Using a DC/DC converter to power the laser

[IgorT]

Since i like to make my lasers small and portable, but want my output power constant, even when the battery voltage drops, i believe a step up DC/DC converter could be used to power the laser..

One option would be this one: http://www.dealextreme.com/details.dx/sku.7302
MatajumotorS succesfully used it in his laser project over on CPF.


It makes 5V out of an imput of 1.5V or below. This means a 2700mAh AA NiMh could be used to power the laser, and the enclosure could be very small.

The output might need aditionall filtering, but if it's stable enough over the entire battery voltage range, all that is needed afterwards, is a current limiting resistor and a cap on the LD. The same thing could power a BlueRay from a single NiMh cell, with a different resistor of course.

I would prefer Daedals LM317 circuit after the converter, but the output voltage is too low..


Any thoughts on this? Has someone tried something like this?

 

[Gazoo]

Wow..what a great idea...I never thought of using one of these with the AMC7135! Brilliant. I may have to order a few. BTW, the cap we solder across the diode should provide sufficient filtering.

 

[IgorT]

Wow..what a great idea...I never thought of using one of these with the AMC7135! Brilliant. I may have to order a few. BTW, the cap we solder across the diode should provide sufficient filtering.

Yeah, i was hoping one cap would suffice, if the converter itself isn't already filtered enough...

Otherwise, there is another, better step up converter, which has an adjustable output voltage 2-7V and a current rating of 3A: http://www.dealextreme.com/details.dx/sku.7882  (Again posted by MatajumotorS on CPF)

With this one, it would be possible to set it to 6V and use the 317 for current regulation.


An even simpler way would be to just set the current by setting the voltage, using a 1 Ohm resistor after it for current measuring and the usual cap.

Here the current would drop a little, when the LD temperature would raise, but i believe this difference would be neglectable..


From the battery standpoint, this would be MUCH more efficient, since voltage regulators turn the excess power into heat, while the step up converters can be extremely efficient. I don't know this converter specific efficiency, but you know what i mean..


I just don't know about all those flashing modes and stuff.. Id prefer if it was just a simple converter and nothing else.. But as long as one of the modes is constant on, i'm happy..   The prices also seem reasonable.


EDIT: I found the efficiency to be 85-88%.

 

[rog8811]

This all sounds very good, are you going to try it out Igor? I would be interested in the outcome as it should allow a much smaller overal package....

Regards rog8811

 

[IgorT]

BTW: The first one works with a voltage down to 0.9V. With the second one, this is not stated, but i imagine it should be close..

This means it really sucks the battery dry.

 

[Gazoo]

It might be a better choice, but I like my current to be stable and not change as the battery voltage changes. So I would still want to use it with the 7135, but like you said, I wish they had these in single mode versions.

 

[rog8811]

I wonder what it would take to disable the unwanted modes??

Regards rog8811

 

[IgorT]

This all sounds very good, are you going to try it out Igor? I would be interested in the outcome as it should allow a much smaller overal package....

That's exactly what i want..

I already managed to make a miniature, but powerfull red laser, by using a LiPo, but there my choices were, either to use the LiPo as a 2 cell one and have only 130 mAh, or wire the two cells in parallel, get 260mAh, but the current would drop with the voltage, as the LiPo discharges..

I chose the second option and i'm satisfied with the results, but why not have a miniature powerfull laser with the output stable over the entire battery range?


I will definatelly be ordering a few of both regulators and post the results here, once i get them.


Otherwise, the first one already was succesfully used by MatajumotorS on CPF: http://www.candlepowerforums.com/vb/showpost.php?p=2214397&postcount=254
He also used a LED current regulator from DX after the converter.

 

[IgorT]

It might be a better choice, but I like my current to be stable and not change as the battery voltage changes. So I would still want to use it with the 7135, but like you said, I wish they had these in single mode versions.

You misunderstood me. It would give a "stable" current over the entire battery voltage range. That's exactly what i want..
The only thing that would lower the current would be the LD heating up. The output voltage stays constant down to just 0.9V battery voltage.


But as i said, if you want, you can still set it to 6V and use the 317 afterwards. Or the 7135.

My point is just getting the most out of the battery and having a "stable" output. Even with a current regulator the output wouldn't be perfectly stable, since the efficiency of the LD also drops with the heat..

But with or without current regulation after the converter, it still beats the cr@p out of the current dropping with the battery's voltage.

 

[IgorT]

I wonder what it would take to disable the unwanted modes??

I was looking into step up converters before, for the best possible battery use in the devices i develope.. The ICs are readily available, the coils are the parts that are usually harder to find..

It is possible that these boards use one of the more common step up ICs and you could just look up the datasheet and remove the unwanted components..

But MatajumotorS also said, he "programmed" the first converter to suit his needs. I should ask him how exactly this is done..

 

[Gazoo]

Igor,
I have yet to see one of these maintain the same current over the life of the battery. There is always a first time but I would be surprised. Let us know when you get yours.

 

[IgorT]

Igor,
I have yet to see one of these maintain the same current over the life of the battery. There is always a first time but I would be surprised. Let us know when you get yours.

