You’d be hard pressed to find a better name for an electronic component than the resistor. Quite simply, resistors resist! While that may not sound like much, this basic function makes them one of the most commonly used components in circuits, devices and systems worldwide.
The term “electrical resistance” is defined as the voltage required to make a current of one amp flow through a circuit. If it takes 200 volts to make one amp flow, then the resistance is 200 Ohms (Ω).
Resistors are passive, two terminal elements used to reduce current flow in a circuit. They do this by converting electrical energy into heat, which can be dissipated by various means. This capability also allows them to accomplish many things of use to an engineer, like divide voltages, reduce voltages, adjust signal levels, produce heat on demand, and limit current surges. Resistors can also be used to simulate the electrical load on a circuit. These “dummy loads” effectively absorb power to simulate the working conditions on a system in order to test it.
An amusing way to look at resistors is to think of the current as water, the wire that carries the current as a firehose, and the resistors as drinking straws. In electrical devices, conductors allow electricity to flow, and insulators do not allow current flow. Resistors allow precisely controlled amounts of resistance to be introduced into circuits.
Resistance to the flow of electricity in a resistor is governed by Ohm’s Law, or V=IR, where V is voltage, I is current, and R is resistance (measured in ohms). Ohm’s Law (from German physicist Georg Simon Ohm) describes the relationship between these three values in a circuit, so if you know any two of the values you can calculate the third. Resistor parameters, or values, can range from a fraction of an ohm for current sensing, to megaohms for high voltage applications.
Resistors are also blind to the polarity of a circuit, meaning that they can be installed backwards and still function. Current can pass in either direction with the same resistive result.
There are two basic types of resistors, with each performing specific functions:
- Fixed Resistors – do not vary resistance values depending on a change in voltage or temperature. This allows them to provide a constant resistance in all environments.
- Variable Resistors – offer resistance values that can be adjusted, allowing not only current restriction but also changes in current flow. Examples are dimmer switches and volume controls.
With Variable Resistors there are two main types:
- Linear Resistors - permit current that is directly proportional to the voltage applied across the resistor. Resistance does not change with current variation. There is a linear relationship between the method used to adjust the resistance (physical influence) and the resistance itself. A potentiometer set to half its full rotation will be half the resistance potential of the potentiometer.
- Non-Linear Resistors – can vary resistance values depending on applied voltage or temperature. Examples include Thermistors, Varistors, and Photo Resistors. Non-linear means that the physical influence on the resistor effects the resistance non-linearly
Resistors come either with leads, or as surface-mount packages for production purposes. Resistor values on devices with leads are usually indicated by a series of colored bands on the device. These bands are an IEC (International Electrotechnical Commission) standard coding system that usually specify the device value, tolerance and failure rate. The bands can vary in number (depending on manufacturer) from three to six. At a minimum, two bands on the resistor indicate the resistance value and the other serves as a multiplier for measurement purposes. Additional bands indicate tolerance, temperature coefficient, and failure rate of the device. The first band in the series is usually placed the closest to a lead.
Choosing the resistors needed in a design can be a relatively simple matter of deciding on the resistance value you need (which will require some calculations), determining the power that you need to dissipate in your circuit, and choosing a resistor to match those values. Other factors that may need to be considered (depending on design) include:
- Initial Tolerance: variation from nominal performance values (expressed as a %)
- Power Rating: how much the component can handle (expressed in fractional or full watts)
- Voltage Rating: gives the ability to withstand voltage differentials
- Temperature Drift and Stability: rates the change in performance due to temperature
- Long Term Stability: ability to maintain performance despite aging
- Surge Tolerance: ability to withstand current spikes
Resistors are commonly made from carbon, metal, or a metal-oxide film and enclosed in an insulating casing. Introducing different materials in manufacturing can give them different properties, including high temperature resistance, heat tolerance, increased accuracy, noise tolerance, etc.
If you took apart a common resistor and looked inside you would find a ceramic rod wrapped with copper wire (wire-wound resistor). The number of copper turns controls the resistance. Resistance increases with the length of the wire, so more turns means a longer wire and thus more resistance. In surface-mount resistors, designed for lower power circuits, the copper wire is replaced by a carbon spiral pattern (carbon film resistor). Surface-mount resistors perform the same functions as regular resistors, but are meant for use in modern surface-mount manufacturing processes.