Constant Current laser driver circuit

Hi,

I am building a constant current laser driver using LM317T. I am attaching my schematic herewith. I have simulated the circuit and it works fine, however, when I am making the actual circuit on the board, it doesn't work anymore. I want ~100-850mA in the output and the circuit was designed for that purpose. The input to the LM317 is 5V DC.

Also, I know that the adj and output pin of LM317 maintains a constant ~1.25V, but that seems not the case for the LM317s I am using. My suspicion is that somehow I am frying the IC. Can anyone please give me some suggestions about what is going wrong in my circuit? I know this is a very basic problem but I would really appreciate the help. All the passive components are ok.

What failure are you seeing?
What is the forward voltage of the laser diode?
The LM317 requires at least 2V across it to work and that may not leave enough for the laser.

Hi,

Why R5?
Why R4? What value?

What's the nominal operating voltage of the laser-LED @ 850mA?

Klaus

Confirming what Klaus stated:

You lose 0.85V across the 1 Ohm resistor in series with the laser diode and at least 2V across the LM317. So your MINIMUM input voltage is 2.85V + laser forward drop, almost certainly you do not have enough enough input voltage. I'm not sure what that 1 Ohm resistor is for, you should be safe to remove it, it buys you a little extra voltage back.

Brian.

Hi, sorry for the confusion.

The laser I am using is not concerned with the voltage as long as the voltage is below it's rated value (2.6V). However, I need to vary the current value from ~100mA-~850mA due to operational purpose, so, I used a 10 ohm pot (R4). I added the 1 ohm (R5) to keep track of the amount of current to the edfa. I understand the voltage drop but since I am not worrying about the voltage, I could add R5 there.

The problem I am having is with the LM317T. I have tried several chips, but I am not getting 1.25V between ouput and adj pin no matter what.

The circuit is exactly as shown in the schematic. So, the resistors connected between the two pins (out and adj) should set the current for the load, however, since I am getting a lower(~300mV) V between out and adj pin, my current to the load is also much lower than I designed (~30-~350mA).

This is the thing confusing me. What can be the reason for me not getting 1.25V at Vref? I have tried with several LM317Ts and the result is same for all of them. I have checked the passive components as well and they seem to be fine. I am really at a loss about what the problem may be. So, I would really appreciate any suggestions on this matter.

Thank you for your time.

- - - Updated - - -

I am sorry, but let me add one more thing: my problem is not that the edfa is getting enough voltage, rather it is not getting the intended amount of current. I hope my explanation clears the confusion.

Hi,

What can be the reason for me not getting 1.25V at Vref?

Click to expand...

==>

almost certainly you do not have enough enough input voltage.

Click to expand...

believe it or not.

Klaus

I have also checked by increasing the input voltage from 5V to 10V DC by connecting it to a test load, nothing changed. Also, since I am making a constant current circuit, the change in load shouldn't affect the circuit.

Please correct me if I am wrong, with any input more than the nominal input of >3V, the LM317 should maintain a constant voltage difference of 1.25V between adj and out pin, am I correct?

Hi,

Please correct me if I am wrong, with any input more than the nominal input of >3V, the LM317 should maintain a constant voltage difference of 1.25V between adj and out pin, am I correct?

Click to expand...

3V is not enough.

Input voltage must be at least: Laser_voltage + U_R4 + U_R5 + 1.25V(LM317 Adj) + 2V (LM317 dropout)

so for 350mA with R4 = 0 Ohms and R5 = 1 Ohm:
V_in_min = 2.6V + 0V + 0.35V + 1.25V + 2 V = 6.2V

****

Also, since I am making a constant current circuit, the change in load shouldn't affect the circuit.

Click to expand...

The laser needs some threshold voltage. without the voltage there will be no current flow.

The change in load does not affect current as long as there is enough input voltage.

****
If the current is not like desired there must be something not according your schematic.
The schematic did not change?

Is your 10R pot rated for the current. You spoke of 850/350mA max. Maybe you have burnd it.

Klaus

Hi, thank you for clearing my confusion. The pot is rated for 5W.
I understand what you are saying. I will check again today and make sure I didn't miss any points you mentioned.