Analogue ammeter for tailcap amps

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zipelgas's picture
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Analogue ammeter for tailcap amps

I’ve been using a simple analogue amp-meter for tailcap amp measurements and I thought I’d share a few pictures of the overcomplicated high tech measuring setup Wink
amps.jpg IMG_20160106_215324.jpg IMG_20160106_215558.jpg

I have found it very easy to use and since there are no wires (no resistance) involved, it should give quite accurate measurements.

Every flashlight has different springs and the battery sits in different position inside the battery tube. So the small bent metal thingy is reversible and if needed a little brute force can be applied to reshape it. But it can measure most of my lights with this setup.
IMG_20160109_143640.jpg IMG_20160109_143349.jpg IMG_20160109_143520.jpg

But I’m always wondering how accurate this setup really is? I believe the markings on the right corner of my meter should indicate the accuracy class. So I would appreciate if someone with better understanding could translate what that 2.5 exactly means.

Thanks for reading and let me know what you think of this setup!

Overkill is just about enough!

Edited by: zipelgas on 01/09/2016 - 12:34
luminarium iaculator
luminarium iaculator's picture
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Very nice setup. I think we did not seen this setup yet on BLF. From B158 mod thread you got 4.5A result with 25R while I got 4.4 on HG2 and clamp meter. This indicates that your unit could be accurate.

I am also interest what will more experienced guys say about such setup.


Modding is making something how you want it to be, not how it comes stock...


mattheww's picture
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Location: SW Pennsylvania

I cannot comment on the specifics accuracy of your meter. However the general rule is that in an analogue meter, 5% is about the best you are going to get. There
are a lot of mechanical parts involved. Springs have temperature coefficients, magnets have curie temperatures and the mechanical accuracy of the markers on the dial all play a role. All up it is very hard to get the system accuracy better than about 5% over a temperature range of 10-30C. The bigger challenge with analog ammeters is keeping the impedance of the meter very low so as to minimally impact the readings.

If you operating at 4volts, and the impedance of the meter is 20 milliohms, each amp flowing through the meter will result in a voltage drop of .02 volts. So at 4 volts 3 amps, there is .06 volt drop in the meter itself or about 1.5%. As the voltages rise, the internal voltage drop becomes less of an issue. That same 4 amps at 12 volts will result in the same .06 volt drop in the meter, but that now represents only .5%. The best way to measure current at low voltages
is with a clamp on meter (those with DC capability are a tad pricey), but they have the advantage that they add NO resistance to the circuit because they are not in the circuit. They measure the magnetic field produced by the current!

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This set-up offers a short, direct, large current path which is easily used consistently. If you read the meter well (an art in itself) it will probably get you as close to the truth as your DMM to the tenths and close enough past that. Beyond that lies the variability of components in the light, the inherent accuracy of the testing equipment, the consistency in it’s use, and the correctness of the testing method. For amperage this is probably good enough for all but the few who tweak to the extremes. Your ‘loop wire’ with your clamp meter introduces a variable as does heavy leads instead of the thin ones supplied with your DMM; indeed any practical test method does that to some degree . The most accurate way I can envision to test amperage under actual use conditions would be to section the flashlight body, removing the exact length of the distance between the lead sockets of your newly-calibrated DMM which would be attached to each half as directly as possible while keeping the light’s halfs aligned and held at the same length as it would have been uncut. Now you have a perfect reading but a useless flashlight because you cut it in half .

Yes I’m being facetious but the truth is that if the analog meter is a good one this is probably as good a way to get accurate repeatable amp measurements as most of us will ever need. Even with an inaccurate meter it will still show how much the gains or losses from your mods are which is what most folks are looking for. We’re lulled into a sense of perfection from those easy-to-read digits we see on a DMM display but the truth can be far from that, and if you look at the many different meters and methods everyone tests with here you’ll begin to understand that what probably matters most is that you see something close to what others report and that you get a close idea of what gains your mod work gives you.

Which would you prefer for amp readings, a $10-$15 meter like this that you know is pretty close to the truth or a cheap DMM that you know isn’t likely to be telling you the truth? Those who seek perfection need not apply here- nothing is going to do much better than your expensive DMM setup and this doesn’t claim to reach that level anyway. But it sure beats anything less provided you get a decent meter and I have yet to see anything that looks handier to use consistently so don’t bash this budget-minded solution unless you match it’s price too.


mattlward's picture
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I have noticed that some lights will not turn on when trying to read amps.  I have seen this mainly with FET drivers, but it is not all FET drivers either.  Does anyone know what causes this to happen?  Do I need to lower the resistance through the meter path?  I have 2 decent Fluke meters. 1 HF meter and 1 RS meter and some lights will not work with any of them...

EDC rotation:
Convoy S2+, 6*7135, XM-L2 3D, 10 degree TIR, PilotDog lighted tailcap.
Convoy S2+, H17F, XM-L2 4C, lighted tailcap
Zebralight SC52w-L2
Olight S1A
Olight S1R

DEL's picture
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mattlward wrote:

I have noticed that some lights will not turn on when trying to read amps.  I have seen this mainly with FET drivers, but it is not all FET drivers either.  Does anyone know what causes this to happen?

Maybe too obvious, but are the meter probes plugged into the correct meter sockets?
And are the meter fuses intact? (Most good meters have a dedicated fuse for the high current socket, for cheapies the PCB tracks will be the fuse.)

Edit : Reading your question again, it seems you only have problems with DD lights. So I guess the standard ‘short + thick probe leads’ recommendation applies.

zipelgas's picture
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Location: Estonia

Thanks for all the informative replies!
Nice to hear that there is still some love left for analog meters Smile
Since my el cheapo DMM needs replacement anyway, I’m hoping this GB for Ut210e goes live soon. If I receive it someday, I will report back with the numbers.

But I have to say, that I trust this setup way more than my current digital meter! My DMM gives me different readings almost every time. I have replaced the stock leads with very short and thick wires and it made a lot of difference (higher numbers), but I just can’t get a consistent reading out of it. I think the consistency of readings is the key factor for me. It doesn’t bother me even if the total numbers may be off by a few %

Overkill is just about enough!