Tech: Lighting: Resistor Values for LEDs
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| To limit the current to a level for the LED to
operate safely, a resistor of a specific value must be placed in the
circuit between the current and the LED. To find the resistor value,
the following formula is used: |
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| The voltage source is what your power's current
is. A 9 Volt battery is 9 volts, a 1.5 Volt D cell is 1.5. The LED
Drop value should be listed on the package that the LED came in. Radio
Shack refers to the drop voltage as the "Forward "Supply)
Voltage. The Forward Current should also be listed in the LED specifications
that came with your LED. Usually, for LEDs the forward current is
in milliamps or mA. |
 |
| So, for example, I plan to run the lighting from
a 9 volt battery. The Forward (Supply) voltage is listed as 3.6V.
and the Forward Current is listed as 20mA which is numerically expressed
as .02 |
 |
| First subtract the Source Voltage (9) from the
Voltage Drop number (3.6). That yields 5.4 |
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| Divide that number by the Forward Current Value
of 0.02 |
 |
| The result is 270, which is the value of the resistor
expressed in Ohms. So for my white LED, I need to use a resistor of
270 Ohms. Sometimes the math reveals a resistor value that isn't available
so get resistors to the closest value. |
