Measuring current to flashing LED displays?

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grizedale

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Hello,

We are using an SMPS to supply current to a 24W LED display.

The vin = 24V

The leds flash on and off 400ms on and 300ms off.

There is minimal input capacitance so the 20KHz switching frequency is also pulled from the input voltage supply.

How can we measure this pulsating current accurately?

We only have a TDS210 oscillscope, and also a fluke 179 DVM.

tHE tds210 DOES not allow us to average a current sense resisotr voltage, so is of little use to us

**broken link removed**
 

Sounds easy.. surely just need a current sense resistor and stick your oscilloscope on it to measure the voltage across it? Or have I missed something?!
Alternatively, as far as I recall from an old experiment, Fluke 179s can measure voltage RMS to 20kHz with little error.. possibly that extends to the current ranges too. So you could do a calculation, based on the fact that you're using a square wave, not a sine wave.
 

Dear grizedale
Hi
It is pretty simple . as i understood your current is kind of square wave , right ? so if you use a low value series resistor , such as 0.1 ohm m you can measure the voltage across it , and then use this formula :
Iave=1/T integral over imax dt from zero up to ton .
Good luck
Goldsmith
 
goldsmith
Iave=1/T integral over imax dt from zero up to ton
Click to expand...

i agree but the TDS210 scope doesnt do this calculation
 

What will be the problem if you calculate it by yourself ? it is a simple equation . the result of that will be : Imax*D.C that D.C is duty cycle . is that difficult ? of course not .
 

Hi,

It is easy to meaure the currrent for a flash light system
I = ImaxOntime/(ontimexofftime)

---------- Post added at 18:23 ---------- Previous post was at 18:22 ----------

sorry for the mistake in formula

I = ImaxOntime/(ontime+offtime)
 

THE CIRCUIT IS VERY IRREGULAR, AND THE ON/OFF DUTY IS VERY IRREGULAR, so WE NEED TO MEASURE IT directly, we are thinking of an input current sense resistor, and filtering it heavily and feeding this voltage to a accurate meter
 
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The TDS210 should work fine as the measurement device, no filtering necessary. Just use its averaging automeasurement.
 

TDS210 doesnt average enough for this.

and its record length is just 2500
 

If it's irregular and not a square wave or sinewave, and you have no tool to measure it, why not grab a screenshot of the scope waveform across the sense resistor, and manually calculate the average current across the period of the waveform using pen and paper?
 

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