Amp measurements with my DMM, are they correct?

[thijsco19]

Not so long ago I purchased a UNI-T UT61E multimeter which supposed to be a decent or good multimeter.
But when measuring the amps on one of my flashlights, it looks like that the readings are off.

I have here a convoy C8 with a Qlite driver that has around 10 or 12 7135’s (dont remember the exact number anymore :P).
So it should do somwhere around 4A. The multimeter shows less than 3A, somewhere around 2.5A. That is with fully charged and newly bought Sanyo NCR18650GA.
I know these are tailcap current readings but they should be the same, or at least very close, to the led current.

Also on an other C8 with the FET + 7135 driver on high mode I only get slightly above 3A, also with fully charged Sanyo NCR18650GA’s.

This doesn’t sound right to me?
Would my multimeter be bad or is there something different going on?

[pilotdog68]

Most likely the meter is technically correct in its reading. However, the resistance in the leads and meter itself is causing the current to be lower than it would be without the meter in the circuit. This is true even of the highest quality meters.

Upgrading the leads to some thick, short wires should alleviate that issue up to around 10amps (depending on your meter’s rating)

[thijsco19]

Thnx, sounds logical!

I have some of these (https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=36264) laying around and some nice thick wire, should this do the trick?

[pilotdog68]

As long as those fit your meter, they should do great. I used about 8 inches of 12awg silicone wire so it’s still very flexible.

[thijsco19]

They are a perfect fit :). I used approximately 20cm long 14awg wire.
And now its measuring at lot better, I get slightly above 3.5A on the Qlite driver light (still on the low side?). And 5.5A on the FET light :D. (even a peak of 5.7A!)

BTW, I’m looking to use shunt resistors to measure currents. What would be good resistors to do this?

[ohaya]

@thijsco19

They are a perfect fit :). I used approximately 20cm long 14awg wire.
And now its measuring at lot better, I get slightly above 3.5A on the Qlite driver light (still on the low side?). And 5.5A on the FET light :D. (even a peak of 5.7A!)

BTW, I’m looking to use shunt resistors to measure currents. What would be good resistors to do this?

I always have to remind myself how surprised I am about how much difference the cables/leads (both gauge and length and also connector) make when measuring tailcap current…

[mapache]

Speaking of DMMs and tail cap readings, has anyone compiled a list of flashlights and they’re usual amp range?

[ImA4Wheelr]

If you have a desktop power supply or other way to output a reliable current, you could make an external shunt as discussed in this thread. Really makes a difference.

How to read 10A, 15A, 20A and beyond with a cheap meter

How to read 10A, 15A, 20A and beyond with a cheap meter 08/15/2015 I added an addendum to the bottom of this post with detailed instructions on how to build the shunt resistor. And here is a link to my post #28 in this thread that I hope explains things more clearly . . Up until recently, I have never had to measure very high levels of current. Any measuring I have done were for single emitter lights that had current draws of no more than 5A. For that, I had used a DMM set to the 10A scal…

[thijsco19]

I dont have a desktop power supply or any other way to output a reliable current. (or is a led driver considered as a constant current power supply? Such as these 7135 chips)

But I think I’ll look for a 0.01 ohm resistor and a 0.047 ohm resistor. With these I should be able to measure currents from 10A all the way down to 0.5A with a max voltage loss of 0.1 or less, according to my calculations.