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The primary function of an AC power source used for testing is to replace the commercial utility power source (distributor) used in our environment. The focus for power source system environment should be on consistent voltage, consistent frequency and clean waveform. We can plug into walls and get power to run home electronics, appliances and many instruments used for various testing. However for testing, it is not recommended.

Choosing the right AC power supply for your application requires an extensive definition of specifications, but the basic specifications include voltage, current and power input ratings & output ratings, ripple, and functions. Since most AC power supplies available in market are equipped with DC output mode, to include DC mode output ratings, versatility of output modes for one of the key specifications isn’t a bad idea.

Functions, on the other hand, can be varied by programmability, speed, communication interface options (such as controllability by PC and remote control), measurement functions, switching configurations, and software option for immunity testing (IEC). The function part of assessment depends on the purpose of testing or use. When looking for AC power source for production and inspection line, looking into types of sensor systems and adjustment functions can be helpful.

Another frustrating factor in selecting the right power supply is price. Most of us work within a budget, and price can influence selection. Multiple features and power range will increase the price of most power supplies. It is easy to find a less expensive DC power supply, but it is not easy to find one with the right AC power supply. Anyone can obtain AC power from common outlet, but the quality of waveform is not considered for powering instruments and it should not be. What is needed is a clean waveform, often called a sine waveform. The quality of products you design, develop, test and even manufacture largely depends on cleanliness of AC waveform.

High Power AC Power Supply
There is one common factor in selecting the right AC power supply for high-load (durability) testing, automatic production, and inspection lines, and that is the efficiency of the AC power source. The consumption of electricity is determined by the efficiency rating, and this is usually listed along with other specifications listed on a power supply data sheet. Ideally, you should look for efficiency ratings of 75% or more. Most of AC power supplies that use the PWM inverter method should offer 70% or higher. Compared with linear type power supply, waveforms can be distorted however, and there are many AC power supplies available that are made to resemble the ideal sine waveform that a linear type outputs.

Utilization of the phase capacity (single-phase and three-phase outputs) is also important. Instead of installing both a single-phase and a three-phase system, which adds more cost onto the budget (and take up more space), a “2-in-1” system can reduce the cost and increase project productivity.

Other key functions for production and inspection lines are the sensing function and protection for safety and stability. If a self-testing function is available, this may save you time and money when the unit is out of order. These two purposes are different in nature, but both actually require similar characteristics in selecting power supplies. However, there is one key point unique to each purpose. The quality of final products and reliability of inspection are largely determined by the quality of the AC power source used in the line. The best way to increase productivity is to increase number of power supplies used for the lines. But this means that the increase in the number of motors used also increases waveform distortions by picking up noise as power travels. Therefore, a quality AC source is vital, especially in large scale production and inspection lines.

The reliability of the frequency range is the major key point for high-load testing. If a power supply provides a wide input range and high quality powerful output with less waveform distortion, the possibility of completing testing with quality data is high. Less than 0.5% voltage waveform distortion and 60µs of voltage response speed can be considered high quality.

Figure 1a and 1b: Comparison of the waveforms from a commercial power source from the wall and an AC power supply

Desktop AC Power Supply
The level of critical projects, laboratory projects, and testing often requires compact size and useful functions. For example, testing for batteries used in personal computers and smaller devices can benefit from a compact size AC power source, with the power supply’s maximum output capacity of 500VA being sufficient. Another function that can come handy is the memory feature, primarily for those who need to test sudden changes of voltage and frequency by calling multiple settings.

Options for communication interface can be important, but power supplies that already come GPIB, USB and/or RS-232C interfaces can be more expensive than those that don’t come with those options. Choose wisely according to specific testing and project needs. If communication interfaces can be added later and there is no need for it at the time of purchase, save it until the day it is needed.

Figure 2: An example of a quick reference guide for selecting an AC power supply

High Quality, High Stability
Overall, when it comes to the quality of waveform, a linear type brings the best result whether it is DC power supply or AC power supply. There is a handful of linear AC power supply models with a peak current of up to 4 times the maximum RMS value (current rating) that can be managed with a capacitor input-type rectifier load (switching power supply). The benefit of equipping a high-speed linear type amplifier system is reflected in the output voltage stability. The typical measurements to look for are shown in Figure 3.

Output Voltage Stability
Line Voltage Variation	within ±0.1%
Output Current Variation	within ± 0.1V / ±0.2V (100V/200V range)
Output Frequency Variation	within ±0.3%
Ambient Temperature Variation	100 ppm/°C (typical value)
Output Frequency Stability	within ±5 x 10¯5
Output Voltage Waveform Distortion Ratio	0.3% or less
Output Voltage Response Time	30µs (typical value)

Figure 3

For power supply environment and simulation testing, the benefit of having high quality and high stability AC power supply is the reliability of its test results. Key points to look for in a quality AC power supply are a voltage waveform distortion rate of 0.3% or less (chattering free), and minimum occurrence of a harmonic current.

The main reason for conducting simulation testing is for overall quality assurance. Complication of simulation testing in the current stage and expected needs in the near future, ability to control and access all of the power supply’s functions are important, and will be helpful in terms of further exploratory extended testing. The ability to set the parameters to perform power failure, voltage drop (dipping) and voltage surge are widely used to measure the levels of a product’s quality assurance. A power supply that can be programmed with actual power line abnormality taken directly from its source through an oscilloscope that causes product performance failure is one good feature to have.

For ‘brown-out,’ automated tests, and for creating synchronization with other equipment by setting trigger signals, power supplies that enable sculpting “what if” sequences per address (including the voltage, frequency and execution time) is another helpful tool to have. Consumer products quality testing will need a function that can allow the power supply to output waveforms in which peak of the sine wave is suppressed (similar to Figure 1, where the waveform is from a commercial utility line). If the power supply comes with waveform bank feature (in which a number of waveforms can be stored), it is convenient as well since the distortion of waveform can be varied depending on environment.

Because of the quality of their waveforms, linear type AC power supplies are also ideal for testing to EMC immunity standards (i.e., IEC 61000-4 series). In any EMC testing, the test must be conducted with a clean AC power source because, when noise is detected, the origin of the noise has to be certain. The outcome depends on the quality of the AC power source used for its testing. It is also important that a single phase/single phase three wire output and single phase/three phase configuration can be easily built.

There is a fundamental list of things should be checked before selecting power supplies, such as understanding the power supply’s consumption of power, knowing input current of the supply, and the facility’s maximum input. In some cases, size and price should be understood. Consulting with a facility electrician before installation could save troubles, such as return of a power supply and/or burned circuits. The size and mobility of a power supply is also important for maintenance.

Summary
In summary, there are four key points for an ideal AC power source. They are:

There is no power outage;
It supplies consistent voltage;
It supplies consistent frequency;
It provides clean waveform.

The power we use from any common outlet is far from ideal. Flickering, sudden dip, power outage, inconsistent frequency, and increasing noises are common obstacles of our test environment. In some cases it is harder to detect the cause of failure, whether it originated from the testing object design or the power source used for the testing and evaluation.

There are some power supplies that just do not perform as promised in the catalog or in the data sheet. If the opportunity to evaluate power supplies before purchase is available, it is wise to take the chance to use an oscilloscope and compare the waveforms of each different model. Also trying both low and high ratings of voltage available is another good way to determine its reliability. If that is not possible, then contacting colleagues who have experience with the equipment can be helpful. In the end, having a clear understanding of what is most important for your testing and what you are trying to achieve is a definitive gauge in selecting the right AC power supply. n

Itoko Richardson is with Kikusui America, and can be reached at i-richardson@kikusui.us.