# How Can a BJT Transistor Be Used to Control LED Current?

• Infidel22
In summary, the first article provides instructions on how to design a circuit to drive an LED with a constant voltage, while the second article provides more theoretical information on how BJTs work. Based on these two articles, it seems that you will need to place a resistor in series with the LED to limit the current. ~~
Infidel22
Hello all,
I haver a circuit design problem that is giving me grief. The problem basically asks to use a circuit with a BJT transistor to drive an LED with a certain maximum current. Thus, the BJT needs to behave as a voltage controlled current source, the prof hinted that the design should be an emitter follower but this is a voltage controlled voltage source from what I understand. Any ideas on where to start in designing a circuit to meet these requirements?

http://mgc314.home.comcast.net/emitterfollower.htm

Take a look at the first two circuits at this link. Your LED will have a nearly constant voltage drop. If you put it in series with R8 and adjust the value of R8 to limit the current to the maximum rating of the LED for the maximum voltage input, I think you have what you need. The addition of an appropriately chosen R7 in the second circuit for protection will not be a factor as long as the output is not shorted.

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Infidel22 said:
Hello all,
I haver a circuit design problem that is giving me grief. The problem basically asks to use a circuit with a BJT transistor to drive an LED with a certain maximum current. Thus, the BJT needs to behave as a voltage controlled current source, the prof hinted that the design should be an emitter follower but this is a voltage controlled voltage source from what I understand. Any ideas on where to start in designing a circuit to meet these requirements?
To supplement O-Dan's circuit info, the following link is specific to Bipolar Junction Transistors (BJT), and in particular, covers their special switching applications (which is probably what you're looking for). Check the links below. (First link is main article, in which you'll need to SCROLL WAY DOWN the page to "Switch Applications".)
Article:
http://www.hcc.hawaii.edu/~sdunan/cent112/notes/06FTransistors/
Circuits: (To limit current, place Resistor in series with LED.)
http://www.hcc.hawaii.edu/~sdunan/cent112/notes/06FTransistors/index26.gif
http://www.hcc.hawaii.edu/~sdunan/cent112/notes/06FTransistors/index27.gif

Another Article (more theoretical):
http://engnet.anu.edu.au/DEcourses/engn2211/notes/bjt.html

~~

Last edited by a moderator:

## 1. How do I calculate the current flow in a transistor circuit?

To calculate the current flow in a transistor circuit, you will need to use Ohm's Law and Kirchhoff's Current Law. First, determine the resistance values of each component in the circuit. Then, use Ohm's Law (I=V/R) to calculate the current at each resistor. Finally, use Kirchhoff's Current Law to determine the total current flowing through the circuit.

## 2. How do I choose the right transistor for my circuit?

Choosing the right transistor for your circuit depends on several factors such as the required voltage and current ratings, the type of load, and the desired switching speed. It is important to consult the transistor datasheet and consider these factors before selecting the appropriate transistor for your circuit.

## 3. What is the difference between NPN and PNP transistors?

NPN and PNP transistors are two types of bipolar junction transistors (BJTs). The main difference between them is the direction of the majority flow of charge carriers (electrons or holes). In an NPN transistor, the majority flow is from the collector to the emitter, while in a PNP transistor, it is from the emitter to the collector.

## 4. How do I troubleshoot a malfunctioning transistor circuit?

If your transistor circuit is not functioning properly, the first step is to check for any loose connections or damaged components. Then, use a multimeter to measure the voltage and current at different points in the circuit. This will help identify any faulty components or incorrect values. Finally, refer to the circuit diagram and datasheets to ensure proper component placement and values.

## 5. How can I increase the efficiency of a transistor circuit?

To increase the efficiency of a transistor circuit, you can reduce the power losses by using transistors with higher voltage and current ratings, choosing the appropriate load for the circuit, and minimizing the number of components in the circuit. Additionally, you can use heat sinks to dissipate excess heat and improve the overall performance of the circuit.

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