Current and Power Adapter Reference Workspace User Guide

Current and Power Adapter Reference Workspace Screenshot

Overview

Digilent has created a reference WaveForms workspace for easy visualization and measurement of important signals and quantities of the Current and Power Adapter. The workspace might serve as a starting point for further customer developments and enhancements. This document is intended to act as reference material for the various calculations present within the workspace.

The workspace, equations, and calculations were created and provided by Mircea Dabacan, PhD, Technical University of Cluj-Napoca Romania.

Inventory

Scope Configuration and Settings

The time base can be freely set by the user for convenient visualization.

Channel 1 of the Scope is directly copied in a math channel called v (instantaneous voltage). It shows the differential voltage at the Out binding post. Channel 1 is hidden.

Channel 2 of the Scope is connected to the output of the current sense amplifier, relative to the reference level of 2.5V. Channel 2 is hidden.

A mathematical channel called i (Instantaneous Current) processes the C2 information to consider the Current path HW transfer function:

A second mathematical channel called 𝑝 (Instantaneous Power) multiplies sample-by-sample the i and v channels:

A third mathematical channel computing the 𝑖 𝑅𝑎𝑛𝑔𝑒 based on DIO9 and DIO8:

DIO9 and DIO8 are added as digital signals in the Scope view so that they can be used in the math channel equations.

Measurements

Measurements in the Scope Instrument are defined for:

- i Range displays the (average) value of the i Range channel. It is info for the user, showing the result of setting DIO9 and DIO8 in the “Patterns” instrument. It shows the measuring range for Current for both DC and AC measurements.

DC Measurements

  • Idc is the average value of the i channel (DC value) over the visible time in the Scope view.
  • Vdc is the average value of the v channel (DC value) over the visible time in the Scope view.
  • Pdc is the average value of the p channel (DC value) over the visible time in the Scope view.

AC Measurements

For AC measurements it is important:

  • to include in the visible time range at least one full time period of the signals.
  • to set the cursors 1 and 2 to include an integer number of time periods, ideally starting and ending at moments when v crosses the 0 axis.
  • Irms is the effective value of the i mathematical channel.
  • Vrms is the effective value of the v mathematical channel.
  • P is the Active Power i.e the average value of the p mathematical channel.
  • is the Apparent Power.
  • is the Reactive Power, where T1 and T2 are the times at the two cursors.
  • is the Phase shift of Current relative to Voltage.
  • is the Power Factor.
  • Period is the period of the Volt channel.

Note: The Reactive Power is defined for sinusoidal Voltage and Current:

For non-sinusoidal periodical signals there are more attempts to define the Reactive Power as shown in this document: Power measurement techniques for non-sinusoidal conditions1).

The most used formula used for the Reactive Power does not provide the polarity:

The equation implemented in this workspace is correct for sinusoidal mode and generates positive values for inductive load, and negative values for capacitive load respectively:

Additional Workspace Details

Some advanced features are used in the measurements of the reference workspaces.

  • i Range is the average value of channel i Range.
  • DC RMS Defined Measurements over channels i and c where initially created. At that point, the script code for computing DC RMS was visible, but not editable under “Edit”. Subsequently, the measurement name was changed to Irms and Vrms respectively. That action transformed the measurements to “Custom Channel” type and the script became editable.
  • P, S, Q, and 𝜑 measurements are generated as “Custom Global” which allowed references to multiple channels.

For low currents, the noise might be significant. The “Average” feature in the “Time” control box might be used to reduce the noise, by the cost of increasing the acquisition time.

Spectrum Configuration and Settings

A Spectrum instrument is used to display the Voltage and Current spectra and some associated measurements.

The Spectrum instrument cannot run concurrently with the Scope instrument. Running each of those will automatically stop the other one.

The Traces cannot be renamed in the Spectrum instrument:

  • T1 is the C1 channel (called v in the Scope instrument).
  • T2 is the C2 channel (hidden in the Scope instrument, proportional to the i channel in the Scope instrument).
  • We don't need to apply gain correction because THD math considers the ratio between spectral components.

By default, THD and THD+Noise measurements are set, expressed both in dBc and %.

The user can modify the Spectrum instrument settings and add or remove measurements as convenient.


1) SVENSSON, S. (1999). Power measurement techniques for non-sinusoidal conditions [Doctoral thesis for the degree of Doctor of Philosophy, Department of Electric Power Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg Sweden]. https://core.ac.uk