The resistor is a passive electrical component to create resistance in the flow of electric current. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses.
High-power resistors that can dissipate many watts of electrical power as heat, may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity.
The resistance is measured in ohms. An ohm is the resistance that occurs when a current of one ampere passes through a resistor with a one volt drop across its terminals. The current is proportional to the voltage across the terminal ends. This ratio is represented by Ohm’s law:
Types of Resistor
Resistors can be divided in construction type as well as resistance material. The following breakdown for the type can be made:
⇒ Fixed resistors
⇒ Variable resistors, such as the:
⇒ Resistance dependent on a physical quantity:
- Thermistors (NTC and PTC) as a result of temperature change.
- Photo resistor (LDR) as a result of a changing light level.
- Varistor (VDR) as a result of a changing voltage.
- Magneto resistor (MDR) as a result of a changing magnetic field.
- Strain Gauges as a result of mechanical load.
Many standards exist for resistors. The standards describe ways to measure and quantify important properties. Other norms exist for the physical size and resistance values. Probably, the most well known standard is the color code marking for axial leaded resistors.
Resistor color code
The resistance value and tolerance are indicated with several colored bands around the component body. This marking technique of electronic components was already developed in the 1920’s. Printing technology was still not far developed, what made printed numerical codes too difficult on small components. Nowadays, the color code is still used for most axial resistors up to one watt. In the figure an example is shown with four color bands. In this example the two first bands determine the significant digits of the resistance value, the third band is the multiplying factor and the fourth band gives the tolerance. Each color represents a different number and can be looked up in a resistor color code chart.
Series and parallel resistors
The total resistance of resistors connected in series is the sum of their individual resistance values.
The total resistance of resistors connected in parallel is the reciprocal of the sum of the reciprocals of the individual resistors.