# led bjt design

Discussion in 'Homework Help' started by skusku, Sep 18, 2012.

1. ### skusku Thread Starter Active Member

Aug 9, 2009
70
1
Circuit - http://i49.tinypic.com/mt5lqo.jpg

So the circuit is what i quickly drew as the question states.
"The led is to have a forward current of approx 10mA. Vcc=9V, Vf= 1.6V, Vb=7V and Q1 has hfe(min)=100. Take Vbe =0.7V

Find suitable resistor values for R1 and R2.

What i've done is to say:
Ib=Ic/hfe=10mA/100=1mA [current is the same for series right?]
R1=(Vb-Vbe)/Ib= 6300 ohm

then

R2=(Vcc-Vf)/Ic = 740 ohm

------------------------

Okay so my circuit is very similar to the example from boylestad I have found. Is my calculations correct? For resistor R2 I have followed the book. But in the book they just use the voltage over Rc as Vcc. Is this not supposed to be Vcc-Vce? I cant seem to find any other calculations to get Vce...

Thanks

2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
If you look at the Vce(sat) specs of almost any BJT, you will find that they are specified for Ib=Ic/10. This is called "forced beta", and it is necessary because the beta of a transistor in saturation is much less than it is when Vce is, say, 10V.
See the attachment.

File size:
80.6 KB
Views:
26
3. ### skusku Thread Starter Active Member

Aug 9, 2009
70
1
I saw a graph somewhere from a textbook that when Vce is in saturation mode the voltage varies from between 0V and 0.7 (Vbe).

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Saturation occurs when both junctions (base-emitter and base-collector) are forward biased.