So here’s the real problem and why the constraints are as they are. Over the Holidays everyone in the family got these “stocking stuffers” – miniature Coleman Lantern LED key fob lights.
Now the original designers decided to use 2 cheap 1.5V batteries, either GP189’s or 389A’s. So with 3V they couldn’t use a bright white LED, but had to use a very dim amber/yellow LED that was completely worthless.
So I took one of the lanterns, found 2 3V button batteries that would fit close enough – the 1025’s, a bright white LED and fit a 100 Ohm resistor thanks to a Dremel. Works great – bright and useful light.
And yes the 1025’s each cost as much as the entire lantern – but what’s a few bucks among family.
Probably won’t last long, but may be similar to other typical overpriced key fob LED lights that use 2 2016’s.
What led to the OP was my interest in finding a more efficient longer lasting way to drive the LED given these constraints. I figure I could drill out enough space to add something in a TO-92 package. Would regulating the current give me a longer useful life?
I think the answer is no. The difference is constant current regulation vs. constant resistance. With a fixed resistor the LED is initially driven over 20mA, brighter and dies a slow death. Constant current regulation with a 317T is essentially a variable resistor changing the voltage – on top of a fixed 1.25V drop. I still waste energy, but as heat instead of extra light. Further when the 317 drops out of regulation the LED dims very quickly – faster than with just a resistor at the same voltage. Although on a bench that drop occurs at a supply of around 5V suggesting the 317 regulating current only needs the 1.25V extra to regulate, not 2 more on top of that.
I was really looking for a mathematical expression that would allow me to compare the regulated and non-regulated case. Cheaper than an actual test which would cost me 4 batteries.
Although I couldn’t find internal resistance specs for the 1025 I did find a 30-40 Ohm spec on a very similar battery. So I decided to drop the 100 Ohm resistor too.
The only thing that might really extend the life a bit would probably be a boost converter something like the Joule Thief mentioned. This might also let me keep the cheaper 1.5V batteries. Tiny as it is, not sure I could make it fit but I may give it a shot.
Cheers and thanks for the thoughts
-- Dan