weird effect when changing current

[dark racer]

hey guys. quik question here.

i am building a laser engraver and it's basicly my first time playing around with home made lasers.
and i have a question

tldr.
i made my own laser driver. and when i adjust the current i see a weird effect and i don't know what it means.
also i think i may have wired up my potentiometer in reverse. (turning it all the way clockwise dims the laser) but i tested it before with an led. and then clockwise was brighter and counterclockwise was dimmer.

so the questions are.
what is this weird effect.
and why does it dim when it supposed to be at full power. (note this driver can supply between 125 and 400 mA. the laser diode is in the range of 200-300mA)

this is the driver that i made




on a side note. the effect is when adjusting the current. (so it changes to a set shape for a set current) if i stop adjusting it will stay in whatever way it is at that time.. i have the feeling it's from overdriving the diode and doing so for a long time could destroy it?
(i know i sound like a noob. but i assure you. a noob is far more knowledgeable then me when it comes to how lasers work, i'm just a quik learner and good with google)


thing is. i don't know wich.
is the dimming an effect of overdriving? (in wich case the potentiometer or driver as a whole is working perfectly and as intended)
or is the flickering (changing of the form of the dot) the sign of overdriving. (in wich case the potentiometer is wired up in reverse polarity)

 

[Immo1282]

Most potentiometers are pretty noisy - so you should expect to see all sorts of unpredictable behaviour if you keep the driver powered up while you adjust it.

It's typical and reccomended to set the current of your laser diode driver using a Test-load of which I have attached a schematic as opposed to with the diode attached and powered up. Doing this also reduces the chance of you accidentally overdriving the diode as you measure the current setting before powering up the diode.

How do you plan for the microcontroller running the CNC firmware to modulate the power of the diode? For this and any laser engraver, you need to change the effective power output of the diode in order to burn the material being engraved more or less, so you should look into modulating the beam for that effect.

There are many methods to do this - and one of the commonest ways of doing so is with PWM using a separate TTL input on your laser driver. You will struggle to do fast PWM on an LM317 based regulator with much success unfortunately.

 

[dark racer]

Most potentiometers are pretty noisy - so you should expect to see all sorts of unpredictable behaviour if you keep the driver powered up while you adjust it.

It's typical and reccomended to set the current of your laser diode driver using a Test-load of which I have attached a schematic as opposed to with the diode attached and powered up. Doing this also reduces the chance of you accidentally overdriving the diode as you measure the current setting before powering up the diode.

How do you plan for the microcontroller running the CNC firmware to modulate the power of the diode? For this and any laser engraver, you need to change the effective power output of the diode in order to burn the material being engraved more or less, so you should look into modulating the beam for that effect.

There are many methods to do this - and one of the commonest ways of doing so is with PWM using a separate TTL input on your laser driver. You will struggle to do fast PWM on an LM317 based regulator with much success unfortunately.

well initially it's just an on off system. burn amount could be done with movement speed.
but apparantly. my homemade driver allows me to digitally adjust the laser brightness.
i am powering the driver with a 12v fan output on a 3d printer board.
2 wires with a pwm signal. (0-255)
if i turn the fan on a speed of 1 my laser powers on and is verry dim. (lets call this focus mode)
if i set the fan speed to 255. my laser is on full power.

it's a verry clean and linear dimming and brightning.

 

[Immo1282]

What's the schematic you built that homemade PWM driver to? An LM317 won't do that on its own

 

[dark racer]

the exact schematic is in the OP (the diode is representing the laser)

 

[Immo1282]

Okay - so You're connecting the PWM fan connector in place of the power supply on that driver? A couple of points to note here;

1. You're going to get a seriously large amount of heat coming out of the LM317 here - A linear driver dropping 12V to the forward voltage of a DVD burner laser diode is going to give you a a drop across the LM317 of around 8-10V - really big. The LM317 will need a heatsink for sure as well as the Laser itself.

2. That diode is not going to last long. Laser diodes don't like being overdriven or run without good heatsinking for long, and you compound that problem by switching the input on and off as you have. It'll probably work, but not for long.

 

[dark racer]

Okay - so You're connecting the PWM fan connector in place of the power supply on that driver? A couple of points to note here;

1. You're going to get a seriously large amount of heat coming out of the LM317 here - A linear driver dropping 12V to the forward voltage of a DVD burner laser diode is going to give you a a drop across the LM317 of around 8-10V - really big. The LM317 will need a heatsink for sure as well as the Laser itself.

