Thank you all for motivating me to make a guide like this one. I hope this answers any questions and clears any confusions. If anyone requires any further support please do not hesitate to ask questions.

One of the best items to be able to build yourself is the laser diode driver. The driver is supposed to regulate voltage and current. One of the best things to have in a laser diode driver is variable voltage AND current supply. This is sure to be an item that many would like to replicate and use on all of their home made projects. Also, given the very low component count in this driver, it could probably easily be made to fit into a regular L.E.D. flashlight be using SMD parts and rebuilding the circuit already in the flashlight.

Since voltage is already being provided by the regular battery, let's start with what we really need the circuit to do for the diode.

Regulate voltage:

What?
The voltage source that this circuit is designed to regulate is one supplied by an alkaline/lithium battery. An AA/AAA/AAAA battery supplies approximately 1.5 Volts DC(usually more around 1.2 for lithiums, but I've had a few at about 1.6-1.8!!). A CR123A battery supplies 3.0 or 3.6 Volts DC depending on the battery. This is very useful and very stable for the most part. When a battery is left unused for a while, the first burst of energy dispelled from the battery can spike up to 10 times the average power of the battery! Therefore, for a 1.5V battery, this would add up to approximately 15V. This burst is very small and very short that in most situations would not even kill the laser diode, but there are times when the burst packs some power along with it, supplying well over a watt of energy to the diode. This can spell instant death for most diodes!

How?
To regulate the voltage we can either use a voltage regulator and set it up in voltage regulation mode and limit the supplied voltage(This would still miss some of this spike and still pass it on to the diode) or we can use a capacitor. Capacitors work by charging up when there is a change in voltage. Direct Current voltage turns a capacitor into an open-circuit (a cut wire), and thus any spike or change in line regulation would work first on charging up this capacitor. Capacitors have 2 readings on them that correspond to maximum voltage and capacitance. For most of my applications I use a 47uF 16V capacitor. To be more on the safe side, you can get a higher capacitance and a higher voltage rating, but it all depends on personal preference. A capacitor will work to regulate the line up to the maximum power that it can soak up as fast as it could. Capacitors ADD up when connected in PARALLEL. Therefore, if you want to use mutiple capacitors with different benefits (Max voltage and big capacitance for instance) you need to make sure that BOTH positive and negative leads on the capacitors are connected together. I will explain more detail later on.

Regulate current:

What?
The current supplied by the battery is rather stable, but in most cases, is too much to drive a laser diode. For instance, a DVD-burner laser diode would need about 200-250mA of power at about 2.5-3 volts(for any long life span to actually be expected of the diode). Laser diodes are, unfortunately, a very light load when wired directly to a battery, and an alkaline battery would easily put out about 500mA or more, and lithiums put out 2 or 3 times that much! This too is an instant death situation for our beloved laser diode. Although very simplistic, it is not that common to find a flashlight with the right current regulation, and most of us would like to be able to control the current supply by using a potentiometer.

How?
Current regulation is the tricky part of this circuit. The chip that I like most for current regulation is the LM317. The chip can be used as a voltage regulator and as a current regulator. This guide will use a capacitor as a voltage regulator since the battery's voltage is, for the most part, already regulated and stable, while all we need to do is limit the current that we let flow into the diode.

Required components:
[table]
[tr][td] Component [/td][td] DigiKey [/td][td] RadioShack [/td][td] Approximate Price [/td][/tr]
[tr][td] Capacitor [/td][td] 1 2 [/td][td] 1 [/td][td] ~ $1 [/td][/tr]
[tr][td] LM317 [/td][td] 1 2 [/td][td] 1 [/td][td] ~ $2 [/td][/tr]
[tr][td]Potentiometer [/td][td] 1 [/td][td] 1 [/td][td] ~ $1 [/td][/tr]
[tr][td] Resistors [/td][td] 1 [/td][td] 1 [/td][td] Pennies [/td][/tr]
[tr][td]Silicon Diodes [/td][td] 1 [/td][td] 1 [/td][td]Pennies[/td][/tr]
[tr][td][/td][td][/td][td][/td][td][/td][/tr]
[tr][td][/td][td][/td][td][/td][td][/td][/tr]
[/table]

It has been suggested by Xenodius to use THIS potentiometer. This is a great alternative and a very easily acquired item from most local RadioShack stores. This circuit can completely be bought from your local RadioShack now Thanx Xenodius

This diode driver would be able to drive all kinds of diodes and at all power levels. The LM317 can easily control current up to 1.5Amps but would require sufficient cooling to actually remain operational for a reasonable time. With a respectable regulated power supply, this circuit could power a laser diode without the need for any batteries.

