# Current limiting for transistor base

Discussion in 'General Electronics Discussion' started by Yoshidk, Jan 23, 2013.

1. ### Yoshidk

1
0
Jan 23, 2013
Hello everyone

I have a circuit where my µC (ATmega16) triggers a transistor (ULN2001) to open for a LED
I know i need a resistor between the ATmega16 output and transistor base to limit the current, but i cant figure out which resistor value would be best?
I guess too low would waste power, and too high might kill the signal..

I made a simple drawing of the setup here:

Datasheets:
ATMega16:
http://www.datasheetcatalog.org/datasheet/atmel/2545S.pdf
ULN2001:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00001244.pdf

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Ibase= Iled/beta.
Iled~10.4V/330 Ohm = 30mA (assuming red LED with Vf~1.6V)
beta ~60 (from the transistor datasheet for Ic=50mA)
=> Ibase= 500µA
Ibase= (Vh(cpu)-Vbe)/R2.
Vh = 4.5V (the ATmega should be able to deliver that voltage worst case at Vcc=5V and 500µA)
Vbe = 0.6V
=> R2=3.9V/500µA = 7800 Ohm.

To ensure that the transistor is saturated under worst case conditions use the next lower standard value, e.g. 7500 Ohm.

You can get a quick and dirty estimate from R2=R1*beta*V1/V2 (this neglects base-emitter voltage, Vf of the LED, VoH of the ATmega, but comes close).

3. ### KrisBlueNZSadly passed away in 2015

8,393
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Nov 28, 2011
The ULN2003 and ULN2004 have base current limiting resistors built in. The 2004 is probably the most suitable if you're just driving LEDs with it.

4. ### Harald KappModeratorModerator

10,771
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Nov 17, 2011
I obviously I overlooked that note saying ULN2001 and only looked at the 2N3904. My fault. Kris is right concerning the ULN IC.