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Current limiting for transistor base

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Newbie level 6
Oct 27, 2011
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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:

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Hello Yoshidk,

Look for the datasheet of ULN2001. There you found also ULN2003 for TTL and CMOS -using. There the base - resistor is 2,7kΩ.

Take this value and good is. :-D

If you only have 425µA for base current you need R = (Vb-2xVBE) / IB = (5V - 1,4V) / 425µA ≈ 8,5kΩ.

So use 10kΩ.

All drivers of ULN200x are equal, only the base resistors are different.

– ULN2001 (general purpose, DTL, TTL, PMOS, CMOS)
– ULN2002 (14 - 25 V PMOS)
– ULN2003 (5 V TTL, CMOS)
– ULN2004 (6 - 15 V CMOS, PMOS)


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Use Ohms law!

Q1 will have about 0.65V between it's base and emitter so one side of it has 0.65V on it. The other side has 5V on it so the voltage across the resistor is 5 - 0.65 = 4.35V.
You know the current through it is 424.7uA so using Ohms law, R = V/I, R = 4.35/.0004247 = 10.242K.

However, the ULN2001 is not the same as a 2N3904, it has two transistors in each driver, wired in darlington configuration so there are two base/emitter junctions in series and hence about twice the voltage needed at it's input. If you are using a ULN2001, the voltage across the resistor becomes 5 - (2 x 0.65) = 3.7V so the resistor should be 3.7/.0004247 = 8.712K.

In practise, the nearest standard value would be 8.2K Ohms and it wouldn't normally be critical.


Oh, in the test schematic i just put in some components to show how its wired, im not sure the current is 424µA (this was measured when a 10kOHM resister was in R2s place)
So i guess the best is to just go with the 2,7kOHM resistor, when i dont know this current?
Thanks to both of you for your answers!


yes use 2,7kΩ, because that is also use in ULN2003 for 5V operation.



From desired Vce(sat) you can (I expect) determine a
"forced beta" operating point and from that and load
(plus margin) determine the required base current.
Hopefully this does not exceed pin ratings of the uC.
Then your desired resistor is VOH(min)-Vbe(max@)/Ib.

If you saturate it real hard then switching (off) delay
will be compromised and if your resistor is too high,
rise and fall times will be instead (Miller effect).

Get numbers around your priorities and it should all
shake out neatly (unless you have expectations that
are not supported by hardware realities).

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