Understanding the basics

[laser83]

I've reached a point where I want to start putting lasers together myself. I understand the basic idea but what's driving me crazy is the concept of the driver. I know it's needed to regulate the voltage, that's its purpose. I could buy a driver with a vague understanding of it but to really start to build lasers I need to know exactly what's needed.  I can assemble computers, I know what parts to buy they are easy to put together. I see with lasers having no clue about the parts or what they do is a problem.

I've looked at all the tutorials. The parts in a driver are the: regular, resistor, potentiometer, and capacitor. Now this the issue. I need to understand how the driver works more than anything. What I can't grasp are Ohms, a measure of electrical resistance.  

I'll ask for an example then say I want to build a handheld blu-ray laser that uses a 6X diode at 200mW of power. How does that translate for what's needed in the driver? What must each part specifications be? Ohms, voltage?

Perhaps a better question what do I need to know when buying a driver? I see mA is what's focused on in the selling. What does mA in a driver translate to? I know what mA stands for but what is the difference between 50mA driver setting and a 450mA? Is there a rule that certain amount is needed per mw in the laser?

There is so much information here it is an overload for me. Imagine a book that covers world history when you have little understanding of  history. There are many tutorials here but getting the basic information has proved difficult.

Better yet If anyone with experience building lasers is willing to chat over on an instant messenger program that would be huge help now that I think about it. Sorry for the dumb questions, but this is the only way I'm going to learn. Unless someone has written a "laser building for dummies" book.

Thanks,

 

[ElektroFreak]

V
______

I | R

this little diagram will help you remember Ohm's law

It breaks down like this: V = IR, I = V/R, R = V/I R = resistance in Ohms, I = current in Amps,
V = voltage in Volts.

This is the trifecta of mathematics pertaining to basic electronics. Each of these factors detemines the other, and can be used to calculate the conditions at any point in a circuit.


You've asked some good questions here, but what you really need is to find a basic electronics theory text and read it. There's no way any one person here would be able to properly answer these questions to the level of detail that you need.

Ohms are just a unit for measuring resistance. Amps (or milliamps [mA]) are just a unit for measuring current.

The number of mA being passed through a laser diode determines it's output. Each type of diode is rated to a maximum number of mA (or A). Running them higher than their mA rating will shorten the diode's life.
Since we don't have datasheets for most of the diodes we see here, all we can do is go by the recommendations that can be found in the threads here.

I have an MSN account, I'll PM you the name. I am at work presently, so I can't chat now but if you send me your MSN name then I can shoot you a message when I am available..

 

[chipdouglas]

so you know ma already. one thing important is that the laser diodes (LD) are current sensitive (ma). so like electrofreak said, use the ma that is good for a particular LD. the driver will regulate the voltage. the potentiometer will let you adjust the ma. but to do so you would also need a driver test load to simulate the current draw of whatever LD you use.

michael

 

[laser83]

Thank you both this is extremely helpful information and gives me a better idea of what needs to be accomplished to get a laser running.

As a buyer my main concern has been milliwatts until now I had no idea the extreme importance of the current and its function. Knowing that each diode has a limit on the mA it can accept makes perfect sense, common sense but until now I didn't realize that.

Next question if the driver regulates the voltage its there a size limit on the driver? I assume as many parts as would fit on the driver would be the limit?

Yet I see a driver will burn out if it's takes more voltage than it's supposed to. I finally understand how that happened. Tin foil bypasses the normal method of voltage transmission through the spring in the driver.

What's more important is I see now why a driver will take only so many volts. I have an idea of the concept.

so you know ma already. one thing important is that the laser diodes (LD) are current sensitive (ma). so like electrofreak said, use the ma that is good for a particular LD. the driver will regulate the voltage. the potentiometer will let you adjust the ma. but to do so you would also need a driver test load to simulate the current draw of whatever LD you use.

Ah now this making more sense. How would you do a driver test load? Do mean testing the driver with a machine that creates currents? So we have diode that's optimal setting is 450mA before putting in the diode the driver must be set and tested? If if the current comes out higher than it must be readjusted?

Forgive my ignorance I have no idea what a machine that allows you to adjust voltage called?

I will need a basic book on electronics but that goes into building a driver myself at least now I have an idea of the basic concepts.

 

[Tw15t3r]

If you don't have an idea about electronics and how electricity behaves or works, you need to read alot before you can even begin to try to construct your own driver, or understand how drivers here work. What has been said so far is only scratching the surface of electrical circuits.

I'd suggest the online book: http://www.allaboutcircuits.com
It has 4 volumes, each containing several chapters. Reading the first few chapters of vol 1 will get u aquainted with electricity, and from there, you can skip to the chapters you'd like to understand.

You really need to read alot for this, as it is very different from building a computer. A computer has standard parts that cannot be substituted like, a motherboard, RAM, power supply, etc., and must more or less be connected in the same way.

Electronics on the other hand, like driver circuits, have many possibilities. The range is so large. Worse still, is that you really need to understand each component fully before you know how to make use of them, and a combination of several components could make things rather complicated.

Take it step by step. But if you need a laser fast, the only solution would to be to buy a pre-made driver. (some people just want the laser and don't really want to go through so much trouble to get that. I'd say those are likely to stop the hobby rather fast)

 

[chipdouglas]

The test load constists of (depending on your LD) diodes and a 1ohm resistor. That simulates the draw of the LD on the driver. Go to the search option and type in "Driver Test Load" and set the go back option for like the past 3 months. you will find much more helpful stuff there.


michael

 

[laser83]

Thanks, that like is very helpful I'm going to have to take some time to read it. The idea of actually building a driver is extremely daunting at this point. I'm getting a good understanding though. It also makes senses that to succeed with builds you need to know exactly what you are doing.