I guess i have to start expressing myself more clearly...

I was talking about constant voltage output.. I also mentioned the current would change as the LD would heat up, unless you use a current regulator afterwards..


But then i read this again:

Everyone should keep in mind that the current will not remain constant for the laser diode if it is fed by a fixed voltage. As the laser diode die heats up (as a function of the power delivered to it), its resistance decreases. That must result in an increase in current if the voltage is held constant. The VI curve is quite steep and the effect is very dramatic (i.e. the diode will quickly enter the overdriven zone). That's whay the current sources are so nice - the current is held constant and the voltage regulator allows the voltage to "float".

I read it before, but thought it says the current goes down as the LD heats up.. It's the resistance that goes down..

My interest in current regulation after the step up suddenly increased.. Who cares about efficiency, it's gonna be a NiMh anyway..

 

[IgorT]

Igor,
I have yet to see one of these maintain the same current over the life of the battery. There is always a first time but I would be surprised. Let us know when you get yours.

I read my previous post to you again, and found what i did wrong..

I wrote: "It would give a "stable" current over the entire battery voltage range".

What i meant was "stable" as in stable +/- the variations caused by the LD heating up..


I really do need to start expressing myself more clearly..



BTW: Gazoo, have you used the AMC7135 before? What's so special about it?

 

[Gazoo]

Igor,
We both know that you can not increase the voltage without increasing current and visa versa. So I stand by my previous post. The boost regulators that I know of will not provide a constant current thus the voltage will not be constant either. Like I said there is always a first time and I would like to read your testing of them when you receive it. The boost regulators would probably be ok for the blu-ray diodes.

After thinking about it some more, the easiest to use and get regulator we have going for us IMO is the 7135. It does require a min. of 3.2 volts so it will run off of one AA li-ion battery, and for those that don't want to fool with li-ion's...three AA or AAA alkaline or nimh batteries. It is a no mess no fuss application and current does stay constant as the diode heats up...I just checked again. However the power of the laser will drop.

I did a review of the AMC7135:

http://www.laserpointerforums.com/forums/YaBB.pl?num=1191898669

 

[IgorT]

Igor,
We both know that you can not increase the voltage without increasing current and visa versa. So I stand by my previous post. The boost regulators that I know of will not provide a constant current thus the voltage will not be constant either. Like I said there is always a first time and I would like to read your testing of them when you receive it.

Oh, i thought you meant the voltage would be constant and the current would change due to heat..
I thought i would get a stable voltage output, using a step up converter like that.


I used a step up converter in one of my devices once, to make it work from a single battery, since it's hard to get boxes with larger battery compartments and i don't like the 9V batteries due to their low capacity.

In that setup it was working perfectly and gave out exactly 3.6V, from a 1.5V battery (down to 0.7V), but it's not strong enough for this application.. The coils would have to be larger and a different IC would have to be used..


That's why i liked the DX step up converters.. They can handle high currents.


Well, i'll order a few of both and let you know, once i test them.

 

[chimo]

But then i read this again:
[quote author=chimo link=1185701612/480#480 date=1194382947]Everyone should keep in mind that the current will not remain constant for the laser diode if it is fed by a fixed voltage. As the laser diode die heats up (as a function of the power delivered to it), its resistance decreases. That must result in an increase in current if the voltage is held constant.  The VI curve is quite steep and the effect is very dramatic (i.e. the diode will quickly enter the overdriven zone).  That's whay the current sources are so nice - the current is held constant and the voltage regulator allows the voltage to "float".

I read it before, but thought it says the current goes down as the LD heats up.. It's the resistance that goes down..

My interest in current regulation after the step up suddenly increased.. Who cares about efficiency, it's gonna be a NiMh anyway.. [/quote]

Here's a graphic example of what I was trying to explain.  This is an image I snagged off the net but it is representative.  I added the red lines and enlarged the image.  There are three Voltage-Current (VI) curves for the same LED for three different temperatures (-40C, 25C, 100C).  
Let's pretend that the LED is driven by a regulated voltage (indicated by the red arrow).  
At -40C the current through the LED would be about 1.5mA.
At 25C the same voltage would result in a LED current of about 4mA.
At 100C the same voltage would result in a LED current of 15mA.

Note that the 10x difference in current between a cold (really cold) LED and a really warm LED.  

In practice, the temperature will not swing that much but it should be extra warning for people using TECs.  A room temperature LED will heat up in proportion to the power pumped into it and the amount of cooling it has.  

Also recall that the LED/LD die will be much hotter than the module it is housed in.  

You can extrapolate from this graph that the current for a LED/LD supplied by a fixed voltage could easily double as it heats up.

To check the change in resistance vs temperature use R=V/I
For -40C curve, V=1.17V, I=1.5mA, therefore R=780 ohms
For 25C curve, V=1.17V, I=4mA, therefore R=292.5 ohms
For 100C curve, V=1.17V, I=15mA, therefore R=78 ohms

NOTE: This is an illustrative example only. Please do not try to apply the curves below to specific cases.