mukundh225
Newbie level 6
Hi all,
My project is to build a circuit to drive 8-different laser diodes. I am using a micro controller to switch between 8 different channels of diff laser diodes. I desiged a driver circuit with a transistor as switching device.
View attachment driver circuit.pdf
My aim is : to provide 2.5V and 45mA to the laser diode.
My design is : as follows
hfe=150, Ic=45mA =>Ib=45m/150=.3mA
Rb=(5-0.7)/.3m =14.5k Ohms.
Voltage from DAC= Voltage across( resistor RL + laser diode + Vce(sat) )
==> 5 = VRL + 2.5 +0.3
VRL= 2.2 V ==> RL=2.2/45m = 48 Ohms.
My Issue :
The voltage across the laser diode does not raise above 2.1 V according to this design and when I try to increase the voltage from DAC, the voltage across the Collector-Emitter terminal raises proportionately making the voltage across the laser diode remain at 2.2V. And also my current through the laser diode is like 40mA and not 45mA which atleast is negotiable for its operating value..
Could some one please tell me the reason behind this. As to why my Vce is not constant at Vce(sat)... Sometimes it raises upto even 3V.
Or is anything worng with this design ?
It would be really helpful if some one helps me out...
Thanks
Mukund
My project is to build a circuit to drive 8-different laser diodes. I am using a micro controller to switch between 8 different channels of diff laser diodes. I desiged a driver circuit with a transistor as switching device.
View attachment driver circuit.pdf
My aim is : to provide 2.5V and 45mA to the laser diode.
My design is : as follows
hfe=150, Ic=45mA =>Ib=45m/150=.3mA
Rb=(5-0.7)/.3m =14.5k Ohms.
Voltage from DAC= Voltage across( resistor RL + laser diode + Vce(sat) )
==> 5 = VRL + 2.5 +0.3
VRL= 2.2 V ==> RL=2.2/45m = 48 Ohms.
My Issue :
The voltage across the laser diode does not raise above 2.1 V according to this design and when I try to increase the voltage from DAC, the voltage across the Collector-Emitter terminal raises proportionately making the voltage across the laser diode remain at 2.2V. And also my current through the laser diode is like 40mA and not 45mA which atleast is negotiable for its operating value..
Could some one please tell me the reason behind this. As to why my Vce is not constant at Vce(sat)... Sometimes it raises upto even 3V.
Or is anything worng with this design ?
It would be really helpful if some one helps me out...
Thanks
Mukund