LPC836 Laser Diode Problem

[Adamtrim]

Hello all,

I am in the process of building a laser around the Mitsubishi LPC836 650nm Red Laser TO18 5.6mm LD Diode 350mw 500mW CW. I am powering it with an LM317 and a wall wart which outputs 12 VDC at a max 2.0A. According to various videos online I saw you could make a current limiter around th lm317 and a 3.3 ohm 1 way resistor across the adjust and output. Both my lm317 and diode are getting pretty warm after a couple of seconds and the diode is not even close to outputting the amount of power it should. What am I doing wrong?

-Adam

 

[paul1598419]

Your diode needs to be in some sort of module to dissipate the heat. The Vf of this diode is so low that at +12 volts in, the IC will need to be heat sinked well. Even if you decrease the supply voltage, the current you want going through the diode is 370 mA to 420 mA.

That said, you should introduce yourself in the welcome sub forum and tell us a little about yourself.

 

[Adamtrim]

Hi Paul,
I have it seated in an Axiz module which I bought frcan m eBay. I pulled the driver as well as the other diode out and reseated the lpc836 where the diode was. I am using a 6 volt power supply as well. I will be sure to stop in the welcome forum and introduce myself a little, thanks!

 

[Adamtrim]

Also, I have the lm317 heat synced as well. I have an old cpu heatsink on standby but do not feel like drilling the cavity at the moment. Could this be my only issue?

 

[hakzaw1]

Did you touch the diode to see how warm it was??
ESD is a silent diode killer as well is powering with batteries w/o proper driver.

 

[paul1598419]

You will want to put that nickel plated brass module in some kind of heat sink. The module alone is not enough mass to pull all that heat away from the diode. You absolutely need a constant current laser driver set to no more than 400 mA. If you insist on using the LM317, it has to be heat sinked on a TO-220 heat sink. The best way to check the driver's current is to put three silicon rectifiers in series with a 1 ohm 5 watt resistor and measure the drop across the resistor. Use this in place of the laser diode. It should measure 400 mV or less.

 

[Adamtrim]

Paul,

In my previous set up I did have the lm317 heat synced, but with a smaller heat sync. I will tryna larger heat sync on both the module and the lm317.
I’m thinking there may be something wrong with the power supply but I will run it with the test load you recommended to see what the output current is.

 

[Adamtrim]

The diode is set in an axiz module. This is what I touched to see if it was warm. When mounting the diode and working on the driver I was connected to an esd bracelet properly grounded so I don’t think that is the issue.

 

[Adamtrim]

By Silicon Rectifiers do you mean SCRs? or a silicon diode

 

[paul1598419]

A silicon diode, like a 3 amp 1000 PRV rectifier. Place the diodes anode to cathode and at the last cathode in the string add the resistor. Use the free anode as your positive connection and the free end of the resistor as your negative connection. Use this in place of your red laser diode and measure the voltage drop across the resistor. Each mV = 1 mW. So, you are looking for 400 mV or lower but at least 370 mV.

 

[Adamtrim]

I used an ESD bracelet when mounting the diode. It is also sitting in an Axiz module. the LM317 is heatsynced but the module is not

 

[Adamtrim]

After making the 3 diode 1 resistor setup and measuring my output across my resistor I found a voltage of 1.4 volts...and rising.... I am no dummy when it comes to electronics as I am an electrical engineering student. None of my leads are shorted and everything has a proper solder connection. My only thoughts could be that the lm317 died before this test but I am not sure if that is probable. I am thinking it is either that or the 6V 2A max psu I am using. Am lost though ¯\_(ツ)_/¯

Update-I found I am using a 1.6 ohm resistor by accident. I do not have any true 1 ohm resistors on Hand. Regardless isn’t this power much to high?

 

[paul1598419]

What you are attempting to measure is current, not power with this dummy load. And, yes, the 1.6 ohm would have a lower voltage drop. You need to replace the LM317 and your sense resistor between the sense pin and the output pin controls the amount of current used by the driver. You should be using the sense pin as the output in this configuration to be a constant current source.

 

[Adamtrim]

Annnnnnnd I just realized I’ve been using the vout pin as “current out.” Re measuring with the 3.3 ohm resistor across the adjust and vout across the 3 diode 1 1.6 ohm resistor yields 520 mV. Now all I have to do is order more diodes -_- (and increase resistance). I knew something wasn’t right.

 

[paul1598419]

I had a feeling that was an important item to stress. Glad you got it worked out. Good luck with the rest of your build.

 

[Adamtrim]

Thanks Paul I appreciate the help. I just ordered another lpc836 but it isn’t coming in till mid August :/ do you know of any good laser diode distributors? I am in New York, the diodes do not have to be lpc836 but I am not super familiar with lasers yet and don’t wanna stray super far along the path.