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What size of resistor is required to operate the LED from a 9V battery?
Wiki User
∙ 14y ago
In order to determine what size of resistor is required to operate an LED from a 9V battery, first start by knowing the current and voltage required for the LED. That information is available in the LED's specifications.
For discussion purposes, lets assume a typical LED at 2.5V and 50mW. The translates to a forward current of 20mA.
Build a simple series circuit containing a 9V battery, a resistor of an as yet unknown value, and the LED.
By Kirchoff's current law, the current in the LED is the same as the current in the resistor, which is also the same as the current in the battery. This is 20ma.
By Kirchoff's voltage law, the voltage across the LED plus the voltage across the resistor equals the voltage across the battery. This is 6.5V. (9 - 2.5)
By Ohm's law, resistance is voltage divided by current, so the resistor is 6.5 / 0.02, or 325 Ohms. The nearest standard value to that is 330 Ohms.
Cross check the power through the resistor. Power is voltage times current, or 6.5V times 0.02A, or 0.13W. A half watt resistor is more than adequate for this job.
Wiki User
∙ 14y ago
Wiki User
∙ 14y agoThe resistor required for a 9V power supply for an LED would be 270 Ohms.
This assumes that the LED requires 25mA at 2V. The drop across the resistor would be 7V. (9V-2V) By Ohm's law, resistance is voltage divided by current, so 7V divided by 25mA is 280 Ohms. The nearest standard value is 270 Ohms.
If you had a low current LED, such as 2.5mA, the resistor would by 2.7K Ohms.
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