The potentiometers in the bundle package MAY have a 100-Ohm pot. I have bought 2 of them and have not found any, but it’s not that difficult to get some from DigiKey…

As about the resistors, I actually prefer getting the package from RadioShack over getting individual resistors from DigiKey.

The LM317T from RadioShack is the same as the one from DigiKey. I personally prefer the smaller package from DigiKey only because it is much easier to pack into small spaces. The downside is that the bigger package has a small heat sink already on the LM317T, thus making it more usable for higher power ratings.

So... let's build it

The easiest way to learn about the LM317 voltage regulator itself is to read the data sheet available here.

Following is the schematic diagram for the laser diode driver.

The pins are not how your LM317 will be... note the labels and not the pin numbers...

To connect this start off my locating the pins corresponding to Vin, Vout, and Adj on the LM317T chip. Also, Make sure you turn the pot to its maximum resistance possible.

Connect the laser diode and make sure the capacitor is SOLDERED to the laser diode pins, be it through wires or directly on the back of the laser diode. If the laser diode happens to disconnect from the capacitor and then reconnect while the capacitor is charged, then that’ll be worse than the spike from the battery itself! It will literally dissipate all the charge almost instantaneously to the laser diode and fry it because it has very little internal resistance…

I personally use solid 20-24 gauge wires and cut the length that I need with some extra room for the part that I would bend into the holes of the board I am using to build on. I like solid wires because they are much easier to maneuver.

Following is a picture of a schematic I built on a proto-board to clarify the wiring and the chip connections for the LM317T.

I have used a 10 Ohm resistor simply because it was sitting right next to my proto board.
I used the LM317T to show the wiring most would use.
I used a different pot from what you would get from the link above simply because this is easier to understand in a picture. Notice that pins 1 & 2 are connected together.
I used a 47uF capacitor simply because it was sitting on the proto board already.
If you want to add another capacitor simply connect it on top of where the other capacitor is.

If you follow the circuit diagram and build the circuit as it is outlined, the only thing left to do is connect the battery and then start turning the pot until you get enough power through to turn on the diode.

Another personal note… I personally like placing a multimeter before the diode to be able to read and verify the current going into the diode. I also usually place another multimeter across the laser diode to check the voltage.

You can grab a momentary switch and place it on the wire connecting the positive battery terminal to the LM317 to make the power control similar to turning on a pen-style laser pointer.

Obviously this whole design can be tuned to be used with a BluRay laser diode or any other kind of laser diode. For further help, feel free to ask questions.

Good luck all and hope this helps;
DDL


A nicer 'picture' schematic was kindly uploaded by aaronX987... here it is for further reference:

Brilliant writeup there DDL and in plain English too

I hope this will be stickied. Thanks for posting this.

Jase

Thank you for your kind comments Tallaxo. I hope it is stickied too as this would definitely be a post to refer to over and over.

I tried to make it as simple as possible and give as much detail without going into too much jargon. It took a while to make, but nonetheless it was definitely worth it. I hope this is what everyone was looking forward to.

--DDL

Wow!!!!
That is excellent!!!!
So, that is what you should use to power the LD's from Senkat, right?
How small can you make that entire package?
Thanks Alot.
What is the max/ min input for the LM317?
Thanks Alot
Jonno

This MUST be stickied.

@Daedal, Thankyou for doing that...I have just ordered some regulators.

When it comes to other components I assume that a 200ohm pot would suffice? Will any signal diode do, I have a shed load of 1N4148's.

This next question will really show my ignorance.....But, if this circuit were to be tested on a standard led (rather than the precious laser diode) would the charecteristics, IE current draw, be anywhere near similar?

Regards rog8811

*Hope the mods stick this in the sticky area with sticky stuff...I will be stuck when it falls down the page *

*edit....years after posting this, and having built dozens of these circuits.. easy to follow picture follows

Hey, I read your 2 guides, they are great and well explained thanks for making them ! However I'd like to know if only a capacitor and a potentiometer could work with 3 AAA batteries, because this whole driver circuit will never fit into a flashlight I have, and if possible I'll try to use this one instead of buying another one, thanks !

well, I will not pretend to answer on Daedal's behalf, but here is my $.02 - if you reduce the circuit, you are reducing hte protection, and risk blowing up the diode....

hmm ok then, might end up putting it in something else than a flashlight... Also I don't want to push it to the limits, just have a nice powered red laser.

