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powering a laser off a variable power supply

thebucketmouse

thebucketmouse

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So I'm getting a variable DC power supply for christmas, and I was wondering what I'd have to do to run a laser off it. I assume voltage and current can be controlled independantly with the power supply, so could I just set the mA to whatever I want the laser to run at? What should I set the voltage to?
 





You dont set the mA, do you?

Thought you just regulated the voltage, and then the diode draws what ampere it needs. (you need resistors to control the Ohm, which controls the ampere drawn.This is on the driver for the laser diode.)

I could be very wrong though... Please correct me if wrong.
 
It tends to be more expensive ones that have adjustable current, most of the ones I have come across for home use are presettable constant voltage.

My one is that type, it displays current so I use it through a drive circuit but read the current draw from the power supply.

Regards rog8811
 
Mouse are you getting one fully assembled, or as a kit?

It's so much fun to get 'em as a kit and build your own -- good soldering & assembly practice too. :)
 
Nah it's coming fully assembled. Thank goodness :) Thanks for the info tho guys, I wasn't exactly sure how it worked. Maybe I'll just find out when it gets here
 
Would you like to post the make and model? Then we can google it to see what it does.

Regards rog8811
 
Even if you CAN adjust the current, you still want to be very, very careful driving LDs directly this way.

Never set the output voltage much higher than the diode requires at the needed current.

Even though the PS may be current limited, the voltage output will immediately increase to the max voltage setting when there is no load. There is almost always a filter cap across it's output. So, NEVER connect the LD to a live PS or it will most likely kill the diode immediatley! The same goes for loose connections, all it takes is a fraction of a second for the filter cap to charge, then discharge into the diode and it's all over. I have a nice little sack of dead diodes to prove this! :'(

The safest thing to do is to connect a resistance in series with the PS, just in case ;).
 
My thoughts exactly, and I'm in college for Electrical Engineering. So I figured it was about time ;D
 
Nice I've seen one like that at Fry's. I think you will be ok. Use a series resistor like Kage said. Set you current to a lower value that the threshold and slowly increases it.

Another thing to note is that if the current range in that thing is like 0-5A than a few degrees of rotation on the knob can equate to tens of mA. I'm thinking you will find it very hard to the current you want exact. Try it on a dummy load first.

-Frankie
 
I don't understand why a resistor has to be used- if the supply is set to a particular current, why can't the laser diode just take that current like it would from a driver?
 
thebucketmouse said:
I don't understand why a resistor has to be used- if the supply is set to a particular current, why can't the laser diode just take that current like it would from a driver?
Click to expand...

If the resistor is in series it will eat some voltage the diode receives and limit the current flow a little. This will act as a safety margin in case the power supply voltage display is not accurate. So if driver is set to 2v the diode may only be receiving 1.9v. As to what value of resistor to use I'm not 100% sure because the V drop across the resistor may change with different diodes. Refer to Ohm's law to figure it out. I think the PS will set a max current level such that it will not let any more current flow once it has reached the set value. This means that if your PS is set to 4A lets say and you hook up a small incandescent light, the light will not draw 4A of current nor will the driver force 4A through the bulb. That is impossible assuming the voltage is constant. Maybe you will also need a parrallel resistor to eat up some of the current too. Maybe you will be fine without any resistors. Again I don't know. It all depends on the accuracy of your PS. I don't think they are designed to be super accurate and stable enough to power such delicate components. That's why adjustable voltage and current diode PS from Thor Labs for example are so expensive. But it's worth a try with some cheap diodes I think. Ask if you have any more questions.

Good luck ;)
 
thebucketmouse said:
I don't understand why a resistor has to be used- if the supply is set to a particular current, why can't the laser diode just take that current like it would from a driver?
Click to expand...

Partly because, as Explorer877 mentioned - the current control knob may be very sensitive, and partly because if any connection to the diode becomes intermittent - it will prevent your diode from getting killed. Still, I would recommend keeping a small zip-lock bag to put your dead ones in, I think most of us end up with at least a few - I'm up to about 20 or so now - ;)
 
Alright thanks a lot. But would a test load still work identically to a laser diode, for testing the current settings??
 
thebucketmouse said:
Alright thanks a lot. But would a test load still work identically to a laser diode, for testing the current settings??
Click to expand...

No, but it will be very close. I recommend using a DDM to measure the current as to gauge how accurate your PS is. Although it should tell you on the box. ;)
 
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