Example Transistor Circuit with LEDs

Put together a very simple circuit which uses LEDs to show how transistors work

A Transistor (1) 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 (2).

Transistors in Action

Pictured below is a very basic circuit incorporating a transistor. A 12V LED Spotlight (3) 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 (4) (labelled Dim LED).
The LED lights, but very dimly because the current through it is so small thank to 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.

Transistor Circuit Analysis

A number of measurements were made in the example circuit constructed above. When switched on the power supply voltage was measured as 11.39 Volts. The voltage across the LED at 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 (5)) that the current flowing through the resistor (and therefore through the dim LED and into the Base of the transistor) is 7.83/10,000 = 0.78mA . Within the transistor this is amplified with the LED spotlight bulb receiving a current of around 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. According to the manufacturer's specifications, this transistor has a gain (amplification factor) of 200 @ 2mA of base current (our gain was 58/0.78=74), a maximum collector current Ic of 100mA (we used 58 mA), and a total power rating of 350 mW (our LED spotlight used 645 mW!).

While the chosen transistor coped fine with the 645 mW of the LED spotlight, a higher power rated transistor should have been used such as a BC549B (rated to 625mW), or even a power transistor.

What is the Advantage of this Circuit

You may well ask what is the point of using a transistor in this circuit. The switch could have been placed inline with the LED spotlight bulb and the transistor was not necessary. Well in this example that is probably true however, if there were 100 spotlight bulbs and a power transistor in the circuit, the total collector current would be five amps. Therefore any switch used with the transisitor would also have to be rated at up to five amps.

Low current switches cost pennies - for example, 11p gets you a PCB mounted slide switch rated at up to 30 volts DC and 0.2 Amps which is just 8.5 x 3.5mm in size. 5 or 10 Amp switches are far larger and much more expensive. Transistors are key to reducing the size and expense of electronic circuits.

Web Link References

(1) http://www.reuk.co.uk/What-is-a-Transistor.htm
(2) http://www.reuk.co.uk/What-is-a-Transistor.htm
(3) http://www.reuk.co.uk/buy-12V-LED-SPOTLIGHT-BULB.htm
(4) http://www.reuk.co.uk/buy-WHITE-LED.htm
(5) http://www.reuk.co.uk/Ohms-Law.htm

Article from REUK.co.uk:
http://www.reuk.co.uk/Example-Transistor-Circuit-with-LEDs.htm
Published: 23rd April 2007
© REUK 2009