Hello Daedal,
Beautiful and easy to understand write up. However I do want to clear some confusion. A capacitor does not act as a voltage regulator. It is the LM317 that regulates voltage or current depending on how it is used. The capacitor is acting as a surge suppressor. I am sure you understand this, and I don't mean to nit pick, but since your write up is so well done making this small change will make it perfect

Damn, Daedal. This is an awesome guide. It seems like something even I can do! ;D

I will definitely be builiding a few for my diodes.

Thanks again!

No it won't work because the potentiometer can't handle that much current when it is wired directly to the current source. Now here is the good part. You can get by with just a 1 watt resistor and a capacitor. I used a 5 ohm resistor connected to a three cell battery pack using Nimh batteries, and was driving the diode with 175ma's. But I had no specs on this diode so it is just a ball park figure. Every diode will be different, even those from the same batch will difer a little.

The great part about using the regulator circuit, aside from the fact it is adjustable, is that it will provide the same amount of current to the diode no matter what the input voltage. However keep in mind the voltage must be at least 2 volts above the planned output voltage. So if we need 2.5 volts to drive the diode, the input voltage must be 4.5 volts or more. Any more voltage and the voltage will stay at 2.5 volts at the output. Anything less than 4.5 volts and the voltage will start to drop to less than 2.5 volts at the output. The same applies when using the LM317 as a current regulator.

Everyone needs to have a DMM (Digital Multimeter). Don't try to do this without one or you will end up ruining the diode.

First of all thank you all for the nice comments and for your support. I really hope this answered most of the questions anyone had in mind, but I will surely add some more to make it even more complete. With your help, this shouldn't be too difficult

Now... let's answer all the questions:

This can be used to power any laser diode with a power input of up to 37V and up to 1.5 A! The LM317 is a very versatile device capable of so much given its compact size. I use the term compact size because it is in fact a LOT smaller than trying to regulate things using single devices all around. Also, This whole thing can be made to fit inside a laser pointer if need be. All you need to do is get a bunch of surface mount devices and have a VERY steady hand. If you wire this up right with SMD's this can be as small as the driver in the leadlight... maybe smaller

Great question rog. A 200 Ohm will work just fine. Make sure it is something you can fine tune to a reasonable level as that gives you better control over the current supply. I use a DMM and test the resistance as I wind it up and down. Be very careful using only one Pot though as pots tend to have the horrible ability to short out when you get near the end of their turn and that would instantly kill your diode... :'( Either put a couple 1 Ohm's or something similar. I am not very sure about the ability to use the 4148 in this as I have never tried it. I just glanced over the datasheet very quickly and still am not very sure. It is in fact a high-speed diode rather than a rectifier diode. It is probably better used in a switching power supply than to rectify a signal.

This is something I had forgotten to include in my guide, and will include it soon. YOU CAN USE LED'S TO TEST YOUR CIRCUIT BEFORE YOU PUT THE LD IN THERE! Thank you for pointing that out and for reminding me. Effectively and due to how the LM317 works, you can test the current by wiring the circuit directly to the DMM because it is not affected by light load...

Here's my 2 cents about this setup... If you do not want your diode to live any kind of expected life or do not mind, do not regulate the current. I am using a direct connect to LD in my MagMod but that is only for fun. The diode I am using is a 9mm and I got it for cheap and have 9 of them! To me these are used to test the circuit before I put another diode in there. So even after it burns out, and becomes an overpriced LED, it is still useful to me...

As about fitting it into a flashlight... I'll include a list of SMD parts from DigiKey to help those in need of making their own mini-regulator. With that it might be possible to fix up the circuit on most flashlights and even add one where one is not present

Thank you Gazoo for trying to clarify a few points and correcting me. You are right, a capacitor doesn't regulate the voltage, it just dampens the voltage spikes I also hate to be retaliatory if this does actually sound like that, but I have explained all of that in my guide. This is not meant to regulate voltage at all. Only clear out as much of the noise as possible to make it well suited to run a laser diode.

Thank you all again, and hope to hear more from you soon;
DDL

I had to leave this out... Thank you very much for sticky-ing this and my other guide. I was hoping they would and that was a very fast turnaround.

--DDL

Thank you Daedal,

The following statement may confuse a newcomer:

"How?
To regulate the voltage we can either use a voltage regulator and set it up in voltage regulation mode and limit the supplied voltage(This would still miss some of this spike and still pass it on to the diode) or we can use a capacitor."

I realize you go on to explain and again I don't mean to not pick (even though I know I am ;D) or sound retaliatory either. However even with your detailed explanation, if I did not understand your explanation, I would be thinking, "but didn't he just say we can regulate voltage with a capacitor???"

