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[SOLVED] PWM current

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mosfet123

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Hi

How can i measur pwm signal current by means of multimeter
 

stenzer

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Hi,

if your PWM frequency is within your DMM's measurement bandwitdh you can use the DMM in current measurement mode. Keep in mind that a PWM signal consists of of several sinusoidial waves by means of the fundamental frequency/signal and its harmonics. Have a look at fourier series of a square wave (PWM with 50 %) respectively a PWM signal.

So you have to ensure that the fundamental signal and a couple of harmonics (e.g. up to the 5th) is within your DMM measurement bandwidth. If your DMM is a TRMS multimeter you can easily calculate the current sourced by your MCU or your PWM driver.

BR
 

mosfet123

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I have True rms dmm and i measure ac mode it appear 0,4A but DC mode 1,3A. Which one?
 

KlausST

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Hi,

0.4A in RMS mode
and 1.3A in DC makes no sense.

What type of DMM?

Klaus
 

KlausST

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Hi,

My small fluke114 true RMS meter does "true RMS measurements" in AC mode only ... but it blocks DC.
This means it´s "true RMS without DC"
The datasheet says 45Hz ... 500Hz.
So neiter the HPF of 45Hz is good for you nor the 500Hz upper LPF frequency.

I doubt yo can get good informations from the FLUKE179.

Do a test:
Find a source with 0V / 5V square wave and 50% duty cycle.
* a true RMS inlcuding DC will show: 5V x 0.707 = 3.535V
* a true RMS excluding DC will show: 5V/2 = 2.5V
* an average measurement will show 2.5V (sadly the same value as above, it will differ with duty cycle)

Klaus
 

mosfet123

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as far as i understand. Dc mode shows RMS voltage value , Ac mode shows RMS Current value. Right ?
 

KlausST

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Hi,

you don´t know the difference beween AC and DC?

Klaus
 

Easy peasy

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Unless you have a multimeter with a BW of > 100kHz, you simply can't make the measurements you desire - at some point to do real power electronics - you need a 'scope ... and shunts, and CT's and 100x probes ....
 

mosfet123

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İ sent oscilloscope photo. I measure its current DC mode 1,4A Ac mode 0,4A . Could you say me what is it power.
 

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stenzer

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Hi,

why aren't you using your scope in combination with a low valued resistor to determine the current sourced by your PWM source, as suggested in post #10 ?

By investigating a waveform with a scope you should use a low Volt/Devision setting, which gives more insight by means of filling the screen in y-direction as much as possible (e.g. 500 mv/Div instead of 5 V/Div). Further, according to your recorded waveform it looks like your PWM never reaches the off-state (0 V). Also your PWM freuency is seems to be pretty low, is it your actual implemented freuquency?

BR
 

mosfet123

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its led driver output signal. just how do i interpret this. I need a value, roughly
 

stenzer

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Hi,

decrease your V/Div as well t/Div setting, so your voltage resolution as well as your time resolution is increased (showing e.g. 2 or 3 periods). Nevertheless, I assume you are measuring the voltage at the output of your LED driver, which does not give any feedback about your sourced current.

Use a low valued shunt resistor and measure the corresponding voltage drop, which gives you the current by applying ohm's law.

BR
 

    mosfet123

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d123

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Hi,

For a pretty rough estimate, multiply peak DC voltage by DC current then multiply by duty cycle. I think that's right.

If you read a bit about true RMS and average AC, some other useful formulas will come up about converting an AC to a DC value.
 

    mosfet123

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KlausST

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Hi
İ sent oscilloscope photo. I measure its current DC mode 1,4A Ac mode 0,4A . Could you say me what is it power.
You ask for current, show a scope picture with voltage and in the text you talk about power.
All three are different electrical measures. One measure can not replace the other ... (without circuit details)

I guess there is a misunderstanding of basics.....at least it is very confusing.

Klaus
 

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