M140 post-mortem: Was this driver a bad choice?

Hi,
I'm new to this forum, lots of great info here and a nice community. Maybe someone can help me understand the error in my ways that led to a dead LED (M140) and maybe spare some others from the same fate.

My first attempt to drive the M140 was using a home built circuit based around an LM350 set up to deliver ~1A constant current. That worked pretty well actually, but it wasn't overall a good fit for my application.

So next I tried using an off the shelf driver that looked good by the specs (PDF attached, LUXdrive A009 made by LEDdynamics). This seems perfect for my application, high current, high efficiency (i.e. less waste heat) and easily dimmable.

I used a 100K pot in parallel with a 15K resistor between the dimming inputs, and verified with an ammeter and dummy load that the max current was about 1.5A. Next I tried some high power white LEDs and finally the M140.

As I ramped up the current the M140 failed suddenly at about 1.1A. One person in the know suggested that this driver is for regular LEDs but not suitable for laser diodes (which are more sensitive) and that I post a question here.

So I'm wondering if anyone here has had similar experiences or any insights into why this combo failed when it looked good on paper. I'm not dead set on using this driver, but if there are some tricks to making it M140 compatible that'd be great to know too. Otherwise, I'd just be happy to understand what went wrong and hopefully avoid a repeat.

A little more info on my setup that might be helpful:

Power supply was 12VDC/1.8A wall wart with 6ft cable.
A009 driver was wired to power supply
Diode outputs wired to M140 with ammeter in series
Dimming was controlled by 100K pot w/15K parallel resistor (to set the max current)

One possible clue, the A009 doc says to use a 100uf bypass cap across the power supply if the power leads are several feet long. Also, the whole 'testbed' made liberal use of alligator leads, about 12" and the doc says that the driver and LED should be as close as possible.

It's tempting to try again using a bypass cap and shorter leads, but as these diodes don't exactly come cheap I'd feel better if I had more to go on than a hunch

Thanks in advance for any advice, caveats, explanations, theories and all that good stuff!

Cheers,
Joe

Quick and dirty of it is that laser diodes are just more sensitive, due to the fact that not only do they have an active material turning electrons into light, but unlike LEDs, they are also a precision optical device that has optical faces aligned a certain way. These mirrors aren't anything special; in fact, they are usually formed by cleaving, since the selected material will have a weak plane that is oriented correctly. But as such, when these mirrors are damaged, there is no more resonance, which is part of what makes the device a laser and not just an LED.

Alligator leads are also a big no-no, primarily because of the way that switching regulators work, and how unsecure alligator leads can be. The leads can lead to resistance in the load, but also can disconnect the load from the output of your regulator for just long enough for the output cap to charge too much. Then if load reconnects, it gets blasted with much too much voltage.

You might consider posting what your requirements are, and folks here can suggest some better drivers for your application It looks like mainly you are looking for easily dimmable high-current driver that can run off of ~12VDC supply. Might involve swapping out a pot for one that is rated for more turns. Someone will probably chime in with a driver recommendation.

Welcome to LPF! I know you just came for an answer, but you might get stuck here if you browse the forums too much :san:

That driver says constant current so your probably OK with that driver.

The M140 is a pretty tough cookie but it can not take high voltage spikes and I am betting that is what happened
since you are using alligator clips, that is a big no no with laser diodes you really need to solder the connections
even if they are temporary and your just testing you still really need to solder all the connections !

Why you say well if you move the alligator connection it can possibly disconnect for just a few milliseconds and that is
long enough for a high voltage spike to build up on the output cap and it's this high voltage that probably killed your M140.

Good luck...

Thanks for the replies, so far blame seems to be pointing at my sloppy test rig so for my next act I may try again with the A009 driver, a bypass cap and clean wiring. It'd be great if I can get it to work reliably with this driver, it is just about a perfect fit for my project and pretty cheap, $20.

BShanahan14rulz suggested that I post more about my requirements, so here it is. My goal is to mount the M140 to the Z-axis of my mini-cnc mill. I've got a large heat sink that I'll attach the host and driver to, then attach that to the mill. The mill has 24VDC so I'd like to use that for the LD driver as well.

I'll repurpose the spindle signal to turn the laser on and off and I'll manually set the power using a pot. The A009 also works with a 0-10V dimming control, so eventually I might try to use that feature to be able to set the power from GCode.

My 'official' purpose for all this is to make PCBs at home. I will probably start off with isolation milling, but I saw some cool posts about using lasers, which seems to have a lot of advantages. A blank board is sprayed with a layer of black paint, then etched with the laser, then chemical etched. So that's the official reason, unofficially I intend to explore my creative side on various bits of wood, metal, paper, glass, etc

Thanks again for the replies. I'll probably give this A009 another try with a better setup, and if I do I'll post back with the results.

I'm wondering how clean the output of that driver is. If the output is a PWM pulse train then I'd look for a different driver. Do you happen to have an oscilloscope? Hook up a white LED or two and make sure that driver is putting out nice steady DC. And yeah, no alligator clips :yh:

Good idea WizardG, I'll scope it later and post the results.

