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Pulsed current measurement across resistor is different from Digital Meter

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xmen_xwk

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I have a current sensing resistor of 0.05R at low side of inductive load. The inductive load is being pulsed by N-Mosfet also at low side with 200Hz frequency.

At duty cycle of 46% the voltage across current sense resistor is 25.8mV measured by 2 different digital meters. Since the measurement looks stable I believe they are average of entire pulse period.
So if I'm correct, the current is 0.0258 / 0.05 = 0.516A but if I connect digital current meter at low side of load it shows me current of around 1A. These measurements are different at almost all duty cycle values but if I set the duty cycle to 100%, the sense resistor measurement matches with current meter.

Now, my question is, how do I measure correct current using the sense resistor ?
 

Hi,

Maybe both are correct.
May one shows trueRMS while the other shows average current.

You have to decide which one is the correct method.
Example:
* If you want to calculate the dissipated power in the indutor's wire --> use RMS
* If you want to calculate the magnetic force of the same inductor --> use average

Another example.
Lets say you have a
* stable supply voltage with value = Vs
* an about ideal resistor with value = R
* and an about ideal switching device, that switches the resistor at the power supply ON and OFF with a duty cycle of d_c
Then
I_avg = d_c × V /R
But
I_RMS = sqrt(d_c) × V / R

At 0% d_c both currents show the same value.
At 100% d_c both currents show the same value.
At 50% duty cycle I_RMS shows a 41% higher value than I_avg ....but both values are correct.

If you measure the current to calculate the power dissipation of a
* resistor --> use I_RMS
* zener --> use I_avg


Klaus
 

KlausST said:
Hi,

Maybe both are correct.
May one shows trueRMS while the other shows average current.

You have to decide which one is the correct method.
Example:
* If you want to calculate the dissipated power in the indutor's wire --> use RMS
* If you want to calculate the magnetic force of the same inductor --> use average

Another example.
Lets say you have a
* stable supply voltage with value = Vs
* an about ideal resistor with value = R
* and an about ideal switching device, that switches the resistor at the power supply ON and OFF with a duty cycle of d_c
Then
I_avg = d_c × V /R
But
I_RMS = sqrt(d_c) × V / R

At 0% d_c both currents show the same value.
At 100% d_c both currents show the same value.
At 50% duty cycle I_RMS shows a 41% higher value than I_avg ....but both values are correct.

If you measure the current to calculate the power dissipation of a
* resistor --> use I_RMS
* zener --> use I_avg


Klaus
Click to expand...

I think I got where is the problem. When duty is 46%, in rest of 54% the current is flowing through flyback diode. Thats why current meter is showing 1A while resistor is turned off for 54% of pulse.
 

Hi,

Why didn't you tell that you measured at different places?
That's why I so often ask for a (hand drawn) sketch.

Most probably with a sketch you didn't even need to ask...because the problem becomes visible..

The benefit for the forum is that hopefully many users see, that a pencil and a paper is so useful...


Klaus
 

sc.png

seems like its not possible to measure current using sense resistor at that location.

- - - Updated - - -

KlausST said:
Hi,

Why didn't you tell that you measured at different places?
Click to expand...

I didn't know that either until I inspected the circuit more carefully.
 

Hi,

Bith measurements are suitable ... but for different things.

What is yout measured current used for?

Klaus
 

KlausST said:
Hi,

Bith measurements are suitable ... but for different things.

What is yout measured current used for?

Klaus
Click to expand...

There are some calculation need to perform on flowing current but for that to happen I need to know the current.
 

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