Keep in mind, your situation is quite rare, i’m talking generally here, you have the real need, but not all does 
That’s why i said if you don’t have the real need, serious rl scenarios, its not a must. Cheers
If your TK75 is running 4,68A at each emitter. Assuming that is what you mean. You are looking at 14 amps divided on 3 emitters. That is a whole different heat level than the 3A or more from a modded TK61.
Why would I not want to run a 1600lm 500kcd to run for 15 minutes? Or at least know its up to the task. Why would you want to dim down all that glorious throw if you have the runtime for it? :D
If im up on the mountains when its dark its useful to have good range. And I can use all the range I can get. Same if you are in a boat. You can never have enough light or range from a handheld flashlight. I even enjoy being able to light up mountainsides when Im at trails beside them.
Even just for fun I still enjoy pushing limits and lighting up things in the distance. Why have 1-2 minutes of fun when you can keep going longer...
Doing some extreme testing indoors is useful to know if it will handle the heat. I never post anything about a resistor mod unless it seems to be somewhat reliable in a bit of extreme testing. When that is said. I cant guarantee anything about reliability really. But I can do tests that can help others to get a better picture of what the light is capable of or how it acts.
Like Dale said, I actually expect my flashlights to handle some work if its put to the task.
What is the point of buying a Ferrari, get in tuned, and then barely use it above 4000 rpm and never run it faster than 50 miles an hour or slower? I like to be able to use certain things. Id like to know something is up for the task. Any task its supposed to handle, and if its not. I will improve it. That is what modding is all about to me. Improving lights. If I have made an unreliable light, then its not a good mod. Id rather find the limits of a light indoors, than outdoors when its actually needed...
Pushing a light well beyond 100 Celcious might be a bit over the top. I tried to make an omelett on my 9 XM-L light btw. 0:) Maybe I made a multi purpose camping light. You know, cooking stove and flashlight all in one. :D As long as emitters are mostly below 150C and as long as the batteries are below 60C, and as long as driver circuits can be replaced, I don't have issues testing limits. I even enjoy it.
Need at least 3000 for a walk.. 1600 lumen is so 2013.. lol
dazed1, not rare at all for the 20 million people living in Texas. Just sayin…
Hahahah you made me laugh for sure with the 2013 part 
Anyway, i guess i’m like this because i use 95% of the time my lights for urban areas i live in the capitol city 
BTW i never said i don’t use my lights on turbo, i’m just more for different turbo usage, turbo few mins > high > low, then after 10 seconds again turbo and so on…
So yea, its similar runtime, just not constant. BTW about the amps, not really sure how that works, but i guess its ~ 4.6/4.7A per led? i’m confused by all of this driver (regulation)>cells>parallel>series things, so i’m not sure but i think you are right, its 4.7A per led, resulting in 14A….
Also one more thing, i heard that when volts drop, the amps rise, but is this drain from cell only? i’m guessing the amps to the leds never increase more then 4.7A right? sorry still just a noob
If it can't hold up to 10 minutes turbo, it's no good. This is a $200 (modded) super thrower and it should be able to run until it gets too hot to hold or until the cells die.
No reasons to test before. Its built to handle 4 hours and 20 minutes on Turbo when stock. Or about twice that time if you have the battery extension kit of whatever its called. Basically its built to handle full output all the time. Like pretty much all lights...
I did two runtime tests after resistor mod.
I hope to give you some exact numbers regarding the modified TK75 in the distant future. ;)
Yupp.. Thats pretty much what I tried to say when I wrote that wall of text above. :D
On a side note. I bought my light from WB. TK61 is now back it stock from BG and slightly lower price than earlier. A tad cheaper than WB after coupon code are used in both stores it seems.
So let me get this straight.
You get the light to 4.8A from 3A at stock, with only resistor swap right, nothing else added?
And you did 2x runtimes ~ 30 mins each on turbo (one of them indoors you said?) without any issues at all? that’s totally impressive, knowing that it was suggested to me to not go over 4.2A for safety reasons (and even that can be dangerous)
Anyway, just to be on safe side i will wait for RMM and maybe your findings about new transistor (s) you guys have any idea of what can be used for reference? thanks.
Yupp. As seen in OP.. More like 4,7x amps to be precise with one R030.
Pretty impressive huh.. Not so useless info to know about afterall. Although, that does not mean its not dangerous for the driver. :p I just means it went fine when I did it. :D Its worth to mention again that I did it without any thermal pads or heat sinking done to the driver. So with thermal pad + normal outdoor use you are probably much safer.
(btw, both tests were indoors. First were around 15-20 minutes or so. I did not time it.. Turned off the light when it felt "hot enough".)
I know some details about the transistor/MOSFETs mods that vinh is doing.
Ill let RMM continue with his MOSFET testing, and let him do the talking. Technically its beyond me...
Thanks for the great answer
I understand that every part is different, and that even on stock can be burned, that’s why are electronics, but still you get very good results.
BTW, assuming on stock TK75 gets to 2850lm, (Fenix is spot on with ANSI) with this mod, 3.0A >4.6/4.7A, the 4400 lumens should be led lumens, and OTF should be around 4300? or driver efficiency comes into play additionally compared to stock?
