an LED.
Why do we use resistors?
Resistors on a circuit board.
Above, we’ve highlighted some of the typical applications of resistors. We’ll expound further on them below while listing their other primary applications.
Current Limiting
As mentioned earlier, the only way to control the volume of current flowing to a component (current rating) in an electric circuit or PCB is via a resistor. Excessive current is harmful and can cause the burning of parts.
Therefore, for a circuit with constant voltage, tweaking with resistance can decrease the current flow according to the resistance value of your resistor. This explains the profound use of current-limiting resistors in all circuits.
Voltage Division
A Voltage Divider Circuit.
Take a case whereby you have various components in your circuit with different voltage requirements.
You may opt to have additional voltage sources, each matching the rating of the respective element, but this would be messy and not feasible in most cases.
The ultimate solution to such a dilemma is having resistors in your circuit, which play the role of voltage dividers.
They split the voltage according to the needs of the respective components, thereby eliminating the need for different voltage sources.
Pulling Up or Down Floating Voltages
Take a case whereby you use a microcontroller such as Arduino, which features more than one input pin. If you do not connect any component to an input terminal, the microcontroller does either of the following:
- It may pull the pin to the VCC, meaning that the floating voltage will be rendered high.
- Alternatively, it could pull it to the ground, lowering the pin state.
The solution to these floating voltages is to connect a pull-down resistor for the first case and a pull-up resistor for the latter.
Feedback Circuits
An opamp illustrating the feedback concept.
Operational amplifiers function in a loop system, which feeds the output back to the input.
This mode of operation leads to a problem, especially when dealing with an infinite output gain.
The solution is to include a negative feedback resistor which regulates the gain back to safe operating levels.
Terminating Load
Testing the load ratings of a circuit requires a resistor. For example, take a case where you have a 1k load and would like to test if the load truly adds up to that figure.
All you need is to connect a 1k Ohms resistor across the load.
This principle also applies in the communication field, whereby electrical engineers use the technique in facilitating recessive bit transmission.
Gain Controller
How do you control the gain in a common collector (CE) circuit? Simply connect a collector resistor to the circuit, and its value will be directly proportional to the overall circuit gain.
Therefore, you can increase the gain by using a collector resistor with higher resistances.
Heating
An electric heater.
The heating aspect (thermal energy) in electric circuits is possible thanks to the heat emission properties of resistors when conducting current.
This is primarily the operation principle behind lighting in incandescent light bulbs.
Resistors, therefore, are capable of converting electrical energy into heat.
Other applications of resistors in heating include microwave ovens and electric stoves/ heaters.
LEDs and Transistors
LED and resistor in a series circuit.
Unlike their incandescent counterparts, LEDs cannot withstand extremely high temperatures.
They primarily require little electrical current to operate, and the only proper way to regulate the amount that gets to the diodes is by incorporating a resistor into the circuit.
The same also applies to transistors which require relatively low currents to function.
Summary
Like capacitors, resistors are must-have parts in any electric circuit thanks to their wide range of applications.
Most components would burn in their absence due to excess current or malfunctioning.
We’ve covered all their applications and hope you know everything about resistors and when to use them.