Simple LED Calculator

This calulator will help you figure out the proper resistor(s) needed to safely hook up one or more LEDs to power supplies of various values.

Enter the LED layout, number of LEDs, supply voltage,the Vf, and If LED values. The Vf and If values are found on the LED packaging. Press the button and the schematic will appear showing how to arrange the components. The resistance value will also be calculated.

LED layout (?):	Building List...
Number of LEDs:	Building List...
Supply voltage (?):		Building List...
LED Vf (forward voltage) (?):	Volts
LED If (forward current) (?):	milliAmps
Resistor value (?):
Resistor power rating (?):
Total current draw (?):

LED Calculator Using Constant Current Circuit

This calulator will help you figure out the proper resistor needed to safely hook up one or more LEDs to power supplies of various values. Even though this version is slightly more complex, it provides better control over the current no matter what charge state the supply is.

Enter the LED layout, number of LEDs, supply voltage,the Vf, and If LED values. The Vf and If values are found on the LED packaging. Press the button and the schematic will appear showing how to arrange the components. The resistance value will also be calculated.

LED layout (?):	Building List...
Number of LEDs:	Building List...
Supply voltage (?):		Building List...
LED Vf (forward voltage) (?):	Volts
LED If (forward current) (?):	milliAmps
Resistor value (?):
Resistor power rating (?):
Total current draw (?):
LM317T Power Dissupation (?):

Other Useful LED Information

• LED Wiring
  The below picture shows the schematic symbol and physical LED.
  
  The positive (+) anode lead is longer than the negative cathode (-) lead. Also, the casing of the lens has a flat spot on the negative side.
  
  If you still can't tell which lead is negative, you can temporarily hook the LED to a 9v battery along with a 330 ohm resistor.
  
  There's really not much to worry about if you do accidentally hook it backwards: it just won't work.
  
  
  
• LED Formulas
  Below are the formulas being used in this page:
  
  LEDs in series:
  Resistor Value = (V_supply - (V_f X LED_count)) / I_f
  Resistor Power = I_f² X resistor_value
  
  
  LEDs in parallel with common resistor:
  Resistor Value = (V_supply-V_f) / (I_f X LED_count)
  Resistor Power = (I_f² X LED_count) x resistor_value
  
  
  LEDs in parallel with seperate resistors:
  Resistor Value = (V_supply-V_f) / I_f
  Resistor Power = I_f² x resistor_value
  
  
  
  Where:
  resistor_value = Resistance in ohms
  V_supply = Supply voltage in volts
  V_f = LED forward voltage in volts
  I_f = LED forward current in amps
  LED_count = Number of LEDs used
• Using a Linear Voltage Regulator
  The formulas in the first calculator above assume a steady voltage, but in reality, a battery will have three voltage states: fully charged voltage, nominal voltage, and discharged voltage. For instance, if you are using a 4s lipo pack, these voltages will be 16.8v (4.2v/cell), 14.8v (3.7v/cell), and 12v (3.0v/cell), respectively. If a resistor is chosen based on the nominal value, the LED will be brighter when hooked to a fully charged battery, and dimmer when the battery is nearly discharged.
  
  This can be fixed by the use of a linear voltage regulator. This would keep the output voltage constant no matter the battery charge level. So, if a 5v regulator is used on a 3s lipo pack, the output will remain a constant 5v even though the battery will fluctuate from 12.6v down to around 9v.
  
  RadioShack carries two inexpensive versions; the LM7805 (5v) and the LM7812 (12v). The LM7805 would be used if using battery voltages between 2s-4s lipo (6-12 NiMH cells). The LM7812 would be used with 5s lipo (15 NiMH cells) or higher. The idea is to choose a regulator whose output is at least 1.5v higher than the minimum battery voltage.
  
  Below is a simple schematic diagram showing how to hook up one of these regulators:
  
  
  
  FYI: Any of the regulators listed above can technically be used wired to have adjustable outputs. Two extra resistors would be required to "program" the voltage. However, this is beyond the scope of this page, and one of the LM78xx series regulators will work fine in 99% of the cases.