Physics - Worked example: LEDs
Worked example: LEDs
An ideal LED has the characteristics shown in the graph below. This LED is
connected into the circuit shown. The resistor protects the diode by
ensuring that the current flowing in the circuit is not too high. The LED
is glowing, indicating that a current is able to flow through it.
* What voltage exists across the 40 ohm resistor?
* What current flows through the resistor and LED?
* A 50 ohm resistor is now connected in parallel with the LED. The LED
still glows brightly. What current must be passing through the 50 ohm
* Since the LED is operating, it must have a voltage of 0.8 volts across
it (the graph gives this as the switch-on voltage). This means that 2.2
volts are available to the 40 ohm resistor.
* Using the values known to occur for the resistor:
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