Resistors and Resistance Resistor ● A Resistor is an electronic component used as a limiting device to adjust and set voltage and current levels in an electronic circuit. Just as the Hoover Dam controls water flow, a resistor controls the amount of current flow in a circuit.
Resistor and Resistance ● Resistors are the most common component in an electrical circuit. Resistors provide an opposition to current flow. ● The larger the resistance, the less current is able to flow. ● The letter reference designator is “R”. ● The unit of measure for resistance is OHMS; represented by the Greek letter Omega.
Resistors Types FixedVariable There are two types of resistors: Fixed and Variable Fixed Fixed resistors have a specific value. Variable Variable resistors are those whose value can be adjusted.
Resistor Types ● General purpose –General purpose resistors are the most common and are used to limit current flow in all parts of a circuit. They are cheaply made; most are carbon. They are identified by color bands. ● Power –Power resistors are used in circuits with higher currents and higher voltages. They are wire wound, and have their value printed on their bodies. fixed ● There are three categories of fixed resistors: general purpose, power, and precision.
Resistor Types fixed ● There are three categories of fixed resistors: General purpose, power, and precision. ● Precision –Precision resistors have a.01 percent tolerance. They are composed of metal film, and are used where accuracy is a must, such as in medical or test equipment. The value of the precision resistor is in printed code on the body of the component.
Resistor Types RheostatsPotentiometers ● There are two types of variable resistors: REMEMBER: REMEMBER: Variable resistors are those whose value can be adjusted.
Resistor Types ● Potentiometers- ● Potentiometers- Have a carbon composition, and are used when circuit current is low. They are used for such things as volume control and light dimmers.
Resistor Types ● Rheostats ● Rheostats are wire wound, and are used when circuit current is high. Rheostats are used as temperature controls for soldering irons and electric griddles.
Resistors Characteristics Three Resistor Characteristics: ● Power rating –The flow of current through a resistor generates heat. In fact, one of the resistor’s jobs is to dissipate heat. If more heat is generated than the resistor can dissipate, the resistor will burn up. A larger resistor has more surface area for the heat to escape. The larger the resistor, the higher the power rating. Power rating is measured in Watts.
● Ohmic Value RESISTOR COLOR CODE: –All fixed resistors have a fixed ohmic value. This value is determined by the manufacturer. All carbon film and carbon composition resistors have color bands. By reading these color bands, you can identify the ohmic value of the resistor. Below is the RESISTOR COLOR CODE: Three Resistor Characteristics: Resistors Characteristics
Three Resistor Characteristics: 1098765 4 32 Read bands from left to right: ● The 1 st Color is the 1 st significant digit ● Example: violet=7 ● The 2 nd Color is 2 nd significant digit ● Example: green=5 ● The 3 rd Color is the multiplier ● Example: orange= x 10 3 (# of zeros) ● The 4 th Color is Tolerance ● Example: gold= 5% (see next slide) ● Ohmic Value ● RESISTOR COLOR CODE:
Resistors Characteristics Three Resistor Characteristics: ● Tolerance ● Tolerance is the amount that the ohmic value can vary from the actual ohmic value. The tolerance rating of general purpose resistors aren't exact (usually between 5 and 20 percent allowance for accuracy). 10%5%20% (default)
Resistor Faults ● Open –An open is a break in the current path in a circuit. Just like a drawbridge 'opens' a roadway, an open resistor prevents current from getting through. If you were to measure the output of an open, you would see the applied voltage of the circuit along with infinite resistance. ● Short –Technically a resistor can short, but only for a split second; then it opens! A short is the opposite of an open. Instead of the applied voltage and infinite resistance, you would measure zero voltage and zero resistance. In other words, maximum current would flow through. If this happened to a resistor, the heat from the increased current would quickly surpass the power rating of the resistor, destroying it.
Resistor Faults ● Changed Value ■Changed values can cause errors, fluctuations, drifts, or even catastrophic outages. The voltage, current and resistance in an electrical circuit must be accounted for between all components. If calculated values versus measured values are different, you could have a changed value.
Summary ● Virtually every piece of electronic equipment contains resistors. Competently troubleshooting them will allow you to isolate outages much faster than pure guesswork. ● A resistor is an electronic device used to limit circuit current. ● Resistors are either fixed or variable. ● There are three general categories of fixed resistors: general purpose, power and precision. ● There are two types of variable resistors: potentiometers and rheostats. ● The most widely used resistor is made of carbon, and has color bands on the body to indicate their ohmic value. ● The three common faults in electronic components are opens, shorts and changed values. Resistance is Futile!