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Understanding AC Power Source Measurements, Part 1: Voltage and Current

Posted by Grady Keeton on Mar 1, 2016 10:46:53 AM

Programmable AC power sources are primarily used to provide a low distortion, precisely controlled sinusoidal voltage to a unit under test. Increasingly, however, AC sources, such as the California Instruments I-iX Series II, are being used to perform measurements as well. The tight integration between AC source (stimulus) and measurements (response) provides several benefits:

  • The measurements are optimized for the output range and power level of the AC source providing optimal ranging, which results in better accuracy and resolution.

  • The integration of the measurement system into the AC Source eliminates the need for external test equipment in most ATE setups, reducing overall system cost and complexity.

  • The internal measurement sense points eliminate the need for external wiring, which is prone to noise pick-up and often results in signal degradation.

Consequently, evaluating the measurement performance of an AC Source can be as important as assessing its ability to provide sufficient power and current.

Voltage and Current

The most fundamental measurements that can be made at the output of the AC Source are those for voltage and current. Of key importance is that voltage and current measurements are true RMS (Root Mean Square) values, not average readings. The RMS value of an AC current is equivalent to the DC current that would produce the same amount of power dissipation in a resistor.


Figure 1. True RMS value of AC current.

As shown in Figure 1, you can visualize this by breaking up the current waveform into infinitesimally small time slices and determining the power dissipation (P = I2 * R) at each moment in time. The power is equivalent to the sum of all these values divided by the number of time slices. Since the resistance R is presumed constant, it can be taken out of the equation, resulting in the following formula for the effective current:

Ieff = √(I12 + I22 + …. + In2 ) / n, where n is the number of time slices.

This value is the effective value of the AC current since it is equivalent to a DC current that would produce the same amount of power dissipation in the resistor. Changing this calculation from a sum to an integration over time (dt) will produce the RMS current value. This value is not the same as the average value which is the sum of the currents divided by the number of time slices.

For the special case of a pure sinusoidal waveform, the average voltage is equivalent to the RMS value divided by a factor of 1.11. As long as the signal to be measured is sinusoidal, this ratio can be used to calculate the RMS value by performing an average voltage measurement. For non-sinusoidal signals however, this fixed ratio no longer applies and there can be a significant difference between the average value times 1.11 and the RMS value.

While the output voltage of an AC Source is typically sinusoidal, the current may not be. Many loads these days are non-linear and will draw harmonic currents. To accurately measure current, not only must the measurement system of the AC Source measure true RMS, it must also have sufficient bandwidth to cover higher order harmonics. For many AC power applications, harmonics up to the 40th can be relevant which implies the measurement system bandwidth should be at least 40 times the highest fundamental frequency used. AC sources with arbitrary waveform generation capabilities must use true RMS measurement for voltage as well as current since either one may not be sinusoidal.

For more information, download the application note “Understanding AC Power Source Measurements.” You can also contact AMETEK Programmable Power Sales at 858-458-0223 or email the Sales Department at sales.ppd@ametek.com.

Topics: AC measurement, AC Power Sources

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