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Calculate Resistor Value for LED
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This calculator is used to find the value of the dropping resistor required to limit the current of an LED to the selected milliamps. The Vf defaults to 1.95v and is typical for most LEDs, except blue and white which will be slightly higher. The R1 value is reported in ohms.
The wattage of the resistor used for R1 must be greater than the value reported by the calculator. Example: if 0.07 is shown in the Wattage field, then a 0.125 watt resistor (1/8w) or higher is acceptable.
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Enter the current at which the LED is to operate and the calculator will find the resistor value. Alternately, you
can leave the LED current field empty and the calculator will report the milliamps being used by the LED.
The Voltage Supply and LED forward voltage values must always be entered into the form. The wattage for
the R1 resistor is the minimum that should be used. Select the next higher wattage resistor. A 1/4w resistor
is 0.250 w, a 1/2w resistor is 0.500 w, a 1w resistor is 1.000 w and so on.
20 milliamps is the typical maximum current for most LEDs. Select a value less than 20 unless using a special LED. A current of 5 milliamps should be bright enough for most LEDs, and 2 or 3 ma is plenty for a blue color indicator.
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I tested LEDs at 3ma. and recorded the forward voltage drop for use in the calculator. When possible, I used different type LEDs for each measurement. Example: For the red pair, I used a 5mm round for the first test and a rectangular shape for the second test. Same with the green and yellow. |
| LED Forward Voltages | ||
| Red | 1.98v | 1.96v |
| Green | 1.94v | 1.92v |
| Yellow | 1.90v | 1.92v |
| Blue | 2.76v | 2.78v |
| White | 2.78v | 2.76v |
| Pink | 2.84v | 2.84v |
| Ultraviolet | 3.10v | 3.16v |
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