View Full Version : Laser Diode Constant Current Driver

Mad Professor
18-07-2010, 01:10 PM
Good Day All.

I am looking for help and advice we regards to the best way to drive a low powered laser diode.

This is a basic design I have come up with:
I have yet to work out all the vavles needed for R1 and VR1.

But would this design work ok?

Thanks for your time.

Best Regards.

18-07-2010, 02:50 PM
Not sure what the laser diode needs but a simple current source can be a 7805

Hope this will help

18-07-2010, 03:20 PM
It could work, but whats the laser diode current you expect?

Most sub-200mW laser diodes are somewhere in the 60 - 150mA rating, so asssuming thats the range you want R1 = 1.2/0.15 = 8 ohm (use 7.5) and VR1 is 1.2/0.06-R1 = 12.5ohm (that could be tricky, but RS do a 25ohm 1W (http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=5028542) ). Power dissipated in each is ~0.2W (R1, use a 0.5W) and ~0.3W (VR1, use a 0.75W or 1W)

Most laser diode have a forward voltage of 1.5 - 2V. The LM317 needs a 1.5v headroom and you need 1.2v in the R1/VR1 combo... so the input to U2 needs to be 2 + 1.5 + 1.2 = 4.7v... a tad close to the 5v out from U1. Also in this configuration, at say a diade current of 100mA, U1 is dissipating (24 - 5) *.1 = 1.9W and U2 only (5 - 3.2)*.1 = 0.18W. The disspipation in U1 is close to needing a heatsink (2W). I'd even things up and use a L7808 or L7809, which would then dissipate (24-8) * .1 = 1.6W and U2 would dissipate (8-3.2)*.1=0.48W and you'd have more headroom if you had a particularly high forward voltage diode.

Personally I'd scrap U1 and drive U2 from the 24v rail, dissipation is then (24-3.2)*.15 (worst case) = 3W so will only need a tiny heatsink, anything better than 25degC/W - clip on jobbie will do.

What is the purpose of D1 and D2?

Peter's solution would work, the 7805 driven direct from the 24v rail would dissipate (24 - 7) *.15 (worst case) = 2.55W so again a small clip on heatsink would be advisable. 7805's are cheaper than LM317T!

Mad Professor
18-07-2010, 03:57 PM
I know most pepole use DVD or Blu-Ray writer laser diodes.

But as I was scraping an old HP Laserjet 1100 I tought maybe just maybe I might be able to use the laser out of the.

I have no idea of the power output of the laser, but here are a few pictures of it.




I am only looking at cutting gasket paper at this point in time.

18-07-2010, 06:52 PM
A bit of googling suggests this is a 5 or 7mW device with a forward current of maybe 50mA tops and forward voltage of 1.9V running in the infrared... but how to verify this without frying it, I dont know...

Laser Diode experimenters website (http://www.repairfaq.org/sam/laserfaq.htm#faqtoc) has a lot of info on laserdiodes and power supplies. It seems the only way to know if you're using the right current is to measure the laser output power against the diode spec... which isn't helpful... but this section (http://www.repairfaq.org/sam/laserdio.htm#diodct1)of the site explains how to test the laseing action in a controlled manner...

Mad Professor
18-07-2010, 07:13 PM
I did have a look on google my self, but could not find alot of info on the laser diode.

If the power output is indeed that low, theres it no real point in wasting time with it.

So lets go down the route of buying all new parts, so we know what the power output is ment to be.

As said before I am only looking at using the laser to cut out paper gaskets, if I can also cut cork gaskets even better.

I don't really want to go down the route of a CO2 Laser, I would like to keep with a simple ish laser diode setup if possible.

So can you please advice what sort of power output and laser wavelength.

Thanks again for your time.

18-07-2010, 07:24 PM
lasers aren't my field, but that website has a lot of detail.. I suggest a read of that first.... there is a section on cutting various material with CO2 laser... its not a cheap activity...

19-07-2010, 09:42 PM
Mad Professor,

For info 'Tweakie' has added CO2 capability to his machine. Am I allowed to link to another forum? :redface: (albeit Mach3) - please remove if not !

It's had over 5000 reads, so he must be doing something right. In the interests of spreading information, here it is:

21-07-2010, 06:01 PM
Mad Professor,
It's had over 5000 reads, so he must be doing something right. In the interests of spreading information, here it is:

Wow, that is a good read... Thanks for pointing it out.