Help choosing transistor...

Jed

Newby with transistors. I have a project I'm working on for the company I work for. I need to switch a load and need to choose a transistor for it. I found this page online showing how to calc the specs needed for the transistor and resistor. I think I've got everything figured out, but I'd appreciate if someone could spot check me on this:
www.kpsec.freeuk.com/trancirc.htm#ic

Current from IC pin is 40mA (I've been given an Arduino to completely this project with)
IC supply is 5V regulated down from a bank of 9V batteries
Load Supply is from a bank of 9V batteries
Load resistance is 32.4 ohm

Step 1: Transistor's Lc(Max) must be > 9V/32.4ohm or 278mA
Step 2: Transistor's hFE(min) must be > 5 x [noparse][[/noparse]Lc(max)/40mA] or 34.75
Step 3: Chose NPN 2N3053 with lc(max) of 700mA and hFE(min) of 50
Step 4: Base resister = [noparse][[/noparse](5V x 50) / (5 x 0.7A)] or 71.43 Ohm

Can someone verify that a 2N3053 with a 82Ohm base limiting resister will work for my load and keep my IC safe? I'm a little confused on step 4, whether to use units of A or mA for current. Thank you!

Mike Green

Ohm's Law is your friend!

The IC output is 5V less the voltage across the IC's output transistor which is typically 0.3V. The IC's datasheet will tell you (Vih - voltage output high). Any junction transistor will have a base voltage drop of about 0.6V. The 2N3053's datasheet will give you the actual value. The net voltage across the 82 Ohm resistor then is 5V - 0.3V - 0.6V = 4.1V. By Ohm's Law, you have a base current of 50mA. That's more than you estimated and too much for most ICs.

I'd keep the IC output current to 25mA or less. That's a typical maximum for a microcontroller I/O pin.

With a base current of 25mA and a minimum hFE of 50, that's an output current of 1.25A which is more than the maximum current rating of the 2N3053, so the transistor will be saturated at its maximum collector current and you'll be fine with your load and choice of transistor.

Chris Savage

...and if I may step in briefly...not one to run anything at max I would suggest limiting yourself based on your drive current so that way you don't need to draw 25mA from the I/O pin.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Chris Savage

Parallax Engineering
·

Jed

So I'm good as long as I limit the IC pin current to < 25mA, or with a 180Ohm resistor?

Here's what the website on arduino says about the output pins for the specific board I was given:
arduino.cc/en/Main/ArduinoBoardDuemilanove

Arduino Hardware Info Page said...
They operate at 5 volts. Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms

Are they just not specifying the voltage drop of 0.3V? Will I be safe assuming 0.3V?

Not sure how to read the datasheet on the NPN to find out the base voltage drop, I'm sure it's there in the numbers, but again, am I OK assuming?
www.futurlec.com/Transistors/2N3053.shtml