Electronics Basics: Read an LED Circuit Model

Use polarity and a stated forward-voltage approximation.

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An LED has an anode and cathode. In normal forward conduction, conventional current goes from anode to cathode. For this paper model only, assume a fixed 2 V forward drop, a 5 V ideal source and a 300-ohm series resistor. Real parts require datasheet limits; reversing an LED is not guaranteed harmless. Use diagrams and calculations only. Never use wall outlets, short batteries, open powered equipment or handle charged capacitors.

Worked example

The resistor has 5 - 2 = 3 V across it. Current is 3/300 = 0.01 A = 10 mA. The same current passes through the series resistor and LED. Never connect a bare LED directly to a battery for this activity.

A paper circuit model has a five-volt source, 300-ohm series resistor and forward LED with an assumed two-volt drop. Resistor voltage is three volts; series current is ten milliamps. A labels the anode and K the cathode.

Would you believe it?

LEDs make light without heating a wire filament until it glows. Their light comes from a semiconductor, a different method from an incandescent lamp.

Department of Energy: lighting technologies

Question 1 Under the stated approximation, what voltage is across the resistor? 7 V 3 V 5 V

Question 2 What is the modeled series current? 10 mA 100 mA 1 mA

Question 3 What current passes through the LED in this series model? 0 mA 20 mA 10 mA

Question 4 In forward conduction, conventional current through the LED goes which way? Either way equally Anode to cathode Cathode to anode

Question 5 Does the fixed 2 V assumption apply to every LED in every condition? No Yes Only if the LED is small

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