[Mod] ThruNite TN-31, How to increase the current to the led

Hey guys!

Can someone be so kind to measure how much current is a stock ThruNite TN31 pulling from the battery carrier on highest level? I did a mod to increase the current to the led in my TN31 but in all the exitement to finish the mod, I forgot to take a current measurement of the stock light, I really don’t want to reverse the mod just to measure it.

BTW I soldered a 0.12 SMD Resistor in parallel and the brightness increased, according to the german forum, it should be delivering 4.65a to the led now from the stock @3.7a.

Thanks in advance for your help

Do you have instructions on how to take the reading for this type of carrier Alex ?

Sure! Pics in a moment

Thanks for the pics.

Standard TN31 with XM-L2 measure at 1.38 amps. Battery carrier no load measured 12.63v

Does that seem about right ?

I would think the TN31 on the brightest setting is burning at least 3 amps or more.

Thanks Spasmod! Your stock measurement seems about right, so there is an increase in current with the resistor mod, mine stabilized at around 1.68a which translates to about 4.5 - 4.6a to the led.

ILF its 1.38a x 3 18650’s = 4.14a minus driver efficiency thats how you get to 3.6 - 3.7a

The light does look brighter compared to my other lights than before the mod, not bad! Now I just need to get a XML2 u2 bin for it :bigsmile:

Ok that’s right.

I remember a stock Crelant 7G5 was right at 1.68amps. Times that by 2 3.36amps.

Nice one Alex, I’m very intrigued to know what you think of the heatsinking inside the TN31. I have heard it’s copper inside (perhaps a copper pill)

Would you say there is a fair bit of potential room for increasing the amps, both heatsink and driver wise ?

I hope you put up some beamshots :smiley:

I think the TN31 can safely handle more amps, it has a thick copper round star being held by 2 screws to the aluminum host with some kind of thermal grease or adhesive, Here is a pic, credit goes to TurboBB

when I get my hands on a good XM-L2 I will replace the thermal grease with some Arctic Silver Thermal compound.

So far the mod has been working great and the light does not heat up much, here is the german thread about the mod curtesy of google translate, those Germans sure know how to party! :beer: 8)

The skinny test leads and the resistance of the meter’s internal shunt will reduce the measured current somewhat but the difference may be negligible for this application.

Comparison beamshot

Left Modded TN-31 at around 4.6a to the led, right Modded ZY-T13 at around 3.2a to the led both are U2 Bin XM-L’s as you can see the TN-31 owns the poor modded ZY-T13 which in turn was brighter than the EagleTac XM-L I used to own.

Just did mine too. I didn't have any 0.12 so had to go with 4x 1R0 on top of the existing 2x R082.

image courtesy of taschenlampen-forum

There isn't much room so they had to be staggered to sit on top, not enough height clearance to place them piggybacked.

Calculating it back I now have 0.035 Ohm as opposed to the original 0.041 Ohm This has taken the battery carrier reading from 1.38A to 1.60A

Calculating this back again has taken it up from approx 3.6-3.7A to approx 4.2A at the emitter.

So looks like I've gained about half an amp, not a massive difference but there's a definite intensity improvement in the hotspot.

PS. The pic shows the TN30 driver but the TN31 driver is the same on this side.

The resistance on mine was lowered from 0.042 to 0.030 ohm, It was not a hard mod to do, and the results are noticeable, I plan to play a little with the reflector height over the led, I have a gut feeling that sanding down the thickness of the led centering ring will yield a smaller, more concentrated hotspot, I have some spare led centering rings to file down and try, will report results.

PS. will change the title of the thread to reflect results

Modded mine. Thanks for modding suggestion. Mine with 250mOhm already making TN31 a 19W flashlight. Yours are driven harder.

Alex - did you piggyback your 0.12R resistor on one of those 2 shown in the pic? Or just cut it in with wire, laying it along side somewhere. Spasmod is saying there's not enough height space.

I piggy backed it on top log the existing one, I later bought a R082 SMD resistor from Digikey and soldered it instead of the R120, for a little more current to the led.

The digikey part number for the 1206 0.082 ohm SMD resistor I used is P.082AVCT-ND which is 0.75mm tall and does fit with no issues.

Hi Tom, I should have made it clearer.

Since I used 4x SMD's (due to not having the right ones I had to improvise) it would have meant stacking them three high.

There's enough room to stack a single on top of the existing SMD just not enough to stack them 3 high. :)

The R120 seems to be getting up there already. Do you recall what amps the R082 produced?

Also, do you know if that stock copper star is a direct thermal path type? Didn't pull mine yet, but I'd use a SinkPAD if it's not.

I very lightly filed the center pad with a dremmel just enough to expose the copper, Stock PCB appears to not have a direct to copper path.

Here is a google translate of the German Light Forum where they talk extensively about the mod along with current measurements at the LED


Here is the detailed mods I did to mine

How to open the head up