2-Stage LTC3490 TX0J Arc Mod

I had an old Arc AAA laying around that I had not used since I started carrying my Arc LE so I decided to resurrect it. This is one of the older Arc AAA models with the rubber gasket instead of the O-ring. I wanted something with two brightness levels where one level had comparable runtime and output to the unmodifed Arc and the other would be much brighter. I did a little math based on the datasheet for the LTC3490 and determined the resistor values I would need to achieve the goal. It took a lot more work and time than I had expected but in the end I got what I wanted. I'll let the pictures do the talking.

I cut a 3/8 inch double sided copper clad board to build the positive contact for the two stage switch.


Back side of the board. Can you tell I chiseled the copper away? I didn't have any etching solution on hand and decide to rough it.


Tested the board for the right fit in the Arc head.


Cannibalized a spring from a cheapy Radio Shack battery holder.


Formed connections for the battery contacts and the CTRL/SHDN pin.


Added the spring to complete the two stage switch.


Started assembling the LTC3490 on top of the inductor.


Added the output capacitor.


Bird poop soldering job after struggling to solder the tiny 0402, 715K and 294K resistors.


TX0J Lux 111 mounted to the circuit with a tiny single sided circuit board acting as heat sink and electrical isolation for the emitter.




Filled in all the gaps with Arctic Alumina.



No one is happy without beamshots! Arc LE on the left. Modified Arc on the right running at 46 mA (low setting). It pretty much has no beam, it just a flood pattern. Camera was set to full manual settings to prevent washout and maintain consistency between shots.


Tighten the head a little more and it gets much brighter. Unfortunately, I only had an old 750 mA NiMH cell that had been out of the charger for a few weeks when took this shot but it's still much brighter than the Arc LE.


When I checked this morning using freshly charged 850 mA cell, it was a bit brighter. When I aimed it on the ceiling in a dark room it lit up the room to the same brightness as a Minimag mod using a Q binned Lux 1 driven at 400 mA.

The two stage switch basically connects the CTRL/SHDN pin directly to the positive terminal of the battery reach the 350 mA setting.

Damn, that is a glob of electronics! Cool job tho.

Where do you get the copper clad board from?

jsr said:

Damn, that is a glob of electronics! Cool job tho.

Where do you get the copper clad board from?

Click to expand...

I got the board from digikey, part number was PC44-ND. It's so thin that I could cut it with a regular pair of scissors. The boards I could pick up locally were too thick for me to work with.

Very nice mod. I'm impressed. What are you using for an inductor?

It's a Toko inductor with DigiKey part number TKS5513CT-ND.

Re: 2-Stage LTC3490 TX0J Arc Mod with runtime shots

I finally got around to doing a runtime test to verify whether the low setting on my two level mod will result in comparable runtime to the Arc LE I had. I fed both lights lights with 800 mAh NiMH AAA cells fresh off the charger and set my camera to automatically take pictures every 15 minutes. Initially, the total light output between the two lights were pretty close but the Arc had a tighter hotspot while the mod has slightly higher overall light output. As expected, the Arc's output dropped as the cell voltage dropped but the current regulation in the mod kept a constant brightness throughout the test. The mod was able to run for a solid 6 hours and 15 minutes before the low voltage cutoff was reach. The Arc was able to go on for another half hour before taking the dive. At 7 hours and 15 minutes it appeared that the cell in the mod recovered enough to fire up the circuit again and the brightness was still pretty darn close to what it had with a fresh cell.