All About Resistors on Circuit Boards
Resistors are passive components necessary for use in circuit board assemblies. Rush PCB Inc uses resistors in their PCB assemblies to resist the flow of electric current. Electronic circuits use different types of resistors, such as linear, non-linear, fixed and variable. While fixed resistors have a stated value that does not change, it is possible to adjust the variable resistor from zero to its stated value. Both through-hole resistors and surface-mount resistors are available. Circuit designers use resistors in conjunction with other passive and active components such as capacitors, inductors, transistors, diodes, and ICs to process electrical signals in many ways. The physical size of a resistor depends on its power dissipating capacity.
Types of Resistors
Broadly, the electronic industry classifies resistors into:
Fixed Resistors
As their name implies, fixed resistors are ones that present a non-changing resistance value. Manufacturers make fixed resistors from a variety of resistor materials:
Carbon Composition Resistors
Carbon composition resistors are the oldest type available in the industry. Earlier, the industry used them extensively. Manufacturers made them of powdered ceramic and carbon combination. With better types of resistors being available now, the use of carbon composition resistors has declined.
Wire-Wound Resistors
Manufacturers produce wire-wound resistors by winding a resistive wire on an insulating rod or core made of plastic, glass, or ceramic. The resistive wire is mostly nichrome or Manganine. Wire-wound resistors can usually handle high power dissipation. As wire-wound resistors produce an inductive effect, they are not suitable for use in high-frequency equipment.
Thin Film Resistors
Thin film resistors are typically high-precision. They are suitable for use in high-stability circuits that also require low-noise components. Typical uses of thin film resistors are in testing and measuring equipment, audio equipment, and medical devices to handle low power.
Thick Film Resistors
The construction of thick film resistors is similar to that of thin film resistors, except that thick film resistors have a thick layer of cermet or metal oxides around the body of the resistor. Their construction makes them very popular and affordable. As they can handle higher power dissipation, consumer electronic equipment often use them.
Fusible Resistors
Fusible resistors play a dual role in an electrical circuit. They act as a normal fixed resistor until the current through them is below a specified limit. Once the current exceeds the limit, the resistor acts as a fuse, and opens to prevent further current flow. High-end electronic products such as amplifiers, television sets, and safety management systems use them widely.
Variable Resistors
As opposed to resistors with a fixed value, circuit designers also use resistors whose value it is possible to change while it is in circuit. Unlike fixed resistors that typically have two terminals, a variable resistor has three terminals. The central terminal usually connects to a mechanical wiper that the user can move along the body of the main resistor. This makes them suitable as a voltage divider. They can be made of carbon material, or wire-wound for handling higher power. Wire-wound variable resistors that can handle high power are also known as rheostats.
Linear Resistors
Linear resistors change their current value in a precise linear proportion to the applied voltage. In other words, linear resistors follow Ohm’s law. Linear resistors can be fixed or variable.
Non-Linear Resistors
Non-Linear resistors do not follow Ohm’s law. Current flow in non-linear resistors changes in a non-linear proportion to the applied voltage. Some examples of non-linear resistors are thermistors, varistors, and light-dependent resistors. These can also be fixed or variable types. Variable non-linear resistors are primarily useful for audio equipment. Fixed non-linear resistors are useful for protection circuits in power handling equipment.
Thermistors
Many electronic equipment, like rechargeable batteries, thermometers, vehicles, and consumer electronic products use thermistors for detecting minor temperature changes. The resistance value of thermistors changes with a temperature shift, causing a change in the current flow.
Varistors
Constructed from semiconductor materials, the resistance of varistors changes with the applied voltage. Varistors are very effective in suppressing high transient voltages. Varistors have normally high resistance, but beyond a specified voltage, their resistance drops significantly. This makes them very useful as surge protectors.
Light Dependent Resistors
The resistance value of light dependent resistors depends on the intensity of light falling on them. When in dark, they exhibit the highest resistance, and with increasing amounts of light, their resistance falls. Light dependent resistors comprise cadmium sulfide.
Resistor Networks
Digital circuits often require multiple resistors to connect to data lines. Instead of using individual fixed linear resistors, manufacturers provide resistor networks with many configurations. Users can select from parallel, series, or voltage divider networks.
Surface Mount Resistors
Unlike most resistors with leads and which require holes in PCBs to mount, surface mount resistors or chip resistor have no leads. It is necessary to mount them directly on to the PCB. With very small physical form factor, surface mount resistors take up little space on the PCB, thereby generating very high component density. Most computing equipment, including many other forms of technology, make use of surface mount resistors.
Selecting a Resistor
Selecting a resistor for any application requires attending to a few parameters:
Power Dissipation
The chosen resistor must be able to dissipate the heat it generates while operating. Therefore, the power rating of the chosen resistor must be more than the heat it will generate in circuit. To be safe, it is recommended to select a resistor with a power rating at least twice as high.
Temperature Coefficient
Depending on the material of the resistor, its value may change as the surrounding temperature changes. If the resistor has a positive temperature coefficient, its value will increase with rise in temperature. For a resistor with a negative temperature coefficient, its value will decrease with a rise in temperature. The chosen resistor should compensate for the effects in the circuit when the temperature changes.
Noise
All components, including resistors, generate noise, This is dependent on frequency, and increases with rising frequency. The amount of noise from a resistor depends on its material. For sensitive circuits, it is necessary to select low noise resistors.
Impedance
With increasing frequency, resistors also exhibit complex impedance. This depends on their construction method. Wire-wound resistors show the highest impedance. For high frequency circuits, it is necessary to use resistors with low impedance.
Conclusion
Electrical circuits utilize many types of resistors depending on the application. Characteristics of resistors vary based on their structure and method of manufacture. At Rush PCB Inc, we choose resistors carefully based on their characteristics and their application in the circuit.