# Example Transistor Circuit with LEDs

Posted on Feb 5, 2014

A Transistor is an electronic component which enables a small current to control a much larger current. This feature is very useful in many renewable energy projects and other applications. Click here to read our Introduction to Transistors. Pictured below is a very basic circuit incorporating a transistor. A 12V LED Spotlight bulb (labelled LED

Spotlight in the circuit) is turned on an off using a small switch. When the switch is closed a tiny current (base current) flows into the Base input of the transistor through an ultrabright 10, 000 mcd LED (labelled Dim LED). The dim LED lights, but very dimly because the current through it is so small because of the 10K resistor in series with it. When this current arrives at the Base it is amplified by the transistor into the much larger collector current required to power the twenty 15, 000mcd LEDs of the spotlight bulb brightly. A number of measurements were made in the example circuit constructed above. The power supply voltage was measured as 11. 39 Volts. The voltage across the dim LED was measured to be 2. 72 Volts, and the voltage across the 10K resistor at 7. 83 Volts. The voltage across the large spotlight bulb was 11. 13 Volts. Therefore since the resistor is in series with the dim LED, we can calculate (using Ohm`s Law ) that the current flowing through the resistor (and therefore through the dim LED and into the Base of the transistor) is just 7. 83/10, 000 = 0. 78mA. The transistor amplifies this base current, and the LED spotlight bulb ends up receiving a current of 58mA at 11. 13 Volts (645 mW). The transistor used in this experiment was picked randomly from a box of salvaged components - it was a BC238B NPN type low power general purpose transistor. These are available in bulk from as little as 2p each....

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