Current measuring step-up converter and DSO

[NikWing]

hey all

No idea if this is the correct section to ask.

I want to measure the wattage of a high power LED which is driven by a step up converter

I use a current clamp set to 1:10 and a 1:10 probe, enabled math function of my DSO.
I show measurements as RMS values of each channel.

The boost converter is driven by a 160W PSU, 20VDC. Ground is common for output and input.
I also have a tRMS multimeter here to compare the values I see on the DSO.

What I want to do: adjust the LED's power level using the DSO

Alright. I don't understand this:
If I connect the LED and probe the current with the multimeter, I see around 1.1A on the display.
But if I measure the current with the current clamp I see like 600mA on the DSO.

So I tried to measure the current that flows into the step up converter.
It's the same on DSO and multimeter.
Also, for me, there's no visible difference between the current curve shapes.

Why is there a difference between multimeter and DSO in the LED branch, what's wrong here?

Thanks for enlightment and with best regards,
Nik

[Rick Law]

...If I connect the LED and probe the current with the multimeter, I see around 1.1A on the display.
But if I measure the current with the current clamp I see like 600mA on the DSO...
...

Did you factor in burden voltage?  How did you connect your DMM?

I think if you solder in a 0.1ohm shunt and measure the voltage, you may have a more consistent and more accurate result.

[NikWing]

well, in series xD
set to 10A

I mean, what's the difference between measuring before the step up converter and after?
(between PSU and convert compared to between converter and LED/ballast)

here are some values I measured:

current from PSU into converter:
current clamp: 2.2A
multimeter: 2.26A
voltage: 20.2V DSO, 20.1V multimeter

current from converter into LED:
current clamp: 620mA
multimeter: 1.14A
voltage: 35.3V DSO, 35.1V multimeter


edit: maybe the current clamp has some issues ... it looks like the problem's fault is a tiny air gap if it doesn't close correctly.
it's weird though ... I alternated measuring beween 2 cables and one cable was correct, the other wasn't, the suddenly it did measure correctly and I was like 'ehhh?'
and then it occured again. sucks D:

I really began to doubt >_<

[phil]

Keep in mind that most cheap(er) multimetes don't do well with anything thats not a perfect sine. Does your meter have trueRMS capability?
A LED driver won't have a clear sine output, it's more like a deformed square wave in most cases.
If you want to be absolutely sure about that measurement, find the current sense resistor on your LED driver, probe it with your scope and look at the waveform. If it isn't a sine, don't trust your multimeter. Instead, use the voltage you measured with your scope and calculate the current from there.


Phil

[BravoV]

If it isn't a sine, don't trust your multimeter.

To complement this, even its a pure sine wave, you have to see at what frequency, cause even true rms DMM has limitation on max frequency to measure it accurately.

[NikWing]

I also have a tRMS multimeter here to compare the values I see on the DSO.

yes, true RMS ^^
it's a Meterman multimeter, wasn't -that- cheap

and the step up converter isn't an LED driver, it uses an UC3843A, I don't see any square wave, just a few peaks on a DC voltage 

it's weird, I still believe it's a hard to see, tiny gap (current clamp not completely closed) that causes it sporadically
wouldn't the multimeter (if not tRMS and limited) show less current instead of more?