# Current Measurement

How to measure the output current with the tailcap on?

You will more than likely have to measure the current at the led. This involves un soldering one of the wires of the led and putting your multimeter between the wire and the led pad.

Thanks! That was helpful. I will try when I get myself a decent soldering unit. Right now the tailcap reading (without the tailcap) is the only reading I could measure.

example:
PFlexpro 3.8A p60 drop in / solarforce l2p with a modified switch

Tail cap on: 1187 lumens
Tail cap replaced with copper wire: 1220 lumens
Tail cap replaced with Fluke meter: 1022 lumens

The interesting part:
using a lab power supply and adjusting the amperage to achieve the previous output readings at the set run time:

1187 lumens (tail cap on): 3.76A
1220 lumens (copper bridge): 3.82A
1022 lumens (through meter): 3.17A

In real life, I know I drawing 3.76 amps, but if I try to read it through the tail cap with a meter, I only show 3.17 amps…quite a difference.

thanks, pflexpro, that gives an insight into how different it can be.

I do sometimes current readings at the tail simply by clamping the loose tailcap into the measurement circuit, a bit fiddly but it works (I use solid copper leads for all wiring).

That was great to know. Many variables involved.

There are non-intrusive current meters that wrap around the wire instead of interrupting it, but there would have to be space for it around a one way current path. Might work on a test rig, but not on a working flashlight. They work by measuring the magnetic field, or the change in magnetic fiend when you switch off and on.

I may be wrong but I think these only work with AC current.

Correct, as there is no magnetic flux with a constant DC.
But I think more expensive ones can derive it from the magnetic field generated.
Not sure though…

DC current can be measured (non-intrusively) by using a hall effect sensor. Clamp meters that can measure DC have a hall effect sensor usually at the pivot point.

Here's mine measuring DC current.

I’ve always wondered (I don’t have a fancy clamp-on meter), can you not just clamp the meter around the body of the light [if you have a large enough meter]? Current is carried threw the body to the tail switch where the negative contact is.
Would that not work?

Just underneath the body is the positive, the cell, but anyway I can tell you from experience it does not work. With my clamp meter I can grab a max 22mm diameter.

You can’t put the meter around the whole light, because the current flows in both directions, so it would measure the difference which is zero.
I see that hall effect would measure DC, because it can measure a constant magnetic field. For a toroid to measure DC, one would have to switch the current off or on.

I wish you had not posted a picture of your meter…my clamp meter only reads DC to .1A…now I have to buy a new meter.