MultiMeter question...

LED Flashlights – General Info
1

I am really enjoying this Fluke 179 these past 3-4 weeks, its just a really well made tool. All the switches, buttons and plastic parts just feel much more high end than the older meter it recently replaced. So I was measuring some tailcap currents with different 18650 cells and I accidentally had it set to AC-Hz.

When measuring the current in this mode, I think it "sees" the lights PWM as a square AC wave... and measures the Hz of that wave form... pics below.

The question I have for the more advanced members is why do you measure PWM and strobe frequency on an oscilloscope? Is my DMM measurement below an accurate reading? FWIW this light has an extremely annoying PWM flicker that I can easily see, so it appears to match the ~133Hz readings on my meter.

thanks!!!

HI = 0Hz

MED = 133.4 Hz

LO = 133.9 Hz

STROBE = 8.56Hz

2

... because most multimeters just don't have that funtion :)

And with an oszilloscope you can measure duty cycle, too. Well, then again, some multimeters can do that also.

3

I did the same with the Vichy VC99: https://budgetlightforum.com/t/-/1844 (I measured it parallel to the battery, however, maybe I should've used your much simpler method...)

4

I've just tried to measure PWM through current measurement in the VC99, but it didn't work. Why does it only work parallel to the battery? Dunno.

5

With a multimeter you do not really know what you are measuring, it might be the pwm frequency or it might be double up or something in between. The DMM is best at measuring a clean frequency, but the driver might not always do that.

Here is a example (It is measured in the light, not the tailcap current):

A DMM might or might not count the two spikes, with a scope you can easily avoid the spikes.

Remember to use the DC setting when measuring tail cap current!

6

for our records, i tried the DMM thing with Tank E09 flashlight (known for low PWM frequency) and UT61E multimeter. with either setting (AC vs. DC) the DMM didnt measure any "Hz" or "%". i was measuring tailcap amperage and the light did flash though in all settings ("Hz", "%", "A")

oh well.

7

Why?... am I damaging my meter by measuring AC-Hz across a DC circuit?

Thanks gentlemen for the explanation(s)!!!

I learned something new today, and that makes it a GOOD day indeed!

8

No, but the RMS circuit will give you a wrong reading.

9

To add to the above I tried a couple of DMM's on a light:

The difference in DC current is probably to different voltage drop in the DMM's.

A scope trace of the light looks like this:

10

Thanks for sharing!! I got the same result ("0", "UL") on my UT61E..

:)

11

Thanks to this thread I wont be buying another multimeter just to be able to measure lights PWM with square function ;)!