Tail cap amps - smoke!


I’m measuring tailcap amps of a Thrunite TC15 in turbo mode with a Uni-T 210E. I got smoke and the lens fogged up. My meter read around 9.5A before I broke the connection.

Any ideas why this would be - the torch is still working ok - but I’m only doing the job of the tailcap with the lead running through my clamp meter?

Just running turbo mode with the tailcap installed I don’t get smoke obviously.


maybe changed what some current sensor read, or a temp sensor…

Thanks for the reply. Not sure I understand - what would a sensor reading change cause smoke? And why would checking current change anything?

if it was the temperature sensor, and it falsely told the cpu ’ led temp is fine, give me more current! ’

And why would checking current change anything?—you would have to see the circuit to really know
plus know what they did with the inputs

maybe you shorted power and ground somewhere
you didn;t really say where it came from


Ok thanks. I didn’t short anything as far as I know. I’ve done this before with other torches and it seems to be common practice so trying to figure out how I could avoid this in future?

Do you reckon any damage done to torch? The smoke came from the head of the torch, and the inside of the lens fogged up.


It might be that there’s enough resistance in the tail cap spring to keep the current below smoking point, and the lower resistance from the wire puts it above smoking point.
Question is, where’s the smoke coming from?

(Edit: I realise you just posted “The smoke came from the head of the torch, and the inside of the lens fogged up.” as i was posting but i meant in a more specific way, i wouldn’t expect an LED to smoke though it may happen. I think it’s more likely some contamination on the LED/in the head, or maybe a component. What LED is it?)

Yes that makes sense - it’s the only thing I can think of since otherwise what I’m doing is the same as having the tail cap on. It happened quickly - but the smoke just appeared at the head of the torch.



i mean you have to try it again

maybe can you run it from a current-limited power supply?

set to maybe 10mA, then if that works, 100mA?

i just find that tail cap measurements are always sort of dangerous - close tolerances, nothing is fused, lithium battery can blow up, it may be in a confined tube [-> rocket mode], your meter in current mode is a short, if you happen to think ‘let me measure this voltage here’ OOPS


It’s a Thrunite TC15 with a CREE XHP35 LED.

wle - you mean use a PSU to power the torch? There’s no problem at the lower output levels it was only in turbo mode.

Ok, so 9.5A would be just over 3A @ 12V which is def pushing an XHP35, so could be the LED. I’m not familiar with how the XHP35 destructs.

There’s mention in this thread about them maybe ‘randomly combusting at 3A’ but that’s all i know and don’t know what that constitutes:

unknown circuits - always limit available power when powering up - yes

if you have a power supply that can set a current limit, do that

increase if things seem ok


Thanks both very helpful.

The smoke is basically telling you you have too much resistance and a conductor is melting.

We’re you able to find where the melted metal was coming from.

Specks of dust on the reflector or LED surfaces can turn to smoke at high output. Dust on an LED can damage the silicone or phosphor when it burns, which is why LED cleanliness is critical for high output flashlights. I think residual soldering residue / flux can create smoke as well, at a lower temperature than would melt the solder.

The tailcap bypass probably sent more current to the emitter than normal and something burned as a result.