Its probably around 4500 LED lumen at that current.
Differences in LED Lumen and OTF are the losses you get in reflector, lens, and possible things that blocks light (Bezel, locating ring and such). I don't know exactly, but I would probably expect at least 10-15% lower OTF numbers. Fenix does have very good design, reflectors and AR lenses. So maybe its as high as 4000 lumen OTF?... oh well. Lets try to keep to TK61 in this thread. :)
Hmm 4000 is a bit on a low side, because Fenix get like 950 lumens per led on stock (it was measured)
The loses from lens/reflector should be the same going from ~ 2850 >4000+ lumens or even if a bit more, still it should be around 150 lumens per led loses compared to ~ 80lm at stock and this is big downgrade i estimate, it should be less.
It should be around 4200 imho…knowing that at 4.7 the leds should output around 1500 lumen x3 = 4500 - 200 we are around 4300/4200 at worst imho (with loses/efficiency)
Sorry for offtopic!
If you say stock emitter current is 3A that means about 1120 lumen at emitter based on graph above. I dont know if its 3A stock. But you said so. You said its measured at 950 OTF too right? That is about 15% lower than 1120 lumen.
Graph shows about 1500 lumen at 4,6A. Which means 4500 lumen. Minus 15% equals about 3825 according to the graph above. I believe that match`s numbers (graph you used) were emitter numbers? I cant remember if it was T6 or U2 bin.
You have to take into account that these numbers are just different peoples numbers. They may not always be 100% comparable, or 100% accurate. Unless you measure it with a calibrated equipment we will always be doing guesstimates. People many not even have the same emitter temperature when doing these tests, which also changes things due to heat sag. What Im saying is. Some might say 4000. Some might say 4200. Some might say lower, some might say higher. If I use Djozz charts (which I believe similated OTF but without lens), we would probably be looking at around 4400 OTF with 3x XM-L2 U2 at 4,6A.
Also, on certain Fenix light, some here at BLF measure quite similar numbers, some measures higher with well above 10%.
Whatever you actually are seeing, you wont see the difference between 4000 OTF or 4400OTF anyway. It could be just the same, just like these guesstimates we are doing based on different peoples data. If you say 4200, then Im not going to argue with you against it. Your guesstimate based on available info seems to be as good as mine. When that is said. Relic38 had a giveaway where people guessed OTF numbers on a light based on given spec. You can see how large the differences were. I averaged the closest guesses I believe. This obviously makes me king of guesstimates. :p
Oh well. Enough Off topic. Start your own thread if you want to discuss something not TK61 related. :D
(Topic? "Modded TK75 lumen guesstimates based on assumed emitter current - A non TK61 mod thread")
Mine showed about 2.85A to the emitter, stock, on my meter. I have mine up to around 5.8A but haven't had time to put it back together yet, too busy building other stuff!
Richard, just throw it in a box and send it to me, I’ll put it back together and do some, ahmmmm, testing. Hey, no problem! That’s what friends are for! 
3 minutes on turbo and counting...fingers crossed.
For maximum output I think the way to go is an FET driver, or four Knuckleheads running in unison. 
As a point of reference, my stock light measured 1200 lumens in my sphere, the light at 5.8A with the stock emitter shows 1900.
Fair enough buddy, i will just addd that i’m not sure that the loses are proportional with current bump….imho you wont lose more lumens, if you use the same flashlight, if the loses are already 10% on stock, making the light ~ 40% brighter, wont produce additional 10% lose (but i could be wrong)
On,
WOW 1900 lumens @5.8A? thats sickkkk. I pray for your light lol
Ok, it made it 11 minutes and is still working. Body was very hot around the driver and pill, but it survived! I fried four emitters playing with this driver, but it turns out that most of my tinkering was likely unnecessary. I played around with every MOSFET on the board, but the only one I ended up leaving changed is the "C00C" MOSFET next to Q2 (I don't recall the number itself.)
I removed the stock "C00C" MOSFET and replaced it with four piggybacked IRLML2502 MOSFETs. With only one or two it was too hot to touch in a matter of seconds, with four it will get too hot to touch within about 30 seconds, but it's a big improvement and it's holding for now.
Here's what I have stacked on top of the stock sense resistors (I kept on adding until I got to 5.8A, so you can use different values to get the same effective resistance.)
(2) 100 ohm (1 on each bank for all)
(2) 75 ohm
(2) 25 ohm
The flyback diode is also getting pretty warm, but not too hot to touch. I will run it for a while longer tonight and see if it holds up.
I still think that (4) Knucklehead drivers running at 2A each would be pretty awesome...8A at the emitter, that's what I'm talking about. 
Will it work? As soon as my coils come in I'm going to try some running in parallel with 1 MCU to rule them all!
Oh my oh my, thats some HARDCORE PUSHING! impressive :party:
Very nice info and thanks for sharing.
8amps would be simply incredible. I would love to see that happen! I have to think it’s possible as long as it’s really well regulated and there are Zero spikes.
I have to say I’m pretty happy with mine after adding the R025 but then again, who doesn’t want more?