flashlight AMPS - help!!

Hi guys, I’m a newbie here, but kinda getting addicted to all this (already bought 5 flashlights over a few months :confused: ). So I’m trying to get into some modding and would like to know more about the physics of LED flashlights. I knew how to check the amps at a tail cap with batteries in series, but then I bought a Skyray King and could not figure out how to do that (try to search on here, found nothing useful). After messing around with the light for a bit I ended up getting a measurement. I connected negative lead to one of the screws holding the board behind the tail cap (fully assembled), and the positive lead directly to the driver’s positive wire (going to the LEDs) solder contact (with the head unscrewed). I got a reading of 4.5A. Then I did it again the same way with another flashlight which is a knock-off of Trustfire TR3T6, and got 7A. Can someone please explain to me if that’s the current from the drivers that is being fed directly to the LEDs? All feedback appreciated!

LED+ is connected directly to battery+, so you you have created a direct short on the cells.

If you are using the leads of the meter, you have to put the leads in as part of the circuit, which of course will alter (lower) the actual reading when the light is assembled again without the meter in circuit.

Using a clamp meter is the way to go - a lot of members have recently got one of the UNI-T UT210E’s

I see… so then where would I have to put the positive lead of the meter on SRK to do it correctly? Thanks.

You will have to put the meter somewhere in the working circuit.

In a tube light with tailcap, it's easy to do by putting one lead to the BAT- & the other lead to the body(in place of the tail switch), but in the SRK you will have to either improvise by removing the rear contact plate & joining your meter between it & the body, or de-soldering one of the leads going to the emitter plate & putting your meter in the circuit there.

Oh, Welcome to BLF! :beer:

thanks FmC! I finally got to take a reading at the tail cap like you explained, it was 1A on high, which seems to be pretty low since my SRK gets really hot quick . So would that be the same current at the emitters? If so, is it devided by the the # of LEDs or each one gets 1A?

seems you got luck with a bad DMM which blocks the Amps !

if you connect like that (LED+ to ground) onflashlight you create a short circuit !!

some cells will not like that and can explode in worst case

(good DMMs have near to no resistance)

Yes, that reading is divided between the emitters.

If you are using the standard leads that came with the meter, they will be adding a lot of resistance into the circuit, so you will be getting a lower reading.

Also, you may have damaged the traces inside the meter by shorting out the cells earlier.

Check out this video by member Djozz comparing different methods of current readings, & you can see how much difference the leads alone make.

Yea I see what you’re saying… makes a huge difference. There’s something though that I can’t fully understand. When I checked the amps by “shorting” the curcuit, I got 7A on the TF, and 4A on the SRK. When I checked it at the tail cap, I got 3.5A on TF, and 1A on SRK. TF has 2 batteries, and SRK has 4, so if you multiply # of batteries by amps at the tail cap you would get the total amps acquired by the other method. Is that a coincidence or is there an explanation for that?

The reading from the shorted cells has no correlation to the driver output, other than the driver output on the highest mode may be close to the amount that the cells are capable of.

If you were using a high discharge cell such as a Samsung 30Q, you would have seen much higher readings when you shorted the cells, but would still see a similar reading from the driver when measuring within the circuit.

This applies to current-regulated drivers, which is the type likely to be in your Trustfire TR3T6.

I see. It all makes sense now. Thanks a bunch for taking your time to explain all of this to me. Hopefully my future posts will be more entertaining))

Hey, if you had shorted 4*30Q's, then your first post would have been very entertaining!

a burning DMM :smiling_imp:

be careful with high power LithiumIon cells :wink:
here is a lot to gain in knowledge - and we are easy to entertain :smiley:

haha!! I’m glad that didn’t happen, I felt the wires of my DMM getting super hot, so I only shorted it for a sec or two. But everything seems to be still working fine, so I hope no major damage done, especially to the batteries cause I just bought those (TF 2400mah)

Can’t figure out how to upload the images, but here are the links to my SRK driver (2 pics of the same driver) -


Guys, let me know if there’s a way to bump up the amps on it - it’s only producing 1A on high as is…

if TF means TrustFire - you maybe better discard them…

Not sure… you could start by bridging one of the spare pads at R3, & replace the wires going to the emitters with heavier gauge wire.
It doesn’t look like that driver could handle much current, though.

Your cells are eventually going to be a limiting factor as well, if they are the cheap *fire stuff, plus once you start getting decent current to the led’s, you will have to move to a Direct Thermal Path board, to prevent them from overheating.

By that stage, you will be completely hooked, & probably be competing in this years scratch-build competition…… you have been warned!

Lol that’s the plan anyways.

When you say “bridge” you mean just run solder wire across? Is there anything I can add to the driver? That Q1 slot looks kinda empty.

The cells are actual 2400mah according to multiple reviews I’ve read. As far as the heat, it’s already an issue - it gets really hot after about 5 min on high.

Update: did everything you mentioned above (bridge, 18awg copper wires)… still clocks at 1A and shines the same way pretty much. So I guess I’ll have to use it as is.


I’ve been looking at diff. posts and a lot of folks are adding resistors in the spare slots to increase the current, and also jumping the “000” ones. So if you could kindly explain what all that means, and if it’s something I can possibly do to my driver. Other than that I don’t have any means to fabricate custom parts (pills and such) so there’s nothing else I would be able to do.

If the torch is getting hot pretty quick, as you said earlier, I'd bet it's drawing more than 1Amp.

Do a test; briefly jump a wire from the led - wire to the ground ring, & see how much the brightness increases. That would give you the maximum output that your cells/LED's are capable of.

With the resistor bank, usually there will be some resistance placed there in this type of driver to limit the current, so lowering the resistor value/bridging will usually increase current flow to the LED's. Your driver already had R000's there, so there was probably very little to gain by bridging, but worth a shot. The thin led wires are another form of resistance, but you have replaced those also.

It could be that the driver is maxed, & it looks to me that all of the current has to pass through the tiny little FET at Q3. I could be wrong, but that's how it looks to me.

If that's the case, your next option is to replace the driver with a one of the pre-made SRK drivers available from MTN, or if you are up for something slightly more difficult, you could 'piggyback' a less expensive (but still more capable) driver to your existing one, bypassing the existing driver completely.

Of course, by doing that, you are going to run into overheating issues, as I mentioned earlier.

Another thing to consider, is that you may have lower performing 'fake' LED's in the torch, which will also be a limiting factor.