New Driverboard Schematic

[Double_E]

Does anybody know why a driverboard couldn't consist of a voltage dividing pot driving a transistor with a resistor defining maximum current in series with the current path of the transistor and the laser? I can't seem to figure out why it isn't done this way, since it would be inexpensive and be able to handle a lot of power. The control of the pot might not be linear, but it would still work right?

Thanks for any help!

 

[Arni]

You can only cnage the voltage with the pot. But a Laserdiode needs constant Voltage and adjustable Current

 

[bg2vo]

The current defining resistor should be small so as to drop less voltage(say <0.1V) to maximize the efficiency. Therefore, a simple transistor current regulator is not good enough.

 

[Double_E]

How can you have a constant voltage with adjustable current? Current and voltage are related to eachother linearly. I don't think you can change the current through the laser diode without changing the voltage across it at the same time. Could you give me a more in-depth expaination arni? Thank you for your help.

 

[bootleg2go]

Hi double E,
Use google and read up on ohm's law and also on op amp circuits, current and voltage sources.

Jack

 

[Double_E]

The output of a diode is determined by the current through it, the current through it is determined by the series resistance that limits it. Tell me what is wrong with that statement? I know we want a constant current source, but since that is a ficticious device, we have to apply vary the voltage across the diode-resistor path. Once it is set how we want it, it doesn't have to change.

 

[Bengal]

Voltage = current x resistance
Current = voltage / resistance
Resistance = Voltage / Current

There will be a test at the end of the marking period.
Failure will result in detention.

 

[Double_E]

Is there anybody who is actually familiar with electronics and the driver circuits who can tell me why we can't just use a pot to bias a transistor like a 2222 to run the laser diode with a current limiting resistor in series? Thanks

 

[NewBie]

Double_E, take a look here:
http://repairfaq.ece.drexel.edu/sam/laserdps.htm#dpsldd

Everything you always wanted to know:
http://repairfaq.ece.drexel.edu/sam/lasertoc.htm

 

[Double_E]

Hey, thanks a lot!!! That was exactly the expaination I was looking for.

 

[NewBie]

Anytime!