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Design and Technology Choosing the correct Resistor Standard Values Resistors are only made in certain values. They are based on the ‘E12 series of preferred values’. These are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82. This means that you can buy a standard 68R resistor, 680R, 6.8K, 68K and so on, (but not a 70R or a 67R resistor). Choosing the correct resistor when using an LED For every LED there is a specified LED current (sometimes written IF – the FORWARD current). If a higher current is passed through the LED it will overheat and possibly fail. The LED is a type of DIODE. When current passes through the diode it causes a voltage drop. For every LED there is a specified LED voltage (sometimes written VF – the forward voltage drop). Circuits using LEDs need a resistor to limit the current flowing through the LED to the correct specified value (IF). This is calculated using Ohm’s Law (V = I * R ). To calculate the current flowing through the LED the equation is I = V / R, but the value for the voltage across the resistor is V SUPPLY – V LED R = VS - VLED IF Question 1. What resistor would be needed to limit the current through a red LED with a forward voltage drop of 2v to 10mA when using a 12v power supply ? Question 2. A standard green LED is specified as : IF = 30 mA, VF = 2.8 V. What resistor is needed if this is used in a circuit with a 9v power supply ? Question 3. A 5mm Ultrabright yellow LED has a forward voltage drop of 2v and needs 30mA current. What resistor is needed if a 5V power supply is used ?