Fenix TK40

[soren-hamburgo]

Hi there,


my friend and I are trying to fix an old TK40 after bad cells spilled inside the barrel and ruined the battery cage.

It was an expensive and brilliant light, hence the effort. We were able to clean everything and put things together and now step by step check all system parts.


Using an external power source we could check the head and tail switch. The only trouble is currently the battery carrier.

(There was a good discussion here about various sources of trouble with the TK40: https://www.candlepowerforums.com/vb...53#post3306753)

As you all know, there are both plus and minus on the top and here we get 5,6V. We also checked the power of each "slot" of the battery cage with two AA each. All four slots have 2,6V. Yet the lamp will not work.


We then tried putting some load into the system, a tiny circuitboard with some LEDs on it incidently floating around on our workbench. What happend was that the TK40 battery pack dropped in voltage down to 2,5 V. No wonder the LED would not work.


Before you ask: the load is ok, tried it with 3AA, worked fine, no voltage drop.


So what could be the problem here? Normally, one would assume there is some break or bad spot in the wiring, normally a break of the cable, in this case of the contacts? It is just a guess.


Any help appreciated. Thanks!


Soren

 

[greenpondmike]

Hi. I just want to acknowledge your post although I can't give you any advice because I don't know what to tell you. I could be wrong, but surely there's someone on here that could give you advice on this. I hope you find your answer to this problem and hopefully it isn't a ruined unfixable part due to battery leakage. My advice is don't easily give up on it. I read you review on that other fenix flashlight and you did a very professional review on it with great and practical beamshots. It seems that even though I don't own that light I know it better than I did my wife when I married her.
Best regards....Mike

 

[soren-hamburgo]

Thanks, Mike, for your kind words, you made my day! :thumbsup:

Hi. I just want to acknowledge your post although I can't give you any advice because I don't know what to tell you. I could be wrong, but surely there's someone on here that could give you advice on this. I hope you find your answer to this problem and hopefully it isn't a ruined unfixable part due to battery leakage. My advice is don't easily give up on it. I read you review on that other fenix flashlight and you did a very professional review on it with great and practical beamshots. It seems that even though I don't own that light I know it better than I did my wife when I married her.
Best regards....Mike

 

[greenpondmike]

You're welcome

 

[thermal guy]

All the voltage readings your getting sounds good. Only thing I can guess is that the carrier is shot and when you apply pressure to it when installed it's losing contact somewhere.

 

[soren-hamburgo]

Cool, thanks for the feedback. What puzzles me is that I read on the forum (sorry, cant find that post right now) that the voltage range of the LED is 3.5-6.5 or something like that which is (sadly) why you cannot easily convert it to LiO-batteries (as this will go up to 8V if fully charged). Now Fenix replies to me (to me asking for a replacement battery holder, sadly out of stock for years, if anyone else interested), that the LED-voltage range is 3.1V-3.7V.
This might be a noob-question: but how is this possible if 4 cells are connected in series? This will result in the aforementioned 5.6V?

 

[WalkIntoTheLight]

Cool, thanks for the feedback. What puzzles me is that I read on the forum (sorry, cant find that post right now) that the voltage range of the LED is 3.5-6.5 or something like that which is (sadly) why you cannot easily convert it to LiO-batteries (as this will go up to 8V if fully charged). Now Fenix replies to me (to me asking for a replacement battery holder, sadly out of stock for years, if anyone else interested), that the LED-voltage range is 3.1V-3.7V.
This might be a noob-question: but how is this possible if 4 cells are connected in series? This will result in the aforementioned 5.6V?

Voltage range at the LED is actually around 2.8v (for a dimly lit LED) to something like 3.3v (for a brightly lit LED). Depends on the actual LED, but the range for white LEDs is usually around those levels.

However, the flashlight likely uses a buck-driver (or perhaps even a more simple linear driver, though unlikely), to reduce the voltage from the battery to the working voltage of the LED. The battery will have to provide at least a voltage of 2.8v (or probably higher due to other circuits) for the LED to glow. But it can provide more, since the flashlight circuit will just convert it to 3.3v (or whatever is needed by the LED).

For a 4-slot battery compartment, all the cells will be in series. So, for NiMH cells, you'll get a voltage up to 5.6v (assuming all cells are fully charged and at 1.4v each). For alkalines, you'll get up to 6.4v for new cells. Though alkalines drop their voltage horribly under high loads, so the battery will likely provide less than 5.6v.

Sounds like there's some damaged contact in your battery compartment. High loads spike the resistance there, and the available voltage drops. Can you just ignore the provided battery carrier, and just wire them up in series with another more simple carrier?

 

[soren-hamburgo]

Thanks for the explanation, (just out of interest without wanting to boat this thread: is the buck-driver some kind of step-down, I have these step-up, step-down circuits ready made from the internet and use them to adjust voltages).
I aim is to first spent just a little more time on fixing this damn thing and then go and have something developed on 3d-print. Using your info this new insert could be a simple tube with a LiO-battery inside, I guess, with 3.7V this might be worth a shot...in contrast to having something complicated with multiple AA...

 

[WalkIntoTheLight]

Thanks for the explanation, (just out of interest without wanting to boat this thread: is the buck-driver some kind of step-down, I have these step-up, step-down circuits ready made from the internet and use them to adjust voltages).

Kind of, but they're a bit more complicated than a simple step-down.

I aim is to first spent just a little more time on fixing this damn thing and then go and have something developed on 3d-print. Using your info this new insert could be a simple tube with a LiO-battery inside, I guess, with 3.7V this might be worth a shot...in contrast to having something complicated with multiple AA...

I don't think it will work with a 3.7v cell. It's expecting something in the 5v - 6v range (at least, that's what I'm assuming based on your description, and not really double that). But, it's worth a shot. You might get some light out of it, though unlikely max output. But perhaps it has a good constant-current driver, and can actually work fine with 3.7v. Definitely worth a shot.