# Homework Help: Transistor circuit problem

1. May 8, 2005

### 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?

2. May 8, 2005

### OlderDan

http://mgc314.home.comcast.net/emitterfollower.htm [Broken]

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.

Last edited by a moderator: May 2, 2017
3. May 8, 2005

### xanthym

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

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Last edited by a moderator: Apr 21, 2017