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Transistor issues -- laser diode driver circuit

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mukundh225

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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
 

It's not much of a circuit. Can the opamp drive the current you need? Do the laser diodes really want a simple constant current drive? Usually they have monitor diodes for setting the power level. If you want a fixed current then you cannot do it with a resistor and hope the laser diode drop is constant - you need a proper constant current source. What is the purpose of the DAC? To set the brightness of the laser?

Keith.
 

It's not much of a circuit. Can the opamp drive the current you need? Do the laser diodes really want a simple constant current drive? Usually they have monitor diodes for setting the power level. If you want a fixed current then you cannot do it with a resistor and hope the laser diode drop is constant - you need a proper constant current source. What is the purpose of the DAC? To set the brightness of the laser?

Keith.


Hi keith,

Ya the opamp i am uisng is a high current precision amplifier(OPA551).. it can drive upto 200mA. So i don't think there s problem with opamp.

I tested one of the laser diodes and it just needs its correct operating voltage and operating current.

The purpose of the DAC is to give 8 output digital voltages which are programmable. The DAC can provide upto 10V.

I need to focus the beam onto an optical fiber which takes it to a microscope. If the laser diode is triggered I can focus it using a microscope. So even that is not an issue.

I only need to make my laser diode work in the proper operating voltage and current specification.

Please suggest me any other possible modifications to this circuit to make it operate properly in the voltage and current level.

Thanks
Mukund
 

Could you give a part number for the laser? You can only really control voltage OR current, not both. For example, if you need to fix the current then the voltage will be whatever is necessary for that current - you cannot fix that as well.

Keith.
 

I only need to make my laser diode work in the proper operating voltage and current specification.
Laser diodes are usually constant current operated (respectively power controlled through a monitor diode). In this situation, you don't need to care for the diode forward voltage. As long as you keep a safe current (respectively a safe optical output in closed loop), the forward voltage will be within the specified limits as well.

The forward voltage specification is mainly an informational parameter to allow on optimal driver design.
 

Could you give a part number for the laser? You can only really control voltage OR current, not both. For example, if you need to fix the current then the voltage will be whatever is necessary for that current - you cannot fix that as well.

Keith.

Oh I didn't know that we could not set up both current and voltage. The part no of the laser diode is HL6738MG.

So what else can I do ? Because the data sheet of the laser diode asks for both the operating voltage and threshold current to operate it.

I have to design for 8-different laser diodes like the one above and switch it using a micro controller...

Thanks
Mukund
 

That type of laser should be operated with a closed loop feedback circuit which monitors the light output with the built-in photodiode. You are likely to blow them up otherwise.

Try searching for laser driver ICs - that would be the easiest way of driving them. Also, be very careful - they are not eye safe, but hopefully you know that already.

Keith

---------- Post added at 00:14 ---------- Previous post was at 00:00 ----------

I generally design my own discrete laser drivers so haven't used many driver ICs, but have a look at iC-Haus Homepage - keyword: Driver iCs

Keith.
 

Because the data sheet of the laser diode asks for both the operating voltage and threshold current to operate it.
No, it doesn't. You should learn some basics about laser diodes before you design your circuits, otherwise the results are possibly fatal. To give a short hint: The datasheet absolutely requires to keep the maximum optical power of 35 mW. Please look at the Optical Output Power vs. Forward Current diagram and consider how it can work.
 

Just to add to that - you WILL need an optical power meter to set the power. Even if you want to run well below the maximum there is no other way of setting it.

Keith
 

Sorry to disturb you all ... Mukund and Keith

Actually m trying to drive Laser (OPV314Y) or any laser pointer from ethernet output ... do i need any separate LASER DRIVER IC (like MAX3766) or can i drive them directly using some transistors...
 

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