[n00b needs help] Problems with 200mW red laser diode

[kaz]

Hi Doc, thats strange i have built 4 dvd burner diode units using nimh battery laser units without any problems, the batterys put out 2.42 volts max i can not increase the voltage past, 2.42 is the max, i only put a varible resistor in between to adjust the current the threshold of each diode is slightly different on each one, you are compeatly right you must put a resistance between the battery and the diode 2.4 volts at 240 ma was giving me all depending what brand of burner it came out of between 110- 140 mw, idealy it would be nice if the batterys put out 3 volts then i could bring down the ma's to get the same output, and your rohm diodes work a treat at 3.2 volts with no resistor, but they are working on the edge but i knew that when i put them together if i wanted them to last a long time i would have put a resistor in there for more hours of use as they say the candle that burns twice as bright, only lasts half as long

[The_Doctor]

It's pure luck that the threshold puts the voltage of a pair of NiMH's damn close to the drop across the diode at threshold. Because the drive current if increased, puts the diode drop higher to 2.7V or more, it's sort of self-limiting. That explains why you got away with it. Don't rely on that though, it's a VERY narrow margin that ceases to exist the moment you apply enough volts to be sure of getting full power out.

The higher the voltage (and the higher the resistor value you use) the more control you get, but you waste a lot of energy in the resistor. The optimal value for experimenting, and even for commercial builds, is about 5 volts. 4.8V from 4 NiMH's is ok too. You save more energy if you go lower, like the 3.6V of lithium, but it then becomes very important to control the resistance finely, in steps of 0.1 ohm. Keep the supply at around 4.8V to 6V, and you'll be able to cope with cheap easily bought resistors or 1 ohm and upwards.

[kaz]

I hear you Doc and as usal you are completly right and i think your right i have been lucky and sooner or later im going to send one to diode heaven

[The_Doctor]

I hear you Doc and as usal you are completly right and i think your right i have been lucky and sooner or later im going to send one to diode heaven

You won't be the first. I lost count long ago. Thing is, we need to make it count for something when they die. They more they teach us in doing it, the more fun we get out of them. It's actually fun at times, if it happens as a result of a careful prediction after a long and successful test at finding their limits.