Resistance (Physics)
When a light emitting diode (LED) is connected into an electric circuit, it is wired in series with a resistor to ensure it doesn't get damaged by taking too much current. The voltage across a red LED is 1.5 V when it is lit. You wish to supply it using a 9.0 V battery.
What resistance of resistor is needed if a 30 mA current is to flow?
What resistance of resistor is needed if a 30 mA current is to flow?
These calculations seem a bit weird when you first meet them.
The strange characteristic of LED's is that starting from 0V they don't conduct at all until the forward PD across them is below a threshold level (called VF) ... and then once you've reached VF they're suddenly fantastic conductors that will try and pass as much current as necessary to stop the PD across them going above VF... even if that amount current would destroy the LED.
so you've got a 9V battery and a LED that's going to have 1.5V across it... which means the series resistor is going to have 7.5V across it.
cos it's in series with the LED the current in the resistor is the same as the current in the LED and the current in the LED must not exceed 30mA
you must use ohms law to find the value of resistance that passes 30mA at 7.5V
see also
The strange characteristic of LED's is that starting from 0V they don't conduct at all until the forward PD across them is below a threshold level (called VF) ... and then once you've reached VF they're suddenly fantastic conductors that will try and pass as much current as necessary to stop the PD across them going above VF... even if that amount current would destroy the LED.
so you've got a 9V battery and a LED that's going to have 1.5V across it... which means the series resistor is going to have 7.5V across it.
cos it's in series with the LED the current in the resistor is the same as the current in the LED and the current in the LED must not exceed 30mA
you must use ohms law to find the value of resistance that passes 30mA at 7.5V
see also
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