Hi all, I decided to create a K40 mod thread to share information on how to make the K40 into a wicked thrower. Since about 20 people bought it here, it feel like this would be relevant.
The K40 is very similar to the TN31 in terms of design, circuitry, and disassembly. Many have stated that Supbeam, despite its funny name, is an OEM company for Thrunite.
So here’s what I did:
- Dedomed successfully using Vinh’s hotplate + razor blade method
- Increased the current to 6.5A. I used the following resistors in parallel: 0.050, 0.050, and 0.082 Ohms. Shorting the the resistor pads gave a ridiculously high (>8A) current.
- Lapped the PCB to a mirror (sorta) finish.
- Changed the thermal paste to MX-4. (Overkill)
For how to open up the light, please refer to AlexGT’s tutorial: Here is how to mod the ThruNite TN-31 flashlight...
Some differences I would like to point out…
- The head is not glued. No need to boil the light or use strap wrenches. This was a big relief to me.
- I can’t get the driver out. After I’ve removed the contact plate, the driver seems to be stuck. Here’s how ImA4Wheelr did it:
Here’s some tips to get you started:
Lapping:
Some say lapping the PCB will improve thermal conductivity because the PCB makes better contact with the base. I started off with 600 grit sand paper and rubbed it 30 times each on a 90 degree rotation. I then moved on to 1000 grit, 1500 grit, and 2000 grit sandpaper and followed the same procedure.
It should look like this:
I then used MX-4 thermal compound to replace the stock white one. Some spread it out on the PCB, but I like to put a pea sized blob on the plate and set the PCB down and press evenly. This avoids microscopic air gaps.
Reference:
Current tweeking:
The two sense resistors are in parallel: 0.100 and 0.082 Ohms, adding up to 0.045055 Ohms. The stock current was a about 3.455A, so according to Ohm’s law, the voltage is 0.155665025 volts. (Pardon my significant figures.)
You can mix and match resistors to your apetite but keep in mind that they should be closely matched so that the current (and heat) would be spread between them. They do get hot at high currents.
Anyhow, here’s the catch. I found a discrepancy when I was using Ohm’s law to predict the output current. The driver wasn’t perfect, and the output current would vary by the forward voltage. For example, when I shorted the output with my DMM, I read an excess of 8A. When I used a couple 1N5408 diodes, the current would vary by the amount of diodes.
So what I did was used 5 of them to match the forward voltage of an XM-L2 at 5-6A. I them tweeked the resistors accordingly to set the current at 6.5A. I used a 16ga wire across the resistors at first, but that was supposedly too think and the current was to large.
Here’s what I have now:
Dedoming:
This may or may not be the hardest part, depening on your experience.
I haven’t tried the gasoline yet, but here’s “the guide” to gasoline dedoming: A Perfect Dedome?
I was too impatient and uses Vinh’s method of warming up the LED and then swiping it off:
Thankfully, it worked really well.
I’ll try to answer any questions, so please don’t hesitate to ask.
If you can sharing your mods as well, that would be great!
Beamshots, here we go: (Oh and the bridge is 975m across.)
I first would like to apologize to the couple. :party:
Swinging the light around:
I used two exposure settings, 1.0 sec/f3.5/ISO200 for the 6th and 7th photos, and 2.0 sec/f5.6/ISO200 for the rest.
More beamshots: