Ferrite at Work

I'm making a reminder , about the measurements with a DDM for those who have forgotten or the new ones who are joining us.

The question is, can a DMM Measuring maximum and minimum voltage/current in PWM ?

This is mostly a job for an oscilloscope, multimeters are not fast enough. If you can control the PWM it is easiest to measure DC voltage at highest and lowest PWM setting. The min/max function on multimeteres usual requires about 0.3s or 300ms to measure a value, this means it cannot be used even at 10Hz. Some meters has a "peak" function, this may be able to measure 1ms or even 0.1ms peaks.

From the above table it is fairly obvious that even the peak function is too slow when the PWM frequency gets much above 100Hz. 

For power measurement. When working with pulsed current or PWM you cannot just multiply voltage and current to get power, here the RMS (or rather DC+AC) values can be useful. 

 Example: 0V to 10V with 10% duty cycle will delivering 0A at 0V and 1A 10V. It is easy to calculate that it will deliver 10 watt when (Voltage x Current -> 10 x 1) on and with 10% duty cycle the power is 1 Watt. But using the DC values that is 1V (10% of 10V) and 0.1A (10% of 1) gives 0.1 Watt, i.e. very wrong. What about DC+AC values, that is 3.16V and 0.316A and gives the correct 1 Watt.

But there is a caveat on that, the RMS part will include current running both ways at input, this is fine for dimensioning the cable and calculating power for resistors but when the circuit has an inductor or capacitor (reactive circuit) that returns some current each cycle.

In fact, at input if you rely on the power measurement display of your power supply as would reading a DDM, in the presence of reactive circuit, the above power calculations will fail! and for power output measurement, phase angle between current and voltage must be in calculation, this is where an oscilloscope excels. with their new power management software.

When measuring on PWM it is important to know what the result must be used for, average is useful for some measurement and AC+DC is required for other  The bandwidth on multimeters is for AC measurements including DC+AC, but DC measurement usual works at much higher frequencies. For meters without DC+AC it can be calculated this way: DC+AC = sqrt(sqr(DC)+sqr(AC), DC+AC is based on the true RMS value of the AC (I have not seen AC+DC on a NON TRU RMS METER.

ALMOST EVERY RESEARCHER ON THE FORUM HERE KNOW THIS, I HAVE ALREADY POSTED THIS ARTICLE BEFORE AND KNOW HOW TO TAKE MEASUREMENTS WITH A DDM.

Jagau