led bjt design

Thread Starter

skusku

Joined Aug 9, 2009
80
Circuit - http://i49.tinypic.com/mt5lqo.jpg

Boylestad example - http://i45.tinypic.com/20jpta9.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
 

Ron H

Joined Apr 14, 2005
7,063
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.
Boylestad has probably never had a real job designing circuits.
See the attachment.
 

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Thread Starter

skusku

Joined Aug 9, 2009
80
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).
 
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