PHR-805 Bright When Flip Switch And Then Gets Dim

[papermaker82]

Hi everybody, this is my first post here, so I apologize if I ruffle any feathers, but here goes.

I have build a PHR-805 build using a LM317 driver with a 10 ohm resistor as the adjustment, such as described here: Laser driver - It can be done.

I am not using a pot for the PHR-805, just the 10 ohm resistor, and the current is right where it is supposed to be, 120mA. Right now, I just have the driver prototyped on a breadboard, so I know none of the connections are bad. Also, the driver works just fine on my LPC-826 diode, when I adjust the resistance of coarse.

My problem is, when I have the PHR-805 hooked up and flip the switch, the diode is nice and bright for a fraction of a second and then it becomes much dimmer. The diode is still lazing, it is just about 4-5x dimmer. I believe the diode is still good because it does still laze and does this every time I flip the switch, goes bright and then back to dim. The LPC-826 does not exhibit this behavior.

However, when I have my multimeter running between the diode and the driver measuring current (conforming it is at 120mA), the diode stays bright, it does not go dim.

I am running it on a 9V for now just for convenience, but I believe I am not running it long enough (only 1-3s shots) for the battery to drain significantly, and I have also tried with fresh batteries.

Has anybody experienced this before? I tried searching but could not find anything that seemed to relate. I know 405 is difficult to see, but why would it be bright when it first turns on and then dim, but not when on the multimeter?

 

[ApexProxy]

Your issue may be voltage. I suspect that your 9V are more dead than you think and you don't quite have enough voltage to run the 4.5V PHR and the 3V driver. I know it doesn't explain the odd phenomenon with the multimeter changing it but that's all I can think of. You could always drop two 9V batteries in series and heatsink your LM317 and see if it does the same thing. Or try two 9v in parallel. Or find a different power source that you can get 9v+ off of like a 12V wall adapter.

 

[lasersbee]

A cheap 9volt battery will not sustain the current
for very long and it will sag as you describe.

If your driver was built correctly I would suggest that
you use a fresh Alkaline 9V or a freshly charged NiHM
9V battery or two freshly charged 3.6v Li-Ion batteries
in series. Those batteries can supply higher currents
for longer...


Jerry

You can contact us at any time on our Website: J.BAUER Electronics

 

[papermaker82]

That might well be, I forgot the dropout voltage of the driver was 3V. Will the diode in the driver drop voltage as well, like it does in a test load?

Anyway, when I put 2 9V in series to test it, I believe it killed the diode for some reason, it certainly does not work anymore, or even LED. The test load still reads the correct current though, so I don't know what happened, unless something shorted somehow. 18V should be well within the limit of the LM317.
Oh well, at least PHR-805's are cheap...

 

[ApexProxy]

It should have been well within the LM317's abilities as long as you remembered to heatsink it. There may have been something else that went wrong as well. You do have a capacitor on the LD right?

 

[papermaker82]

Yes, I do have a capacitor.

I believe I did short its pins before I attached the diode through, if that's what you were getting at.

 

[ApexProxy]

I was asking for two reasons. To make sure you had protection for the diode and then I was going to ask if you shorted the pins.

Did you heatsink the LM317 before you hooked up the 18V?

 

[papermaker82]

I did not have it in a custom made laser heat sink, but I had it surrounded by some scrap aluminum I had and had a fan on it.

It did not get hot and blew pretty much immediately, so I don't think it was overheating that did it.

 

[ApexProxy]

Then maybe something shorted like you said. Either that or the diode was one of those that is weaker than standard.