# Transistor Equation help

Discussion in 'General Electronics Chat' started by ibigpapa, Jun 18, 2009.

1. ### ibigpapa Thread Starter New Member

May 9, 2009
7
0
So i'm using the site to help determine the calcuations for a transistor i'm setting up. Here is the site.http://www.kpsec.freeuk.com/trancirc.htm

I'll talk about the circuit first.

I have a 9v battery.

The positive terminal connects to an array of leds with a resistor in front of each array. The array configuration is 3 x 8. That is 3 leds in a series and 8 of those in parrallel. The resistor that goes in front of all of those would be a 100 ohm 1/4w. The led has a forward voltage of 2.4v and a forward current of 20ma. That brings the total current of the array to 160ma.

The positive terminal also connects to a voltage regulator that takes it down to 3.3v to power a controller.

I have 2n3409 bjt npn transistor here is the data sheet: http://www.sparkfun.com/datasheets/Components/2N3904.pdf

the led array negative connects to the collector of the transistor
the controller pin connects to the base of the transistor
the emitter of the transistor is then sent to a common ground
There is a figure very similar to the setup i have on the page for the transistor calculations in the section "Connecting a transistor to the output from an IC" http://www.kpsec.freeuk.com/trancirc.htm

The transistor's maximum collector current Ic(max) must be greater than the load current Ic.