Laser Driver v Simple Resistor ?

[Jarrod]

Hey guys,

Firstly just wanted to say that I'm new to these forums and building lasers and your forum has helped me alot. Great work on the tutorials you have.

My question is, what are the advantages of having a special laser driver instead of a plain ol resistor.
I know that the current can't be too powerful as it will burn out the diode, and I'm only looking to run my bluray laser at 100mA.

Will the resistor suffice? Or will the driver dramatically increase the life of it.

Thanks in advance.

 

[robjdixon]

When you turn on the laser, occasionally the voltage spikes, so the resistor isn't effective at controlling current, while a constant current driver always controls the spikes.

For example, if you have a 7.2v power supply, and want 100mA, v=IR, so 7.2/0.1 = 72ohm resistor
But if the voltage spike to say 20v at the start, then with that resistor, the current is:278mA, which will most likely kill the diode.

 

[Jarrod]

Thanks mate. Actually another question, if I don't have a host for an AixiZ housing, will I still need a heatsink? And do the drivers fit easily inside the AixiZ housing?

 

[robjdixon]

It all depends really. If you are using a PHR-803T, and you are driing it at 100mA, then a heatsink is probably a good idea. I tend to use them for even low power diodes, as they provide a snug fit in your chosen host. If you are not using a host, then I would reccoment getting something like this:
Its at the bottom of the grid, below the modules:

http://www.z-bolt.com/MODIIIbg-green-laser-module.html

That could be overkill though. If you a smaller one, PM Dark_Horse, and he can make you a custom one. Here is his website:http://mysite.verizon.net/grall/darkhorsecustom/index.html
He can also anodize them to make them coloured!

As for the driver thing, here are the two bought options that you can use:
Firstly, the Flexdrive:http://www.laserpointerforums.com/forums/YaBB.pl?num=1209418066
This will fit in an aixiz module, and works on 2-5.5v, so you can use a number of battery combinations

Next is the Microdrive: http://www.laserpointerforums.com/forums/YaBB.pl?num=1206947255
For a blu-ray diode you need from 7-12v, so lithium batteries are usually used with this driver. This also fits in an aixiz module.

Lastly, you can make your own driver:http://www.laserpointerforums.com/forums/YaBB.pl?num=1185701612
It is the DDL driver, and uses a LM317 to regulate current. Unless you are amazing at soldering, this won't fit in the aixiz module;

Lastly

 

[erdabyz]

Well, if you had a voltage regulated source (not a battery) you could use a simple resistor... but... they are temperature dependant and not as stable as you would like. IF you use a simple resistor, choose a value with a GREAT tolerance. If your diode can handle up to 200mA, set to 160 with the resistor if you wanna be safe. But a DIY driver costs as much as a voltage regulator would, so use the real drivr and avoid problems.

 

[robjdixon]

IF you use a simple resistor, choose a value with a GREAT tolerance. If your diode can handle up to 200mA, set to 160 with the resistor if you wanna be safe.

The voltage spikes can be a lot bigger than one might expect

 

[erdabyz]

[quote author=erdabyz link=1239790866/0#4 date=1239802519] IF you use a simple resistor, choose a value with a GREAT tolerance. If your diode can handle up to 200mA, set to 160 with the resistor if you wanna be safe.

The voltage spikes can be a lot bigger than one might expect
[/quote]

Read the whole post

I told him that if he was going to use a resistor, he had to add a voltage regulator circuit to avoid spikes.

 

[robjdixon]

Oh my bad. Sorry.

The LM317 is a voltage regulator, so that it maybe how it is used? I'm not completely sure, as I tend to use pre-built drivers

Rob

 

[erdabyz]

Oh my bad. Sorry.

The LM317 is a voltage regulator, so that it maybe how it is used? I'm not completely sure, as I tend to use pre-built drivers

Rob

Well, as I said before, that's not the real safe way, because we are talking about low value resistors, which are temperature dependant, so using a real driver is the way to go. However, for small projects, my method can work.

