DIY Homemade laser diode driver

[Gazoo]

You will love it..the beam is very impressive as is the burning...good luck..

 

[Da_Dude]

Aww Fu_k....
I just broke off the wrong pin an my 200 mW dode.
:-[ :-X :-/ :'( :'( :'( :'( :'( :-[ :-X :-[

 

[aaronX987]

Aww Fu_k....
I just broke off the wrong pin an my 200 mW dode.
:-[ :-X :-/ :'( :'( :'( :'( :'( :-[ :-X :-[

dude, you are in good company. I broke the pins off of mine too but luckily I ordered multiple.
You get a pat on the back from me.

 

[Gazoo]

You might be able to fix it but it will require very steady hands.

 

[Daedal]

Keyword here is 'VERY' ;D

GL... and sorry to hear about your mishap. You're also lucky the other GB is almost done

--DDL

 

[IgorT]

Thanks for all the help; You were right about the fuse IgorT.. I must have blown it when I was building my coilgun. A peice of mechano soon sorted that out though.

The part about the meter in series shorting the circuit, confused me at first, but otherwise, if a meter can't measure current, it's allways the fuse, unless there's something wrong with it..

Those fuses are way too sensitive.. I have mine shorted permanently, but only use this one on low voltage DC. The fuse does have a purpose...

 

[woop]

[quote author=IgorT link=1185701612/510#524 date=1194819746][quote author=Gazoo link=1185701612/510#523 date=1194819557]You will want to use at least a 1 watt resistor.

That is true, my bad...


Thanks Gazoo![/quote]

I say at least because it depends on which diode you are driving and so on. But lets use the GB diode for example. The voltage across the diode will be ~2.85 volts with 250ma's of current. 2.85 X .250 = .7125 watts. Therefore for higher currents I would use a 2 watt resistor.. Since we are building the circuit we might as well use a 2 watt for future diodes and what not. Really this is only necessary for experimentation like for a lab laser since it is easy to calculate the resistor needed if using the LM317 as a current regulator. For the open can diode.... I have run 600ma's into it and my resistors get hot. I need to see what the voltage is on the diode with that current..

Added..I checked the voltage across my open can diode running with 600ma's and it is 3.47 volts. Fortunately I am using 3 watt resistors and a 3 watt rheostat. I have two 1 ohm resistors connected in series with the rheostat so current is limited to ~600ma's when the rheostat is set to no resistance. I did this to prevent accidentally blowing the diode.. [/quote]

hey Gazoo, you have your laws a bit twisted there.
the watt rating of the resistor is how much power it can safely dissipate. using your example, if you have 0.25A running through a 1 Ohm resistor, the voltage drop, V=.25/1 =.25V, power = volts * amps so P=.25*.25 = .0625W
you used the voltage drop over the diode, the resistor is used as a shunt, so its not actually the load, the diode is the load. the example you gave would be the power dissipated by the laser

so you actually could get away with a 1/4W resistor for up to 500mA current, although it will probably get warm. I would still use a 1W. but now you don't need to worry about drawing too much current for the resistor!

this also means that having a 1ohm resistor will only cost your circuit .25V, so it won't really make any difference to the operation of the circuit

Geez i just can't help myself can i?

 

[IgorT]

so you actually could get away with a 1/4W resistor for up to 500mA current, although it will probably get warm. I would still use a 1W. but now you don't need to worry about drawing too much current for the resistor!

I didn't want to admit this before, but in my first laser, i actually used a 1/4W 1 ohm resistor in series with the LD. :-[


After some more talking about it, i got worried, and checked it and it was a bit warm, but not hot.. I didn't change anything since then, and everything still works. The resistor did not burn out, neither did it raise in resistance, while the laser did quite a lot of cutting floppy disks in half..

Actually i think i'm gonna measure it again (just in case), paint it black, and measure it's temperature with an IR thermometer..


Of course it might still be safer to use a higher wattage resistor, even if "just in case" only.. But what is the worst case that can happen? Could it actually lower it's resistance? I find this hard to believe, and think that raising the resistance or even burning out is more likely.. I could probably test it on another one, attached to a PSU.

 

[IgorT]

I took a 5.1 Ohm 1/4W resistor, and connected it to my PSU, set to 30V (with a max current of 3A) and tried to observe the current..

The current limiting did not jump into action even for a moment, and the current did not show up on the PSU. I thought one of my cables was broken (by my cat, again..). Well, the cables are OK, but when measuring the resistor, it went from 5.3 Ohm to unlimited.. It burned out, and i didn't even see any current..


Then i took another one of the same resistors, and limited the power initially, and turned it up slowly. At first it got barely warm, at 1/4W. Later it got so hot, i had to let it go. I did not get a reliable reading at this point, as it's resistance increased.. When i disconnected it, and measured the resistance, it was around 10 Ohm, but it was falling fast to 5.3 Ohm again, as it cooled down.. And even tho it still measures correctly, there is something wrong with it, since i can not do the same thing again..


Anyway, i think this is the worst case scenario.. Burning out, or increasing in resistance.. The 1 Ohm resistor in my laser still measures the same as it allways did, and never get's hot.

 

[aaronX987]

I posted this a while back but I think it got "lost" among the other topics.

Ok, 2 questions...
1) What is the difference between the lm317 and the lm317t or are they virtually the same? :-/
2) What type of modifications do I need to perform to this driver in order to make it Blue-ray worthy?

 

[IgorT]

I posted this a while back but I think it got "lost" among the other topics.

I believe it's the answer that got lost between the other posts, as i'm pretty sure, i saw it a page or two ago.

It's just a different package. It should still be able to do the same thing, but the max. current rating might be different..


In the beginning of this thread, there is a link to the 317 datasheet (PDF), and you look for it's ratings and pin-out in there.



EDIT: The answer you are looking for is on page 34 of this thread, slightly under your first question. The "t" stands for TO-220 outline, and there is also a link to the datasheet there.

 

[woop]

yeah, the worst that can happen is that it will heat up, Gain resistance and maybe go open circuit (same characteristics as a light bulb when it heats up). but you would need to put over half an amp through a 1 ohm, 1/4W resistor for it to get really hot

I think 3W is overkill

 

[Gazoo]

My 3 watt resistors get very hot running 600ma's through them. I have two 1 ohm resistors and a 25 ohm rheostat connected between the adj. and output. The reason I did it this way was to limit the amount of current I can apply to the diode...the rheostat gets touchy as it reaches its least resistance.

The correct way to calculate the wattage of the resistor needed is to take the sense voltage and multiply it by the current. So if I take 1.25 X .600ma's I come up with .75 watts. I admit I was wrong in my previous post but I don't understand why my resistors are getting so hot.. :

 

[a_pyro_is]

That's correct Gazoo, the power dissapated by a resistor across the Vout and Adj pins of a LM317 will be (1.25V/resistance in Ohms)*1.25V = 1W sounds like enough for me.
I too don't understand why a 1W resistor would get too hot to touch.

 

[woop]

Wait a sec. are we talking about resistors for use as a shunt, for measuring current or the sense resistors for the LM317?
for the 317 sense resistors, you are right about the 1.25 voltage.
I thought we where talking about shunt resistors for measuring the current...

 

[Gazoo]

I should have known better, sometimes my brain gets the best of me...or should I say whats left of it.. ;D

The resistors I am using were advertised on e-bay as 3 watt resistors. Now I am beginning to wonder although they are much bigger than 1 watt resistors. Oh well, as long as they don't burn up I am happy..