Instructables

Step 4: Calculating R1

You can find the value of hfe in your datasheet:
Mine says for BC548 its 75 at 10mA at 10V. Its not very precise cause its very difficult to build transistor with a accurate hfe.

hfe = Ic / Ib

We know hfe and Ic so lets calculate Ib:

Ib = Ic / hfe

For BC548:

Ib = 0.03 A / 75
Ib = 0.0004 A => 0.4 mA

Due to Ohms Law:

R1 = U / Ib
R1 = 5V / 0.0004 A
R1 = 12500 Ohm
This is not very accurate to so we use 10kOhm.
 
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bogd3 months ago

Just one question (and please forgive me if you already mentioned this somewhere else): shouldn't it be R1=(U-Ube)/Ib?

With BC548 having a typical Ube ov around 0.7V, this would make it (5-0.7)/0.0004 = 10.75 KOhm.

Which makes your chosen value (10K) even more accurate :)