Resistors Restricting the Electrical FlowBy Douglas Alexander and Brian Steeves
Resistors - Discrete DevicesResistors are the most commonly used "discrete devices" in electronic components and their main purpose in a circuit board design is to create specified values of current and voltage. The first precision wire wound resistor was invented and patented by the African-American Engineer Otis Boykin in 1959.
The following is a discussion on some of the various resistor-based applications commonly in use today.
How Resistors Work and What They DoWhen placed in series, with a tap connection between them, the resistors are used as voltage dividers. Their function is to produce a particular voltage from an input that is fixed or variable. This is one way to derive a bias voltage. The output voltage is proportional to that of the input and is usually smaller. Voltage dividers are useful for components that need to operate at a lesser voltage than that supplied by the input.
Resistors also help to filter signals used in oscillator circuits for video, audio, and many other clocked circuit devices. Used together with capacitors, this is known as an "RC" circuit, and the oscillation is a function of the two interacting to produce a time constant.
Because the flow of current through a resistor converts electrical energy into heat energy, the heat generated from the high resistance to the flow of the current is used commercially in the form of heating elements for irons, toasters, room heaters, electric stoves and ovens, dryers, coffee makers, and many other common household and industrial products. Similarly, it is the property of resistance that causes a filament to "glow" in light bulbs.
Common Resistor ApplicationsCurrent shunt resistors are low resistance precision resistors used to measure AC or DC electrical currents by the voltage drop those currents create across the resistance. Sometimes called an ammeter shunt, it is a type of current sensor. Resistive power dividers or splitters have inherent characteristics that make them an excellent choice for certain applications, but unsuitable for others such as lumped element dividers.
Resistors can be used in stepped configurations when they are tapped between multiple values or elements in a series. In the absence of a variable resistor (potentiometer), connecting to the various taps will allow for different fixed resistance values.
Using high wattage wire wound resistors as loads for 4-corner testing is a common practice in the qualification process of a power supply. By varying the line voltage and the resistive load to all four extremes, (low line, high line, low load, and high load), a power supply's operating limits can be determined.
Typically, a single one mega ohm (1MΩ) resistor is used with an antistatic or ESD wrist strap for safely grounding a person working on very sensitive electronic components or equipment. The wrist strap is connected to ground through a coiled, retractable cable with the 1 mega ohm resistor in series with ground. This allows for any high-voltage charges or transients to leak through to ground, preventing a voltage buildup on the technician's body and thereby avoiding a component-damaging shock hazard when working with low-voltage tolerance parts.
Impedance matching is the practice of designing the input impedance of an electrical load or the output impedance of its corresponding signal source in order to maximize the power transfer and/or minimize reflections from the load. Resistive impedance matches are easiest to design and can be achieved with a simple L pad consisting of only two resistors.
Types of ResistorsPassive terminators consist of a simple resistor. There are two types: (1) a resistor between signal and ground like in Ethernet, or (2) a resistor pair, one from the positive rail to signal with another from the signal to negative rail. Terminating resistors are widely used in paired transmission lines to prevent signal reflection.
Fusible resistors are designed with current limits in mind. When the current reaches a predetermined limit, the resistor opens like a fuse. This type of resistor would be used as a protection device.
Wire wound resistors are also ideal for compensating strain gauge transducers. They offer the necessary accuracy and perform reliably at high temperatures. They are designed to minimize resistance value change, or to change in a controlled manner over different temperatures. Wire wound resistors are made by winding a length of wire on an insulating core. They can dissipate large power levels compared to other types and can be made with extremely tight resistance tolerances and controlled temperature characteristics.
Special Purpose ResistorsMemristor - a passive two-terminal electrical component in which there is a functional relationship between electric charge and magnetic flux linkage. When current flows in one direction through the device, the electrical resistance increases, and when current flows in the opposite direction, the resistance decreases. When the current is stopped, the component retains the last resistance that it had, and when the flow of charge starts again, the resistance value will be what it was when it was last active.
Varistor - (Voltage Dependent Resistor or VDR). A varistor's function is to conduct significantly increased current when voltage is excessive.
Thermistor - type of resistor whose resistance varies significantly with temperature. These are widely used as temperature sensors, inrush current limiters, self-resetting overcurrent protectors, and self-regulating heating elements.
Humistor - Resistance varies with humidity
Ballast Resistor - Compensates for normal or incidental changes in the physical state of a system. It may be a fixed or variable resistor. Some ballast resistors are designed to increase resistance with an increase in current and, conversely, decrease resistance with a decrease in current.
By no means has this been an exhaustive discourse on resistor types or applications. For further detail on resistor technology with testing guidance, please visit us at ComponentsEngineering.com for our latest Resistor 411 or click the images
These 411's are at no cost to you and are designed to be printed and laminated for your quick reference retrieval. In our next imPARTing Knowledge series, we will consider diodes as our featured Component Engineering device.
Douglas Alexander has been working in the electronics R&D and manufacturing sector for over 25 years with experience in all aspects of component selection, qualification, verification, specification control, reliability prediction, and assurance. He was an integral part of Digital Microwave Corporation, WebTV and Transparent Video Systems and holds patent #6907615 for System and Methods for Use in Interconnecting Consumer Electronic Devices. Alexander has also developed ComponentsEngineering.com an online networking, knowledge sharing, meeting place for Components Engineers.