2. That diode is not going to last long. Laser diodes don't like being overdriven or run without good heatsinking for long, and you compound that problem by switching the input on and off as you have. It'll probably work, but not for long.

the lm317 in this setup is a constant current circuit. not a constant voltage circuit.
or atleast it's supposed to be. i added the capacitors later to get rid of spikes of any kind. and i have not measured the voltage on there.
but this diode is being driven by 12v. (i assume the voltage changes when i adjust the fan speed now that i added the capacitors)
but there is a comparably giant heatsink attached to it and 2 fans in the back blowing air right across it.
so that's fine.

(btw where is this schematic you attached? i can't find it)


edit.

disregard everything i said.
adding the capacitors have changed the function of this circuit completely. and because of a busted fuse i cannot measure amps.
ehh.
i now measure 3.5 to 2.5 volts on my laser output depending on the position of the potentiometer. (this used to be 12v before i added the capacitors)
i need to take a closer look at my driver. (also it doesn't matter if the laser only lasts a week. i have plenty of diodes and by the time i use a decent diode i'm going to just use a prebought ttl driver)

i'm guessing that capacitor is the diffrence between a constant current and a constant voltage source. i need a constant current if i don't want to burn out the laser.
so what i am guessing is that adjusting the fanspeed now controls the current instead of the voltage like i had hoped.
wich means if the laser heats up. more current starts flowing wich means the laser gets hotter and even more current starts flowing excetra.

 

[Immo1282]

the lm317 in this setup is a constant current circuit. not a constant voltage circuit.

Yep, but the LM317 is still the component where the majority of the wasted power will be dissapated regardless of its configuration. You will need to heatsink the LM317 if you are powering it with a 12V supply and trying to output hundreds of mA. The laser diode itself will have a forward voltage somewhere between 1.8V and 3.2V in your case. You will need to heatsink the LM317's tab itself.

I apologise, I forgot to attach the schematic - it's on this post. You set it up as the schematic, and then measure the Voltage across the 1 Ohm resistor. I=V/R, when R=1Ohm, means that the voltage across it is the current output of the driver.

 

[dark racer]

Yep, but the LM317 is still the component where the majority of the wasted power will be dissapated regardless of its configuration. You will need to heatsink the LM317 if you are powering it with a 12V supply and trying to output hundreds of mA. The laser diode itself will have a forward voltage somewhere between 1.8V and 3.2V in your case. You will need to heatsink the LM317's tab itself.

I apologise, I forgot to attach the schematic - it's on this post. You set it up as the schematic, and then measure the Voltage across the 1 Ohm resistor. I=V/R, when R=1Ohm, means that the voltage across it is the current output of the driver.

it is heatsinked. the heatsink attached to it is an old graphics chip heatsink. it's atleast 2 or 3 times as big as a regular heatsink designed for an lm317. that's all fine.

i just need this thing to last atleast a day or 2. just for shits and giggles of having something i built myself. and then i'll buy prebuilt stuff made by people who know what they are doing.

 

[dark racer]

ok so i made the circuit in here https://www.circuitlab.com/editor/#

(i am not premium so i can't save them and share them.)
but the driver is working as intended. i just did not know it would drop from 12v on the outputs without a load. to 3.6v on the output with a load.

without the capacitors it's just always a 12v input. and changing the fan speed does nothing other then just turning 12v on an off rapidly.
now with the capacitors it's the same as changing the input voltage. at max power and max resistance on the potentiometer. i get an output of 3.6 volts to the laser and about 300mA
on the lowest resistance of the potentiometer i get about 100mA and 3.1 volts.

then if i lower the fanspeed to 50% i get a hypothetical 6v. and my laser gets about 90 mA with 2.6v

 

[Immo1282]

Yes, the capacitors are acting as a low-pass filter - i.e. The PWM from the fan is being passed on as a varying input voltage.

If your current-regulator circuit takes a varying input voltage and outputs a varying current, it is NOT functioning correctly, and should not be trusted to keep your Laser Diode alive. This is why an LM317 is wholly unsuitable for your application and why I talked about PWMing the laser diode itself, and not the input to the driver.

