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How much voltage and amps does a laser want?

awe

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Hi to all

I am very good in electronics, but not a expert on working with PIC’s
I am new to this laser thingy, but I want to build a laser.
I have built my variable voltage and amperage power. Voltage regular is with some LM317T, and the current regulator is with a MJ2955 transistor with a pot on the base, it was very fast build, not on PCB tho, yet.. I have had some problems with some lame LM317 designs that was supposed to work on a laser diode, but if you disconnect the load, the voltage jumps to 8v+ :wtf:, and I don’t like that…
And have learned the hard method not to connected power source just like that onto a laser


I have a 20X LG DVD writer have is kind of broken, because it doesn’t wane read normal CD’s, it can only read DVD’s, so I have bought a new one, I think the drive is messedup?
My main questions are:

How much voltage and amps does a laser want?
I can’t find a answer for it ? can you?
I know a normal LED works fine with 3volt, but it’s doesn’t blow if you give it to much current, lol. Only if you give it to much voltage
I want to play is safe, I don’t want to waste my diode, I only have one
 

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Different diodes have different voltage and current requirements (red diodes around 3V, violet around 5.3-5.8V). Each type of diode needs to be run at a constant set current, which varies from model to model. It is probably easier to use a LM317 based current regulator, and is not meant to be used "open circuit" - instead, use a test load, such as a string of 1N4001 diodes, to give the correct forward voltage (4 diodes for red, 6 for violet). Also remember, when you run it open circuit, the capacitor charges up, so test it with a test load and discharge the capacitor before connecting a laser diode!

When you say that an LED does not blow with a current increase, you seem to not understand how voltage and current work. As the voltage increases (presuming temperature does not change) so does the current, but because a diode (LED or laser) is a semi-conductor, the relationship between current and voltage is not a linear one. The forward voltage (voltage across an LED or laser diode) can be only slightly different, across a wide range of current, since the "effective resistance" of an LED or laser diode changes depending on the current through it. There are some nice graphs which show this better.
 

awe

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yes, that cap that charges up have turned me off on LM317 based current regulator, my voltage regulator also have a cap, and that doesn't charge up like that..so I got confused and have decided you ask the experts

yea, I kinda not understand that semi-conductor explanation, very difficult for me.
I thing is...a LED does n0t not blow, it only does if you give it to much voltage
but a laser, then does it blow? I've have little almsot none experience with lasers..and first want to get my facts right before connection it up to my laser diode
 
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When you use an LED, by increasing the voltage, you are increasing the current, but the relationship between current and voltage is non-linear, so a small voltage increase could equal a huge current increase. The same is true of laser diodes.

Here's an example of what I mean - see page 3:
http://document.sharpsma.com/files/GH04P21A2GE.pdf

As you can see on the graph, from 3 to 4v, the current increases from 0 to over 120mA. As you may know, any current over 150mA is unsafe for PHR diodes, but this difference between a "good" current and "unsafe" current could only be a difference of 0.1V or less. Furthermore, as temperature increases, forward voltages drop slightly. This is why we use constant-current supply, and NOT a constant voltage. By feeding a diode too much voltage, this causes the current through it to also be too high, but it is the current increase which causes the failure.
 

awe

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to heavy theory for me..but interesting...so I must never go higher then 120mA on any laser?, even on a 20X LG DVD laser?
 

Cyparagon

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INCORRECT. Each laser is different. There is a VERY large range for laser currents. I've got an argon that'll handle 34,000mA, and I've got a HeNe that can't take more than 5mA.

Tell us WHICH laser you're trying to drive, and we can regurgitate statistics then.
 

awe

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the laser I want to drive is a is in a "20X LG DVD writer" everything is still intact..I haven't opened is yet, because i don't want to blow it, I first want to make a driver for it.

I'm currently busy with this guide:
I'm now building that LM317 current regulator circuit
It can be done - Laser driver

am I on the right track?
 

Asherz

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Just from opening it up, your will not blow it.

You need to do some more reading, and then you will realise you need to extract the small 5.6mm diode from the DVD sled in the optical drive, there will be two in there, an IR diode and a red diode.

For us to tell you what current to run it at, we need to check what diode is in the sled.. Unless someone has already harvested from that sled and knows what diode is in there, it could be open can or closed can and there for require different currents :)
 

awe

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hehe yea, I get your point, I have previously opened 5 roms before, I know how a LD look like, they are all cans, and have learned on the hard way that cd roms use IR diodes and not laser diodes :-( no wonder I dint see any red glow... hahahaha
 

billg519

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Current regulators (like the LM317 based DDL design) work by supplying enough voltage to the load that the load will draw the set current. If the load changes its' resistance due to heating, etc., the current regulator immediately changes its' voltage output so that the load can only draw the set current. The only voltage concern is that the supply voltage must exceed the Vf of the LD plus the rquired voltage overhead for the linear regulator used. Measuring open circuit output voltage is useless.

If the output capacitor is a problem because it retains a charge that can blow the LD when you connect the LD, just solder a 470 ohm resistor across this output cap. I'm quite surprised that the original schematic didn't include one, but it never has. The resistor will drain this cap every time the driver is shut off.

Many graphs have been posted here about current draw for various LD's. A search will find you this detailed info.

LED's have a maximum current rating, too. Easiest way to overcurrent and burn them is to run the voltage up with no current regulation. This is why simple round LEDs need a series resistor appropriate to the voltage they will be run on, and why high-power flashlight LEDs need a current regulator board.
 

awe

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isn't there like a fixed resistor value instead of a variable resistor for a 20X LG laser? because If i turn it to much up, then my LD will probably die
 
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Nope, because a laser diode is much more sensitive than an LED, it is normally not safe to use a fixed-resistor limiter unless you are very confident with what you're doing, and are prepared to kill the diode. A spike in voltage from the battery could result in the diode being killed, unless you use a capacitor and ensure your maths is done very carefully. The brightness of the laser (and therefore also the current through the diode) would also drop as the battery voltage drops, whereas a driver ensures that the brightness stays at a set level for most of the battery life, until the battery voltage is so low that the driver stops regulating.

A safe current would probably be around 300-400mA with correct heatsinking. It is well worth using a cheap LM317 based driver circuit, compared with the risk of killing your diode.

EDIT: The sled you have looks very much like a LPC-815 sled, which is considered "safe" up to 400-450mA - I personally use 420mA. The sled is not the same, however, there are a couple of small differences, so there's no guarantee it will perform the same.
 
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awe

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here is my regulator
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the current meter is directly connected to the regulated output, kinda shot circuiting into the meter, and the max i get out of it is 120mA,

Do I need to change that 10 Ohms resistor to 5 Ohms? on the circuit


 

awe

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after changing that resistor to 5 Ohms instead of 10 Ohms, I get a 230mA from my meter, is something wrong here, or is connecting the output to the amp meter direct just plain stupid and giving false info?
 
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To calculate the expected current, divide 1.25 by the total resistance of that "branch". For instance, with a 10 ohm resistance between the first and second pins of the chip, you have 1.25A/10 = 125mA. For 5 ohms you have 1.25/5 which is 250mA. You should be able to do this by adjusting the potentiometer, the fixed-value resistors are simply to prevent the current from going any higher than a maximum value.

It is better to use a "test load" - a string of diodes - to stand in the place of the laser diode, because the diodes behave in a very similar way to a real laser diode (except they don't blow or light up!).
 




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