Thanks too Micheal I'll check that out. I have a much better idea now of basics. I need to read the link on electrics so I can understand the components of the driver. This all been extremely helpful thanks everyone!

 

[Warske]

what's driving me crazy is the concept of the driver. I know it's needed to regulate the voltage, that's its purpose.

Next question if the driver regulates the voltage its there a size limit on the driver?

Its important to emphasize that the purpose of the driver is not to regulate the voltage.

The reason for this takes a bit of work to understand, but its worth it so here goes...

Go to wikipedia and look up "diode", http://en.wikipedia.org/wiki/Diode, then scroll down to "Figure 5: I–V characteristics of a P-N junction diode.  

There you will see the forward biased region on the curve where a laser diode operates.  On this part of the curve (where the "forward" line intersects the "Von" line), a small change in voltage results in a large change in current.  The nearly vertical line also shifts left and right as the temperature of the diode changes, although they don't show that.

OK, now the light emitted by a laser diode is a function of its current.  (It is also affected by temperature, but not as much.)  You can see from the I–V curve that if you try to operate the diode at a constant voltage, and the curve shifts due to a temperature change, the current will change a lot and the diode will either be putting out too little light or it will put out too much and burn out.  If, on the other hand, you operate the diode at a constant current, you can expect the light output of the diode to be fairly stable.

An ideal laser diode driver regulates current to the laser diode, and it gives the diode as much or as little voltage as it (the diode) wants to have at that particular current.

If you can find an I-V curve for a flashlight bulb, you will see that it bends the other way, so you want to power it with a voltage regulator.  A flashlight battery looks a lot like a voltage regulator.  Thus, batteries and flashlight bulbs work well together without the need for special drivers.

Its interesting to note that a battery with a high voltage and a high valued resistor in series with it looks a lot like a current regulator, and that combination can be used to power a laser diode (although there are some issues which I will get to).

The reason that some people seem to get away with hooking a laser diode directly to a battery is that batteries actually do have some series resistance, as does the diode and the wires used to connect them.  The problem is that the resistance is too low to make a decent current regulator.  The diode may eventually get too much current and have a short and unhappy life.

An example should make it clear.  Say you've decided to power your laser diode at 100 mA and depending on the temperature of the diode it is going to need between 3.2 and 3.5 volts.  To make it interesting, figure also that if the diode gets more than 120 mA it will die.

Lets say that at the moment it needs exactly 3.5 volts at that 100 mA current.  If you hook it up to a 4.5 v battery pack and if there is effectively 10 ohms of resistance (in the batteries, the diode, and the wiring), then the diode will get its 100 mA and be happy.  That is because (ohms law here)...

(4.5 - 3.5) volts / 10 ohms = 0.10 amps which is 100 mA

Now suppose its temperature changes and now it needs just 3.2 volts.  It is going to get

(4.5 - 3.2) / 10 = 0.13 amps which is 130 mA

And 130 mA is more than the 120 mA needed to kill it.

Now lets say you have a 90 volt battery and you stick a 855 ohm resister in series with it.  Running the diode with that works pretty well.  With the diode at 3.5 volts, it gets 100 mA again.

(90 - 3.5) / (855 + 10) = 0.10 amps = 100 mA

But now if the diode voltage drops to 3.2 v, the diode only gets

(90 - 3.2) / (855 + 10) = 0.10035 amps = 100.35 mA

And the diode is still going to be very happy and a long way from death at 100.35 mA

So that is what a current regulator tries to do, only hopefully even better.  (The reality is that many of the ones you buy will actually perform much worse.)

Of course, no one wants to deal with a 90 v battery.  They tend to be big and expensive and hard to find now that the age of the vacuum tube is past.  Another problem with this approach is that it is inefficient and most of the power is lost in resistor, which is also getting hot.  Finally, when the battery voltage drops, the current drops along with it.  You need to replace the battery before the battery is actually dead.

(On the other hand, and at the risk of getting flamed, I will point out that if you have a diode that isn't very temperature sensitive, operate it well enough below its maximum current, and use batteries with a relatively stable voltage such as NIMH AA batteries, the resistor approach can work rather well using a red or IR diode and just three cells.  It depends on what you're trying to do.  Just don't admit on this forum that you would ever consider doing this. )

What if you were to use a low battery voltage along with a variable resistor and then monitor the diode current, adjusting the resistor as necessary to keep the current stable?  That is more or less what a linear current regulator does.  If you go back to wikipedia and look up "LM317" you will find a diagram for just such a regulator.  You will also find discussions of it on this forum.  It's been around forever, and a little Google searching should pull up a detailed explanation of how it operates.  It has a relatively high drop out voltage (look it up) which is not a good thing, but if that is a problem you can find low drop out (LDO) designs or look into switching regulators.  What fun!

Since we're on the topic of laser diode drivers, be aware that some diodes have a built in light sensor (a separate photodiode) which can be used to better regulate the light output.  Its particularly useful for diodes that are very temperature sensitive.  On this forum they are mostly ignored.

It can be hard to get an understanding of how voltage and current work.  It may help to play with analogies like water going through a garden hose, and you can use math to solve problems.  It also helps to play around with inexpensive components.  When you see smoke, you realize that your understanding didn't match reality.  With time you will eventually "get" it.  To get started, buy an inexpensive digital volt meter or two if you don't have them already.  Hook up a flashlight to a battery and measure the voltage and current.  Buy a resistor or two from Radio Shack and put one in series with the bulb and one in parallel.  Figure out what happened and why.  Try it again with a cheap light emitting diode.  Measure and plot the I-V curve for your LED.  Measure the internal resistance of an alkaline AA battery and of a NIMH AA battery.  Measure and plot their discharge curves, etc.  Good luck!