Here's the LM317T datasheet:
http://www.datasheetcatalog.org/datasheet2/1/03cgthpfat4t4ly5kfp5lpwladfy.pdf
You have the shematics there. You just need to calculate a resitor in a voltage divider with a formula available in the datasheet to adjust the voltage.

If you need more specific help just ask.

Now i'm trying to design a small and cheap boost driver (those little bastards that can convert 1.5V into 5.5V)... capable only up to 300mA, but if it works it'll be less than $10 per driver. If it works...

 

[robjdixon]

Sounds good. Thanks for the pointers

 

[Warske]

When you turn on the laser, occasionally the voltage spikes, so the resistor isn't effective at controlling current, while a constant current driver always controls the spikes.

For example, if you have a 7.2v power supply, and want 100mA, v=IR, so 7.2/0.1 = 72ohm resistor
But if the voltage spike to say 20v at the start, then with that resistor, the current is:278mA, which will most likely kill the diode.

You are quite right to point out that some bench top power supplies can spike when you turn them on (it's best not to use a supply that does this).  

Just so that the OP isn't confused though, I'll mention that you can't get a voltage spike when using batteries and a simple resistor.  You can theoretically get a tiny current spike if the resistor has much parasitic capacitance.  As a practical matter, this doesn't happen.

For peace of mind, its not a bad idea to wire a capacitor across the laser diode to deal with static electricity issues.  If you do that, it will also take care of any possible problem of paracitic capacitance that the resistor might have.

...you could use a simple resistor... but... they are temperature dependant and not as stable as you would like.

If that were true, the DDL driver http://www.laserpointerforums.com/forums/YaBB.pl?num=1185701612 that many people on this forum use would have exactly the same problem, because it uses an LM317 configured to regulate current, and a low value power resistor to set the current.  In that driver, if the resistance changes, the current changes.  In fact, some people use a variable power resistor to turn it into a variable current source.  The reality is that resistors are actually quite stable and not particularly temperature sensitive if you don't feed them more than their rated power.

The problem using a resistor with batteries instead of using a current regulator is that as the batteries discharge, their voltage drops. The current drops somewhat faster than the voltage, and your laser output power drops somewhat faster than the current.  

The higher the voltage on your battery pack compared to the voltage on your laser diode, the less problem this is.  It is easy to calculate all this out using ohm's law, and if you do that, a resistor can be a viable alternative.

Just another opinion.

 

[Jarrod]

Wow thanks guys for your detailed responses!

...you can't get a voltage spike when using batteries and a simple resistor.  You can theoretically get a tiny current spike if the resistor has much parasitic capacitance.  As a practical matter, this doesn't happen.

... The reality is that resistors are actually quite stable and not particularly temperature sensitive if you don't feed them more than their rated power.

The problem using a resistor with batteries instead of using a current regulator is that as the batteries discharge, their voltage drops. The current drops somewhat faster than the voltage, and your laser output power drops somewhat faster than the current.  

I'll be using a 9v battery or maybe 4 AA's to power my laser. The resistors that I have are capable of up to 240v, and they have relatively high resistance. (I will use maybe a few resistors in parallel to lower the resistance)
I could also add a capacitor, but in the end I think I would be better off getting maybe a Microdrive, because I really to want to have a long lasting diode.

good luck erdabyz on your project.

oh and robjdixon I think that heatsink may be a little overkill haha, but if you suggest a heatsink, I think I may make one out of washers and heatsink glue, which I hear you can do.

Last question? How do I give reputation

Edit:

The higher the voltage on your battery pack compared to the voltage on your laser diode, the less problem this is.

What is the voltage of the laser diode? I'll be using the PHR-803T.

 

[Warske]

Last question? How do I give reputation

You need 50 posts.

What is the voltage of the laser diode? I'll be using the PHR-803T.

About 5 v at 100 mA, a bit more at higher current.

 

[erdabyz]

...you could use a simple resistor... but... they are temperature dependant and not as stable as you would like.

