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Topic: Current Sources (LED/Laser Diode) - Which to consider? (Read 5337 times) previous topic - next topic


Sep 09, 2011, 01:56 am Last Edit: Sep 09, 2011, 02:31 am by focalist Reason: 1
I'm playing with power LED's again, and I've also gotten a handful of infrared laser diodes in the 200-500mW range.

LED's require a constant CURRENT source rather than a VOLTAGE source, as I understand.. and laser diodes are just special-case LED's.

Until now, I've been using LM317's wired as a current regulator :


But I've seen a number of different methods, including transistors and zener diodes, as well as op-amps and switching regulators:
    (Images: Wikipedia)

I use the LM317 circuit because they are cheap, but they do require a pretty high wattage resistor, and the whole circuit wastes a lot of power as heat.  Usually I attach a heat sink otherwise it gets pretty dang toasty.  The photo above shows a 10 watt LED with a hack-job LM317 regulator screwed right into the heatsink, scavenged from an old PC CPU.  Works well, but it gets awfully hot.  I'd like to increase efficiency, but only if I can do it without tripling my cost for the LED driver circuit.

The Bijunction transistor source circuit seems like it may be pretty cheap to assemble also, as do op-amps if using a cheap one.. but I can't seem to find any good commentary on if I am actually going to gain anything other than grief and cost to go to a "better" current source for power LED's and laser diodes.  I'm also a bit confused on dissipation.. I assume that transistors would need to have sufficient power dissipation just like the LM317, in which case a regulator probably has higher ratings than comparably-priced transistors (thinking 2n3904, 2n2222, etc) when coupled with the other components.  The LM317 does have the advantage of only being two components.

The circuits referenced are from http://en.wikipedia.org/wiki/Current_source , but other information seems scattered and confusing.  I'm sure there's expensive and difficult circuits that can do it.. I'm more interested in cheap and simple than necessarily doing it "The right way"... in other words, if the efficiency gain of 5% costs $10, it's not going to be worth it to me to implement it.  Other than it's wastefulness, is there anything particularly BAD about LM317 current sources?



Other than it's wastefulness, is there anything particularly BAD about LM317 current sources?

No, in fact one advantage of using the LM317 is that it includes automatic self-protection shutdown in case of overcurrent or overheating. However it as, well as the other examples, are all of linear regulator type where excessive voltage is dissapated as heat across the active output transistor.

There are avalible now constant current drivers that utilize switching methods to gain higher effiencies. Some even have a pwm input pin to allow varing the brightness of the load led via a microcontroller pwm output.

Examples: http://stores.ebay.com/Sure-Electronics/High-Power-LED-Drivers-/_i.html?_fsub=2171116016&_sid=208644246&_trksid=p4634.c0.m322



Right.. but like I was saying, the example you posted there is $10.99 for a 10w LED driver of that type (even ordered from China), but the alternative LM317 setup :


costs $5 shipped for 10 regulators and another $1.98 for the power resistors, domestic delivery in three days free.  Ten setups, for less than the price of the one.

While I am at it, I'm pretty sure I know better than to ask this next question.. but for the laser diodes, which are particularly twitchy about voltages as I understand, would it make sense to place a standardly-connected LM317 in front? 


I am not sure if you have seen this but it is worth a good read and might answer questions regarding laser driving.

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