My suggestion would be to change that statement to something like as follows:

"How?
To regulate the voltage we will use a voltage regulator and set it up in voltage regulation mode and limit the supplied voltage. And we will use a capacitor to ensure no voltage spikes reach the diode."

Again you have done an outstanding job with your write up and in fact I plan to try it myself. I am looking forward to whatever SMD parts you can find. We do know there is a smaller variation of the LMT317 that can handle up to 500ma's.

BTW, do you know how warm the LM317 gets in your circuit? I mean does it feel hot?

Okay, I have a few questions. :P

In the circuit diagram, the resistor is 4 ohms. You used a 10 ohm resistor in your circuit because you had it close by, but does it make any difference. I am guessing that it doesn't because you included a 4 ohm resistor in the diagram, linked to a 15 ohm resistor at DigiKey, and a 15 ohm resistor isn't included in the RadioShack pack.

So does the value of the resistor matter much?

Pardon my ignorance, and thanks again!

Minor detail, but can you please explain why this is? I would think that when the cap is in the circuit it would only charge up to the voltage that's already on the diode. Then even if the cap were then directly hooked up to the diode alone, wouldn't it still apply, at most, the same voltage to the diode and thus not kill it? Or am I just completely confused here :-/

From what I've been told the main reason for the capacitor in very simple LD circuits (just a cap and resistor) is too smooth out in voltage spikes and give the diode a "soft start".

You are right, it seems that the cap would only charge to the voltage on the diode. I think what Daedal is saying is that you can't just connect 2.8-3.0 volts at 250-300mA directly to the diode, since many of the DVD burner red diodes have a threshold of only 100mA according to the spec sheets I've seen.

The way I understand it is that the capacitor slowly charges and slowly outputs more and more current, so that the laser diode doesn't die a quick death by the high current. If the LD became disconnected from the cap after running for a little bit, and was suddenly reconnected, the normal current you want to run the diode at would kill it.


Wow, I just realized I was talking in circles. :-[

Then again, I have no idea what I'm talking about, and probably am wrong.

I found a seller on e-bay that has some of the SMD's we would need if we wanted to make the circuit smaller. The problem is what would we mount the SMD's to? I think I am going to skip trying this. The parts are extremely small and a pain to work with. I have in the past, but building a circuit from scratch would be very difficult...I would think. I suppose it could be done by super gluing the parts to a small board, then wire them up with small wire. But we would have the problem of heat sinking the SMD version of the LM317.

The LM317 in this circuit is a current source. If the load (LD) disconnects, the Cap will charge to about 6 volts. Reconnect to the LD and you might have an LED The cap at 6 volts might blow the LD. (It's happened to me )

Mike

Oh, so the problem is not with the charged cap temporarily disconnecting, but with the diode itself disconnecting?

Gazoo, thank you very much for your suggestion, although we are NOT using the LM317 to regulate the voltage. In this circuit I am using the capacitor to dull away the spikes from the battery and the silicon diode to provide polarity protection.

As about the LM317T, it doesn't get anywhere near hot at that supply. A smaller one would definitely start feeling a little warm, but this one feels to be at room temperature even after about 10-15 minutes..

Thank you;
DDL

I am very sorry about yet another blunder int he post... I will fix it immediately. Please understand that I did this really late at night... :-[

As about the resistor, if you use a 4 ohm resistor, then you limit the maximum voltage to 312.5 mA. If you use a 10 ohm the maximum that will be passed through will be 125 mA. Therefore, yes it makes a huge difference. I will fix the links momentarily.

Thank you;
DDL

Minor detail, but can you please explain why this is? I would think that when the cap is in the circuit it would only charge up to the voltage that's already on the diode. Then even if the cap were then directly hooked up to the diode alone, wouldn't it still apply, at most, the same voltage to the diode and thus not kill it? Or am I just completely confused here :-/
[/quote]

The reason for the nice spike from the capacitor in such a case if because basically the capacitor is charged up like a second battery. This second battery has very little to no internal resistance and is not regulated by the LM317, in fact it is directly wired to the LD... thus when the LD works as a discharge venue for the capacitor, the electrons are eager to go through the tiny wire and will instantly fry it. In a case like this the LD won't turn into an LED, the hair-thin wire inside the diode will overheat and snap like a fuse wire. The internals of the LD would all be fine, but it would be disconnected...

I am not sure if this explains it any further Pseudo. Sorry if not, and please feel free to ask more if you need;
DDL