If you are planning to only vary the depth of the engraving by moving the laser at different speeds, then you should power the LM317 with a constant voltage. Around 6 volts is better than 12V as the voltage drop across the LM317 will be lower, meaning less power wasted there. But I cannot stress it enough - your current regulator circuit is not working properly if your output current changes with varying input voltage!! This is why testing a driver with a test-load is important. A current regulator should force the same current to flow in the output regardless of the input voltage, output resistance or any other factors.

If you need to vary the optical output power of the laser, you should build a driver capable of switching the laser on and off very rapidly (in the kilohertz range) - That way, the average optical power of the laser can be varied with varying pulse-widths.

 

[dark racer]

Yes, the capacitors are acting as a low-pass filter - i.e. The PWM from the fan is being passed on as a varying input voltage.

If your current-regulator circuit takes a varying input voltage and outputs a varying current, it is NOT functioning correctly, and should not be trusted to keep your Laser Diode alive. This is why an LM317 is wholly unsuitable for your application and why I talked about PWMing the laser diode itself, and not the input to the driver.

If you are planning to only vary the depth of the engraving by moving the laser at different speeds, then you should power the LM317 with a constant voltage. Around 6 volts is better than 12V as the voltage drop across the LM317 will be lower, meaning less power wasted there. But I cannot stress it enough - your current regulator circuit is not working properly if your output current changes with varying input voltage!! This is why testing a driver with a test-load is important. A current regulator should force the same current to flow in the output regardless of the input voltage, output resistance or any other factors.

If you need to vary the optical output power of the laser, you should build a driver capable of switching the laser on and off very rapidly (in the kilohertz range) - That way, the average optical power of the laser can be varied with varying pulse-widths.

what do you mean pwming the diode itself and not the input to the driver?
the whole point of the driver was to protect the laser from the pwm as far as i know.

those are the results i get from a circuit simulator (because my multimeter is broken) results from me specifying probe points when i don't really know how to measure these things correctly (i set a point on the output terminal of the laser wich in the circuit simulator was an unspecified led).
this is a driver used by dozens of people. so instead of the driver being bad. it's probbably my measuring that's bad.
i bet i'm good. and like i said. longevity is not a goal here

i don't need to vary anything it's just something i noticed was possible and felt like info i needed to ad, not really a question i needed help with.
i just thought it was info that might be important to answer my question..
all i really need is to finish the laser so i can focus it. and then engrave atleast 1 thing, that's it.
it's for fun. (just to learn things before buying real parts. like a 2w diode. wich i would need a new driver for anyway. wich i would be buying)
from what i read online this thing produces a steady output. and it's min and max ranges are well within the diode's limits. and even if it isn't it's no problem. (this thing has only cost me litterally 10 bucks up untill now)
i build one with absolute zero knowledge about electronics. and i have learned soooo much in the process. (i'm still a total idiot but the learning is the goal here. and failure is the best teacher)

i know explained the whole situation in the video. but all i asked is what the weird effect is when adjusting the pot.
to me it looks like a few lines jumping around based on how far i turn the pot. (meaning i can predict how it's going to look at a certain setting since it's always the same)
to me. it looks like i am looking at a magnified image of the laser being generated on the actual diode. (some sort of electric arc dancing around depening on the current)


(the diode is already dead btw. i shorted the leads on one of the caps when i wanted to measure something. but i have lots more)

 

[Immo1282]

but all i asked is what the weird effect is when adjusting the pot.

Could be the noise on your potentiometer, seems a little odd. Perhaps if someone else weighs in they might know. How come you've focused the laser out so far? Have you got a lens for that module installed yet?

 

[dark racer]

Could be the noise on your potentiometer, seems a little odd. Perhaps if someone else weighs in they might know. How come you've focused the laser out so far? Have you got a lens for that module installed yet?

no havent made a lens setup yet. got a 5mw module that's adjustable in the mail today. currently contemplating using that setup for focussing instead of making my own colemating lens with the lenses from a dvd drive and a 3d printer.

 

[dark racer]

Could be the noise on your potentiometer, seems a little odd. Perhaps if someone else weighs in they might know. How come you've focused the laser out so far? Have you got a lens for that module installed yet?

well i have a lens now. and i am happy. (i need a better power supply though. currently the entire machine is running off of a 12v 600mA universal adapter.

you can hear the fans fight for power with the laser. (fans slow down when laser is on)

i am using inkscape for the g-code. (the raster extension is a bit lacking though. i am going to look for an alternative when i wake up tommorow) https://hackaday.io/project/4828-raster-2-laser-gcode-generator