If that were true, the DDL driver http://www.laserpointerforums.com/forums/YaBB.pl?num=1185701612 that many people on this forum use would have exactly the same problem, because it uses an LM317 configured to regulate current, and a low value power resistor to set the current.  In that driver, if the resistance changes, the current changes.  In fact, some people use a variable power resistor to turn it into a variable current source.  The reality is that resistors are actually quite stable and not particularly temperature sensitive if you don't feed them more than their rated power.
Just another opinion.

j33z, you are completely right, I made a mistake. The temperature sensitive devices are the diodes. They slightly vary their voltage drop with temperature, not very much, but more than resistors. For example, if you were driving a blu-ray with a 5.5V supply, you'd want a 0.5V drop to feed the diode with the required 5V. If the voltage drop through the diode decreases 0.1V, you might be screwed if you are already driving the laser diode near the limit.
It's also more difficult to set a precise current this way, without risking the diode, because as you have to calibrate each resistor for a specific diode (voltage drop is always within a range, not a "fixed" value for every diode), you can't get a precise current reading with a dummy load.

 

[Warske]

For example, if you were driving a blu-ray with a 5.5V supply, you'd want a 0.5V drop to feed the diode with the required 5V. If the voltage drop through the diode decreases 0.1V, you might be screwed if you are already driving the laser diode near the limit.

That is an excellent example! It shows what can go wrong, and why.

I am going to sound like I'm trying to sell the idea of resistors as current regulators or something, and I'm not. Its more that I'm selling the idea that with a little creativity and an understanding of the fundamentals, you can make things work when other people tell you that they won't work.

So with your example, lets say you pick a resistor that gives you 150 mA with a 5.5 v source when the diode is dropping 5 v across it. Then, as you say, the voltage drop through the diode decreases 0.1V to 4.9 v. How much current does the diode get then? The resistor voltage went from 0.5 v to 0.6 v. Since the resistor is linear, it is now passing 150 * 0.6 / 0.5 = 180 mA of current, and your diode will probably go poof in short order.

Now suppose you use a 9 v power source. Under the the same scenario, where the current is initally set to 150 mA and then the diode voltage drops 0.1 v, the diode current goes up to 150 * 4.1 / 4.0 = 153.75 mA. This is not so bad. And with even higher power supply voltage, the situation gets even better.

If you were actually going to do this using a 9 v battery, you would want to choose a resistor that would be safe for your laser diode when the diode voltage was at its lowest and the battery voltage was at its highest.

Some other things to be aware of would be:

- Batteries have their own series resistance, and you can subtract that from the value of the current limiting resistor you use.

- Batteries have different voltages and series resistances depending on their technology (eg: alkaline, carbon zinc, NIMH, lithium), their state of charge, and their temperature. For example, a common, cheap carbon zinc 9 v battery actually starts out life closer to 10 v.

It's also more difficult to set a precise current this way, without risking the diode, because as you have to calibrate each resistor for a specific diode (voltage drop is always within a range, not a "fixed" value for every diode), you can't get a precise current reading with a dummy load.

That's also a good point if your power supply voltage is low and you are trying to drive the diode to the max. If your power supply voltage is up around 9 or 10 volts and you are driving your violet laser diode at, say 10% under its safe limit, you actually can use a fixed value resistor for every one.

Again, I'm not saying anyone should do this, but it can be nice to know what is possible and why it works.

Thanks for your good ideas!

 

[erdabyz]

I built a 5mW 635nm laser 2 weeks ago, using the switching voltage driver that came with the host. It was giving 3 stable volts out of 1xAAA battery. After testing my diode's voltage drop with a rkcstr driver and calculating the required resistor I got it working. With the required tolerance margins and always playing within the datasheet specs, this particular diode has been lasing for more than 2 or 3 hours so far, and still doing fine. It needs 40-50mA and i'm giving it between 45 and 53. Stable enough.
Yes, you definitelly can make things work in a "not so good way" for pretty cheap, and with a reasonable safety. I just didn't want to buy a $23 lavadrive for a 5mW diode...

PS. +rep