Here are a couple of scope photos. The A009 driver is driving two white LEDs at about 125mA. The first photo shows the output voltage, pretty solid at 6V with not much discernible ripple. The second photo is with AC coupling, 50mV scale and 5uS/div.

From the second photo you can see about 100mv of sawtooth ripple, each cycle is about 5uS so it looks like the driver is switching at 200KHz. The same ripple shows up on the input supply, so a bypass cap will probably be a good idea.

Using a power resistor as a load I let the driver run to its maximum current by letting the DIM inputs float. It held pretty close to its rated maximum of 2100mA (measured ~2120 mA), and ripple was much less.

These results actually look pretty good, better than I expected, and I'm still using test leads for some of my connections. I'll take another look once I've built a cleaner setup with a bypass cap and short, permanent connections.

Cheers,
Joe

Current waveforms are the thing to look at. I would be more concerned with what is happening at startup
and shutdown. Switch bouncing has killed many a diode. Also if your pot is dirty, it will cause the output
to jump around. That is why we usually do all of the adjustment on the dummy load and leave it alone
once the diode is attached.

jameadows said:

Using a power resistor as a load I let the driver run to its maximum current by letting the DIM inputs float. It held pretty close to its rated maximum of 2100mA (measured ~2120 mA), and ripple was much less.

Click to expand...

I presume this is the cause of the problem: If you have some ajustable current source that will actually put out maximum current when the input is left floating, you stand a large chance of frying the laser diode while adjusting the current setting.

Potmeters are basically contacts of metal that you can rotate across a surface, and will 'bounce' when you adjust them, causing momentary disconnects as you do so. If the driver is wired such that disconnect means deliver maximum current, that'd be bad.

As these disconnects usually are very transient, it could help to fit a capacitor between the potmeter slider and ground. A pull down/up (depending on how the driver works) may also work. You should basically design it such that the laser diode doest get the full brunt if the potmeter you are adjusting shortly disconnects from the circuit entirely.

Hi Lightning Stalker, thanks for those tips. The scope images I posted were to show whether of not the driver was using PWM and it does appear to be a true constant current driver.

I don't have a current probe so instead I set the scope across a 1.2 ohm 5W resistor and ran the driver at 1.5A. The scope showed a steady 1.8V which is the right current for 1.5A and there wasn't much ripple. I did notice that at about 0.25 amp there was some oscillation, about .3V p-p. I've got a 15K resistor in parallel with the pot, so if the pot does have a dead spot the current should be limited to 1.5A.

I built a clean setup (no alligator leads!) and added a big filter cap at the power inputs. I'm also using the dimming input to turn the laser on and off, if you short the two pins together it shuts down the current, so I wired this to the spindle control relay on my CNC driver board (also in parallel with the pot).

I ran the laser intermittently for a couple of hours as I tweaked the CNC setup (Mach3 and LinuxCNC) and the LD survived and I'm really impressed by the power of this little sucker!

At this stage I'm inclined to say that this is a pretty nice little driver for $20. Eventually I may take advantage of the dimming capability (0-10V) but for now I'm just setting the current manually and letting the CNC software turn it on and off.

Thanks again everyone for the help and advice.

Best regards,
Joe

That's great you got it working. Just for future reference, a non-inductive resistor should be used when
doing tests with switchmode drivers (buck and boost). It can play havoc with the output of a driver and
could explain some of the oscillation you were seeing. Most wirewound resistors are inductive. You could
even make your own from a length of stainless steel welding wire. Just keep it straight or in a single arch
to minimize inductance.

>> Most wirewound resistors are inductive
>>
Ah yes, the real world, thanks for the reminder

Benm said:

You should basically design it such that the laser diode doest get the full brunt if the potmeter

Click to expand...

Thanks, yes, that's what I have going on now. There's a 15K resistor in parallel with the pot so current is limited to 1.5A even if the pot opens up.

Cheers,
Joe

Hi, I'm wanting to do the same thing. Are there are updates or progress on your build?

PM coming........

madmike8 said:

Hi, I'm wanting to do the same thing. Are there are updates or progress on your build?

Click to expand...

Hi Michael,

Since wiring it up cleanly and adding the filter cap it's been working well for many hours of runtime. I've got it bolted to my mini-CNC rig and have been using it to engrave wood and make cutouts from paper and card stock.

I found a variant of LinuxCNC that is adapted for laser instead of spindle cutting, it uses the Z axis to turn the laser on and off. I can't remember right now where I got it but if you're interested I'll find the URL. This software also has support for PWM but I haven't put that to use yet. My CNC driver uses a relay for the spindle and that's what I'm using to turn the laser on/off, but the relay wouldn't be fast enough for PWM. The driver module also has a power control (0-10V), right now I'm manually setting the power with a pot (usually set to full power).

Hope this helps, good luck on your build, feel free to ask me any more questions and let us know how it works out!

Cheers,
Joe