Constant current or voltage power supply?

OK, for you pros who test the Luxeons, what is the cheapest power supply that one can set to run at a particular current (i.e. settable current regulation) or particular voltage (i.e. settable voltage regulation)? Obviously this will be useful for testing the luxeons to figure out what Vf it really is etc.

This is a question of HOW you guys do it.


Thanks in advance for any help.
Mo

Normally if you go the current way, you shouldn't have any problem, if you go the voltage way, make sure you know what's the exact voltage that gives you the right current for your LED. That means lots of adjusting and so on. One prob with this is that you might encounter thermal runway (search around the forum for it). Of cos if you have heat sinked the LED well, you shouldn't worry about it.

A good old LM317T can be easily used as an adjustable voltage regulator and also as an adjustable current regulator. They are tough as nails, easily tolerate reverse voltage, etc. Nice part to work with.

If you want to you can even hook up one of them in voltage-control mode (dialable with a simple potentiometer) and another one in series with it in current-control mode (also dialable with a simple potentiometer).

As long as you have a decent DC power supply of about 12V or so (you can build this too if you like) then you can easily make it both voltage and current regulated by the addition of just two LM317Ts and a few simple passive components.

The circuits are very simple and all of the details for using the LM317T in these two modes is included in the LM317T datasheet. You can find this datasheet most easily at National Semiconductor's website, or from Digikey.

best of luck!

/ubbthreads/images/graemlins/smile.gif

A.

A second on using a 317. I just built a current supply this week - using 4 switches, I can add in 30, 100, 300, and 600mA (and any combination of them). With this, I can charge batteries, test LEDs for a given current, and blow-up incandescent bulbs! /ubbthreads/images/graemlins/grin.gif

About the blowing up bulbs thing - While LEDs like constant current, incandescent bulbs are afraid of it! Just try forcing 600mA into a 3 cell PR bulb, and you'll see what I mean!

-Darin

Also check the LM723, I think it can do both, and is common in power supplies.

http://www.national.com/pf/LM/LM723.html

And, for $200-$300, you can get a current/voltage reg'd power supply. Maybe there's a cheaper bargain out there. Check some of these places http://www.qsl.net/k6ls/test.html

They're really handy for testing leds and bulbs, charging batteries, etc.