### Author Topic: Transistor  (Read 1873 times)

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#### Rolf

• Jr. Member
• Posts: 11
##### Transistor
« on: May 16, 2007, 12:38:15 PM »
Hi. I am using a Motorola BC547 transistor to control a series of LEDs from a microcontroller. I bought if from futurlec, and the datasheet can be found here :

http://www.futurlec.com/Transistors/BC547.shtml

The problem I have is calculating what current I have to supply the base to make my LEDs work. I want to use as little current as possible to save power. I will connect the base to a 5V IO-pin and controll the current with a resistor.

I read through the datasheet, but did not understand how to calculate the base current. Does anyone know how to calculate this? The current running through the LEDs, when active, is about 30mA.

Also, do you know anything about how much current/voltage you can run through a normal flat ribbon cable? I wonder if i can use this for my LEDs.

#### andreahmed

• Jr. Member
• Posts: 37
##### Re: Transistor
« Reply #1 on: May 16, 2007, 03:35:52 PM »
Rbase = Vcc - Vbe saturation / IB
choose ib ..
Ic = VCC - Vce saturation / Rc <-- load

#### pelibot

• Beginner
• Posts: 4
##### Re: Transistor
« Reply #2 on: May 17, 2007, 10:28:57 PM »
You can definitely run 2A threw a ribbon cable wire.
So no problem with LED's.

Regrading the transistor, you can saturate it's base with few hundreds uA.
A series resistor in the current path of the LED should make the real current limit you require threw the LED.

Now, if you also need a printed circuit for this, come visit a tutorial I made about how to make your own PCB at home using a laser printer and some ferric chloride at
http://www.epages.co.il

Good luck !

• Jr. Member
• Posts: 11