I just built a 750mA LDO linear current regulator for driving Luxeons. I built this so I could treat my 5W luxeons a little more gingerly. I have a 5W with an exceptionally low Vf (6.24V @ 700mA). I chose a linear regulator because a LDO will give about the same efficiency as a step-down (or step-up) converter, but will be dead simple to build, and cheap.
I used Doug Owen's world torch circuit as the starting point (an LM334-based current regulator). The only difference is that I swapped the 2n3906 transistor for a Zetex FZT968 (sot223 format). To make soldering easier, I broke off the collector pin on the one side, and used the tab as the collector connection. The sense resistor (the coil of wire) is 0.08 ohms.
The PCB is a 1" circle. I made the pads by cutting channels in the copper cladding using a dremel and a cut-off wheel (takes a steady hand).
With full input voltage, at 750mA, the minimum dropout voltage is very near 0.1V. With reduced input voltage (out of regulation), at 400mA, the dropout voltage was down to around 0.06V (so practically in DD).
This circuit kicks the pants off using a resistor, and is quite cheap to build - about $4 in parts. The best part - it worked exactly as it should the first time!
I'll definitely be building a few more of these for the rest of my lights.
I used Doug Owen's world torch circuit as the starting point (an LM334-based current regulator). The only difference is that I swapped the 2n3906 transistor for a Zetex FZT968 (sot223 format). To make soldering easier, I broke off the collector pin on the one side, and used the tab as the collector connection. The sense resistor (the coil of wire) is 0.08 ohms.
The PCB is a 1" circle. I made the pads by cutting channels in the copper cladding using a dremel and a cut-off wheel (takes a steady hand).
With full input voltage, at 750mA, the minimum dropout voltage is very near 0.1V. With reduced input voltage (out of regulation), at 400mA, the dropout voltage was down to around 0.06V (so practically in DD).
This circuit kicks the pants off using a resistor, and is quite cheap to build - about $4 in parts. The best part - it worked exactly as it should the first time!
I'll definitely be building a few more of these for the rest of my lights.