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Current Source Circuit Question

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EDY19

Tue Mar 04 2008, 08:34PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Well, I've started becoming interested in lasers, and since I live in an apartment, I figure that this may be a safer hobby then tesla coils, at least for right now. Anyways, I started designing my own laser supply for solid state lasers of various sorts, and I'm just checking if the schematic has any errors or recommended additions/changes. The source is adjustable from 0ma-800ma and a maximum voltage of approximately 8V across the load, power supply using a LM7810 voltage regulator. The only thing i would still like to add in is a photodiode feedback circuit, but i haven't really looked into how to do this yet. In the picture, the 1 ohm and 10k resistor are actually a potentiometer to adjust current, i just got lazy so the resistances don't add up to 10k

. I also added a soft start circuit so the current through the load ramps up to full value in about 1 second.

Sulaiman

Tue Mar 04 2008, 09:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I'm not an expert with laser diodes but I have destroyed a few !

It takes a microsecond or less to destroy a laser diode due to over-current so;
Any feedback-controlled power supply must be stable and have no overshoot
so unless you are well versed in feedback systems (I guess you're not since you asked this question)
stick to the simplest solutions.
e.g. Battery and variable resistor,
or battery, potentiometer with wiper to base of a pnp transistor with an emitter resistor to +V, collector to laser diode.
or battery, potentiometer with wiper to base of a npn transistor with an emitter resistor to laser diode , collector to +V
etc.

I have seen 78xx regulators oscillate, op-amps overshoot and ring, .........

I'm not saying don't do active feedback control because ultimately you will be
sensing the laser light amplitude to stabilize it with a feedback system
which is probably when you will discover all the ways that things don't necessarily behave as designed ;<)
Get some cheap laser diodes to experiment with first!

...

Wed Mar 05 2008, 01:17AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I would repeat what Sulaiman said, the best way to drive a laser diode is the simplest way to drive a laser diode. A basic npn transistor (or pnp if that is your fancy) is a decent current source, and really I wouldn't worry too much about feedback. With a basic laser diode, the power is quite constant for a given current (it changes slightly with temperature, but really for most work just a constant current source is just fine). Of course it gets more complicated when you are using more complex lasers, ie dpss modules.....

BTW, one or two generic silicon diodes in series make good test loads for testing laser diode supplies, if you have a scope you should test any circuit to looks for overshoot when you power up the driver (this is what single shot mode is for)

c4r0

Wed Mar 05 2008, 04:35PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Here's my driver I invented and made one week ago. It seems to work well. I'm using it to drive dvd laser diodes and green pointers. You just have to connect and disconnect diodes when power supply is off (or driver output is short-circued) to prevent killing the diode. LM358 has two op-amps so i decided to make two drivers on one pcb :)

lpfthings

Wed Mar 05 2008, 09:47PM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

Laser diode drivers arent hard to make, they are quite simple even with a LM317. Just make SURE, that if your circuit has a capacitor for spike protection, make sure it is discharged fully before reconnecting the diode, if not, your diode will be dead without you even knowing till you power it up. I have killed quite a few laser diodes, ive killed about 7 DVD diodes, 2 IR diodes, and a Blu-ray diode, not cheap. BTW

sells DVD burner diodes for a pretty good price, thats where i get mine from.

EDY19

Wed Mar 05 2008, 11:14PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

Thanks for all the replies! I chose that current source basic schematic from a TI? Application note, as it was titled "Precision Current Source" I have access to some nice digital storage oscilloscopes at school, so I'll prototype one out and give it a startup test and see how that looks. I changed a couple things around, one of which is a capacitor across the laser diode, zener regulator across the power supply, a fine and coarse adjustment knob (Coarse- 0-1A, fine, 0-100ma). I'm just trying to find a nice old/dead DVDRW computer or recorder to take apart and mess around with

lpfthings

Thu Mar 06 2008, 05:24AM

Registered Member #1361 Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305

Like i said, check the link i posted, cheaper than ripping a new drive, and you can buy them already in the modules too.

EDY19

Thu Mar 06 2008, 04:50PM

Registered Member #105 Joined: Thu Feb 09 2006, 08:54PM
Location:
Posts: 408

I get a lot more satisfaction out of making my own stuff, even when its probably cheaper to buy the whole thing assembled with a PCB

But thanks for that link, and they definatly look like the place to find the DVD-RW diodes if i cant salvage an old computer from somewhere.

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