# Measuring peak of a pwm signal

Hi all,

I need to measure the peak current of a sinusoidal PWM signal.
This signal is a PWM sine wave, with a 4KHz carrier. The signal is too small to add a LPF.

Any suggestions?

If you know the load resistance, you can just measure the voltage and calculate the current.

Its a motor, so current will vary with load.
I guess I have to somehow synchronize the signals and attach an interrupt to trigger the adc.

If you know that it's a sin wave, can't you just look at the current being drawn by the whole circuit and multiply by about 1.4?

KenTF:
If you know that it's a sin wave, can't you just look at the current being drawn by the whole circuit and multiply by about 1.4?

A PWM sine wave?

Sort of oxymoron.

Paul__B:

KenTF:
If you know that it's a sin wave, can't you just look at the current being drawn by the whole circuit and multiply by about 1.4?

A PWM sine wave?

Sort of oxymoron.

BUT only sort of

Hi, to make it plain so everybody knows, are you using a VSD, (variable Speed Drive) to run a motor and you want to measure the peak current.

Now is this the peak current, as in the peak of the current waveform, or peak current as in the peak starting current?

The VSD should have an analog output scaled to motor current.

If it the peak of the AC current you need then it will be 1.414 times the RMS current, the fact that it is PWM at 4khz makes no difference as the carrier current is smoothed by the inductance of the motor.
You will have the variable frequency current to deal with, as it is a sinusoid peak current = 1.414 time RMScurrent

Tom......
PS, have you got the manual for the VSD if you are using one?