nubie ? UF T20

Not shure where to post this, but I just bought a UF T20 with a red led for a hunting light. I like the light but when I turn it on high it is bright then in a couple seconds it dims quite a bit, very noticeable. The battery is a orbtronic 3400mah and it is reading 4v. Is this normal, I’m not shure what driver is in there the info said the led is a cree xre q5 and the modes are High, Low, and Strobe. I have looked around here some and found tons of good info, any help is appreciated.

Welcome to BLF!

My first thought is: bad heatsinking of the led while at the same time driven too hard by the driver. You could check if the ledboard is fixed onto the pill well enough, and if they bothered to add some thermal paste. If the led board is floating in air instead of clamped onto the pill (with or without thermal paste) you can expect the behaviour that you see.

You can also skip fixing your light and straight go to making it better by using a XP-E2 led on a copper board, with a driver that drives it at about 2A . You do need some solder skills for it or ask a BLF-member near you to do it for you.

There could be a short with the reflector touching the led or wires. Was it working when you bought the light?

Li-on batteries when fully charged read 4.2v and empty read 3.6v.

Poor heat sinking? The emitter overheating because of poor contact with the pill?

I'm not familar with the construction of the T20...

An XR-E isn't going to be drawing a lot of current from your cells...

If you have a multimeter, check the tailcap current of your light and see what it's doing when the light dims.

Is there any way you could post pics of your setup?

I think your led pcb is floating / not contacting the body. The fact that your emitter seem to be thermally saturated within 2s highly suggest this.

Either that or the emitter was poorly reflowed onto the MCPCB.

If it’s a red emitter, it’s not a CREE XRE. Cree never made a red XRE. I went through the same situation when I got an HS-802, and mine appeared to be a clone emitter. See this post :

As was mentioned earlier, a Cree xpe-2 on noctigon/sinkpad is what you want. The UF-T20s I have modded were between 1.4 and 1.75A, and performed well with decent runtime.

As far as your stated problem, it sounds like the emitter isn’t properly affixed to the pill as was mentioned earlier, or it could be a bad driver like I found in the HS-802.

If you want to disassemble, loosen ring around the lens. After lens is removed, you can get to the LED and pill, which is threaded as well. Take it apart and snap some pics for us.

Let me know if I can help.

The led board is attached to the pill with some white “glue” I’m assuming its thermal paste. I have a meter, how do I check the tailcap current?

Set your meter to DC Amps. Move the red lead to the Amps location. Remove tailcap, and put red lead on the end of the light body (the shiny part), black lead on the batt-. Light should come on. Read amperage.

Ok the tail cap readings are 167a on high, 35a on low, and 110 on strobe.The light does not seem to dim when its on with the meter and the #s don’t drop. when I put the cap back on it dims again, only on high. Working on pictures

Sounds like the Tail switch is flaking out at higher current.
Take it apart, braid the spring, clean everything and re assemble.

Please explain, braid the spring, I did take the tail switch apart and cleaned the surfaces. no change yet

Sorry for not explaining,
What I meant was solder braid the tail cap spring. It is actually called de-soldering braid and can be purchased at Radio Shack and a lot of other places. You solder it on the inside of the spring at the top and bottom, it creates a short cut/least resistance path for the electrical current and usually improves output.

But even with out braiding the tail cap, it should run on High. It sounds like you have a defective tail cap switch.
If you have another light that will thread on to the body, you could try that, otherwise you could always short the tube with a piece of bare copper wire to get it to run and take the wire out when you are done. Would work as a Redneck temporary solution.

Sorry I do not have better solution, but from the info you have given it appears to be a switch issue.

I cant figure out how to post pictures. also I’ve ruled out the tail switch, I tried the redneck solution, still dims. So its probably bad heatsinking. I would like to go the new led and driver route like mentioned by djozz, do I put them on my pill or make a whole new “drop in” setup. I would also like to try to fix mine to learn more.

When you swap out your emitter make sure it’s on either a noctigon or a sinkpad. Then make sure you use some good thermal paste or grease. This will take care of your thermal issues. Nothing you can do will make this light have zero thermal sag, there simply isn’t enough mass to do that. The XP-E2 with around a 2amp driver is a good solution. Fasttech will have a 2amp driver.

I prefer going another rout and that is an XP-G2 de-domed with a 5 amp driver. I have done about a dozen of these so far and for me I prefer the extra lumens that the XP-G2 puts out and by de-doming it this brings the die size down but of course there is no red XP-G2.