Page 1 of 2 12 LastLast
Results 1 to 10 of 18

Thread: Help with 1 MHz Analog Laser Modulator

  1. #1
    Join Date
    May 2013
    Posts
    12

    Default Help with 1 MHz Analog Laser Modulator

    Hi,

    Have anyone built this driver before?http://www.repairfaq.org/sam/laserdps.htm#dpsldd317

    I just finished building it but my output is at a constant 0.3A.

    Can any help me with this?

    Thanks

  2. #2
    Join Date
    May 2013
    Location
    Fort Mill, SC
    Posts
    556

    Default

    What resistor did you use on the output of LM317?

    What output did you expect to see?

    Was the output constant with modulation or was this measurement taken with no modulation? AKA laser always on?

    Or are you looking for a different type of help with this circuit? Please provide more detail on what you need help with.

  3. #3
    Join Date
    May 2013
    Posts
    12

    Default

    Hi.

    Okay apparently I must divide 1.25 with the amount of current I want. Thus 1.25/1.5 = 0.833.

    I build two of these drivers one using a 1ohm 1 watt resistor. And the other using a 8.2 and a 1 ohm in parallel (0.888).

    With both I don't get more than 0.3 - 0.4A. I'm using a dummy load of 6 diodes (3A) and one 1 ohm measuring the current over the resistor.

    My other concern is that the lm317 gets unbearably hot withing less than 20 sec upon maximum.

    Currently I'm not applying any modulation.

    Thanks in advance

  4. #4
    Join Date
    Jan 2007
    Location
    Florida
    Posts
    4,382

    Default

    What kind of diodes? 6 diodes e.g. 1N4007 will drop about 4.2V. What kind of laser diode are you going to drive with this circuit?

  5. #5
    Join Date
    May 2013
    Location
    Fort Mill, SC
    Posts
    556

    Default

    1.5 Amps is going to be too much for the LM317 if I remember the datasheet correctly.

    What was your Pot set at when you took these measurements?

    What are the voltages in reference to ground on the inputs to the Op-Amps?

    What is the voltage going into the Adj pin on the LM317?

    What do you mean you are measuring the current over the resistor? To measure current your meter must be in series with the circuit. If you are measuring over the resistor that is a voltage measurement. I assume you meant that you are measuring in series with the resistor and the current measured through the resistor is 0.3 - 0.4A?

    Also just what is the input voltage you are feeding this circuit?

  6. #6
    Join Date
    May 2013
    Posts
    12

    Default

    Hi.

    Okay I took new measure measurements.

    Input voltage =12V
    Pot halfway at = 550 ohm
    Adj pin on lm317 = 3.38
    1st Opamp input pos and neg both = 1.62
    2nd Opamp input pos = 2.38
    2nd Opamp input neg = 2.1

    Current through diode measured = 0.12A

    All using the 1ohm in series with diode.

    Diode dummy load and measurement done like this http://www.rog8811.com/test%20load01.jpg

    The 1ohm in series gets really warm. I will add 2x 100ohm in parallel
    Now that lm317 is heatsinked it's more stable.

    Still very strange that I can't power more than 0.4A with pot at max. With the lm317 I should get at least 1.2A right?

    Did I do the 1ohm resistor calculation correct? (like explained in my previous post)

    I'm driving a M140 445nm diode.

    Shot
    Last edited by roekeloos; 05-31-2013 at 11:08.

  7. #7
    Join Date
    Jan 2007
    Location
    Florida
    Posts
    4,382

    Default

    Ok, the LM317 has thermal protection so if it gets too hot it will protect itself by shutting down. It needs a heatsink. You need to put 5 volts on the input of this circuit. It probably won't provide any output if you don't. If you put three 1 ohm resistors in parallel you will get 0.333 ohm which will probably be a bad thing for this experiment. The dummy load was built to test blu-ray diodes? not sure if that'll work well with the 445 diodes. If you put 2 100 ohm resistors in parallel you'll get 50 ohms.
    The current capability of the LM317 is 1.5A iirc.
    I build two of these drivers one using a 1ohm 1 watt resistor. And the other using a 8.2 and a 1 ohm in parallel (0.888).
    Where are these resistors located? Is this R11? R11 is supposed to be 8.2 ohms if I'm reading the schematic correctly. What are you using for a power source (please don't say a small "wall-wart")

  8. #8
    Join Date
    May 2013
    Posts
    12

    Default

    Hi.

    Sorry I didnʹt mean 3 x 1ohms. I meant I will use 3 resistors so that the heat can spread evenly. I will use 3 x 3ohms.

    Yes the resistors are for R11. Not sure if this have to be 8ohm. The circuit have extra space for for adding in parallel until required output is set. In the explanation it says I must use the 1.25/A formula.

    I also realized my pot is very crap and will now use a trimming pot.

    Also I will now try to add the 5v input. Thanks

    Using a 12-24V 120W universal laptop power supply. Its got quite stable output but is still a cheap solution. Will this be a problem?

    Does anyone know what's the perfect dummy load for the M140 diode?

    Thanks for all the help thus far.



    Regards

  9. #9
    Join Date
    May 2013
    Location
    Fort Mill, SC
    Posts
    556

    Default

    From my understanding of the LM317 datasheet for a constant current source you are trying to have the adj pin be as close to 1.25V as possible to produce the constant current that is limited by the 8.2 Ohm resistor. I also agree that if you have nothing hooked up to the modulation pin that could be an issue. The op-amps are looking for voltages on both inputs. The zener diodes are basically limiting the inputs to 5V so that no matter what Vin is there will be 5V on the Positive input of the first Op-Amp then the modulation will be somewhere between 0-5V as well. For testing tying it to 5V and taking measurement and then tying it to GND and taking measurements would be my next step. I believe this circuit is doing a difference Amp so that the output of the first Op-Amp is somewhere between 0-1V. (Don't quote me on that but when I did the math very quickly a little bit ago that is what I got with the chosen resistor values) The second stage is the same kind of setup basically it is a difference Amp so the voltage coming from just past the 8.2 Ohm resistor is compared to the voltage coming from the Pot. (I did not do the math on that stage because I don't know the specs on your diode but maybe you can figure it out now that you know its a difference Amp. Then the difference between these is fed to the Adj pin.


    When you turn the pot do your values on the adj pin change?

    Also keep in mind that if you did overheat this component for longer than it can handle you may need a new one to complete the testing. The LM317 is pretty robust but just saying. A cooked chip is a cooked chip there is no going back.

  10. #10
    Join Date
    May 2013
    Posts
    12

    Default

    I tried to keep only the 8ohm but the output is now very very small and does not respond to any signal. It seems I must stick with the 1ohm.

    Yes the Adj pin voltage increase as I decrease pot resistance. Its not even close to 1.25 though.

    When I add input signal of around 4.5 V it doubles the output.

    Max Output now around 0.6A with 1.25ohm as R11. Resistors still getting really warm.

    Thanks for info on differential op amp. I kind of grasp the concept but can't do the maths around it. I'll experiment further and keep you posted.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •