The consumption of electric power for this resistor becomes 0.1A x 0.1A x 70 ohm = 0.7W. The resistance value of the resistor becomes 7V / 0.1A = 70(ohm). Here are a few ways to find out how much current the circuit demands.Īssemble the circuit and measure the actual current used with a multi-meter.Ĭheck the component's current use against a standard table.Īssume the current consumed is 100 mA (milliamps) in the following example.ħV must be dropped with the resistor. However, if you try to drop the voltage from 12V to 5V using only a resistor, then you need to calculate the power rating of the resistor as well as the resistance value.Īt this time, the current consumed by the 5V circuit needs to be known. When powering a light emitting diode, a comparatively large current flows through the resistor, so you need to consider the power rating of the resistor you choose.įor example, to power a 5V circuit using a 12V supply, a three-terminal voltage regulator is usually used. 1/8W is almost always used in signal circuit applications. Resistors in electronic circuits are typicaly rated 1/8W, 1/4W, and 1/2W. If the maximum rating of the resistor is exceeded, it will become extremely hot, and even burn. Power is calculated using the square of the current ( I 2 ) x the resistance value ( R ) of the resistor. The maximum rated power of the resistor is specified in Watts. Just like you wouldn't use a 6 volt flashlight lamp to replace a burned out light in your house, you wouldn't use a 1/8 watt resistor when you should be using a 1/2 watt resistor. The power rating indicates how much power the resistor can safely tolerate. For example, a ±5% tolerance would indicate a resistor that is within ±5% of the specified resistance value. The tolerance of a resistor denotes how close it is to the actual rated resistence value. The "tolerance" and the electric power ratings of the resistor are also important. The resistance value of the resistor is not the only thing to consider when selecting a resistor for use in a circuit.
There are other types as well, but these are the most common. The typical resistor is made of either carbon film or metal film. They are also classified according to the material from which they are made. There are two classes of resistors fixed resistors and the variable resistors. Resistance value is designated in units called the "Ohm." A 1000 Ohm resistor is typically shown as 1K-Ohm ( kilo Ohm ), and 1000 K-Ohms is written as 1M-Ohm ( megohm ). This symbol is used to indicate a resistor in a circuit diagram, known as a schematic. The resistor's function is to reduce the flow of electric current.