Download leds and resistor values

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 ?