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SIMPLE Laser PSU

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LAN_403

EEYORE

Sat Jun 14 2008, 07:22PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello all,
I am unsure whether it was right to post a new thread here, or to simply add to my thread about C mount Laser diodes...
Anyhow, I am thinking about a laser power supply. Batteries are getting aggrivating! I will need to be able to go from 0-4amps at least of current. I know there are plenty of circuits out there, so my question is:
1) Can I use a simple voltage reg. and use resistors in series with the laser diode? Is this a safe way to run a laser?
For example: A simple higher current adjustable voltage reg. with power resistors in series...I can turn up the voltage and measure current through the laser...(Laser drops about 2 volts across itself, a resistor will limit current).
2) Are there any potentially dangerous hazards of regulators? Laser diodes are super sensitive! I will eventually power an $$$ 3watt 808nm laser diode with this, but would also like to run low power lasers as well...
3) I want to stay away from complicated circuits...I dont ever plan to modulate the laser either, so I dont need a special IC...
So I guess what I am getting at is: Can I run my laser diodes from a variable voltage regulator with s resistor in series to limit current, and say a zener diode across the laser diode to protect in case the regulator fails somehow?
THanks!
Matt

...

Sat Jun 14 2008, 08:41PM

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

You could try building the die4drive with a bigger heatsink for the transistor (and different current limit resistors).

If you aren't up for that, yes it would work to used a fixed regulator and a resistor. You would want to watch out for oscillations, but if you are using a fixed variable regulator (ie, lm138) you shouldn't have too much trouble. It would also be possible to wire the regulator as a current mode regulator, although I might suggest sticking with a voltage mode regulator and dropping resistor just because it is less prone to exploding diodes due to dirty connections (commercial drivers have circuitry to disable the output if the detect an open circuit, simple drivers like this just charge up the output capacacitor as fast as it can and blows up the diode).

EEYORE

Sat Jun 14 2008, 09:10PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

So you say fixed regulator? Any way to get it to work with an adjustable regulator? (HATE having to constantly wire in/out resistors to limit current. WOuld much rather be able to sweep the voltage up and down across this resistor+diode series combo...
Anyone know of a good reg. with greater than 3amps? The LM350T seems perfect, aside from its 3amp limit...Could I use two of these?Perhaps I justneed to build it and try it out on cheap lasers...
I dont need any sort of current monitoring or output monitoring for now since these diodes can handle a bit. I will keep run times low to watch out for overtemp for now...The 3watt 808nm likely will need some temp monitoring though at a later time....
Thanks,
Matt

Hon1nbo

Sat Jun 14 2008, 09:33PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

with that expensive of a laser it should come with feedback, so work that into the design... if you are skeptical of the design, there are cheap power supplies available online for basic needs... try Sam's Laser FAQ

EEYORE

Sat Jun 14 2008, 09:48PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

DaJJHman wrote ...

with that expensive of a laser it should come with feedback, so work that into the design... if you are skeptical of the design, there are cheap power supplies available online for basic needs... try Sam's Laser FAQ

It does have a photodiode as well inside...Im not worried about over driving it much. My circuit wont accidentally jump the current up 1amp...Small fluctuations in current/power out arent a problem. A constant current is just fine.
I guess a better way to ask this question is:
Has anyone used an Adjustable Voltage Reg. and run into problems with noise, or failures? If the thing failed, would a zener diode be enough to save the laser? I will be monitering the laser while it is in use; there will be current meters and voltage meters in place. (A moderate rise in current wont be too bad for a high power laser). Voltage spikes and whatnot or osscilations would be a problem though.
KISS prinicple here

Matt

Hon1nbo

Sat Jun 14 2008, 09:54PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

I have used one for a 250mW 650nm laser, but it recently had a sudden lapse in power, so I am uneasy about it... but it did take some abuse... before that it was just fine, but never operated at that kind of power.

EEYORE

Sat Jun 14 2008, 10:13PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

DaJJHman wrote ...

I have used one for a 250mW 650nm laser, but it recently had a sudden lapse in power, so I am uneasy about it... but it did take some abuse... before that it was just fine, but never operated at that kind of power.

Perhaps the lapse in power was overcurrent? Got a dvd laser? I see alot of people on ebay list them as 250mW when that means pulsed. They call em CW though

So I was looking at the LM350T data sheet, and it says 4.5amps max current. Sounds more than sufficient! I tihnk I will order one, and whip up a driver and see what happens

So, for some protection, I will add a cap. and resistor across the laser diode, as well as a 4-5volt zener diode to clamp voltage incase the regulator goes...I will size the current limit resistor such that 5volts across it and the laser diode wont be too much current before I shut the thing off. When the 3watt laser is used, I will add a thermometer circuit so I can watch temp.
Can anyone see anything wrong here before I go ordering stuff? Its super simple, one step above annoying battery power.Matt

...

Sat Jun 14 2008, 10:31PM

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

You might try the lm138 I noted in my previous post, it is good for 5a continous or as much as 12a peak

EEYORE

Sat Jun 14 2008, 10:38PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

... wrote ...

You might try the lm138 I noted in my previous post, it is good for 5a continous or as much as 12a peak

Wow, 12amps peak? Must need a monster heatsink! Thanks for the tip!
Matt

Carbon_Rod

Mon Jun 16 2008, 03:28AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

When designing a supply for the 35 watt bar I came across little useful documentation.

Tips:
1. Temperature regulation is very important (water block cooling or TEC)
2. Optical output monitoring is only important if you are trying to stabilize the output
3. High power Schottky diode reversed across the laser
4. Voltage should not exceed 2.1v for most circumstances (some lasers are damaged above 2.3v)
5. Carbon resister to load the specific SMPS output prior to keying on the Laser
6. Use a good Power Mosfet key the laser
7. over-voltage shunt for the spike during SMPS power shutdown
8. Keep the high current wires short (a difference of 0.1 ohm can be significant)
9. Optical output is typically 30% efficient (1watt optical power = 3watt of supply power approx.)
10. Safe output current is typically 2-4 times the measured threshold current.
11. anti-static... short-out the laser with a key-switch when not in use

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