Power supplies and electronic loads are forms of test equipment that go together like signal generators and analyzers. Power supplies generate precise amounts of energy to power a device under test (DUT) while electronic loads provide the means to measure the output of an energy source, such as an uninterruptible power supply (UPS), an inverter, or even a battery. Power supplies and loads are available for AC or DC testing, over a wide range of power ratings and capacities. Finding the right match for an application is a matter of understanding the specifications for each type of instrument and the performance levels needed for the measurements in mind.
AC and DC power supplies are defined by a voltage range, maximum voltage, current range and maximum current. Some supplies may also be rated for a short-duration, peak current level above and beyond the maximum rating, as might be used for inrush current testing. In addition, AC power supplies have adjustable frequency ranges and may have standard frequency settings, such as sinewave outputs at 50/60 Hz, to simplify testing of standard AC-powered devices. AC power sources are usually differentiated in terms of their signal purity by the amount of distortion at a standard output setting, such as 50/60 Hz for ease of comparison with other AC power supplies.
Summarizing AC Supplies
For example, the Chroma 61511 and 61512 series programmable AC power sources provide selectable single-phase or three-phase outputs, with adjustable voltage ranges of 0 to 150 V, 0 - 300V (Chroma 61511), or an automatic setting for adjustments under software control. The frequencies for each of these units can be set at DC or from 15 to 1500 Hz. The two series of AC power supplies are rated for maximum power of 12 (Chroma 61511) and 18 kVA (Chroma 61512), respectively, in rack-mountable enclosures. The AC power supplies can be equipped with various digital interfaces, including GPIB, RS-232, USB, and Ethernet interfaces, for use in ATE systems.
Chroma’s 62000H Series of DC power supplies includes units with maximum power levels of 5, 10, and 15 kW, in which the starting points in selecting a DC power supply for a given application include the voltage range and the current range. In the case of the Chroma 620000H DC supplies, the voltage can be set from 0 to 1000 V while the current can be adjusted from 0 to 375 A, depending upon model. Of course, for truly high-power testing requirements, some may feel that 15 kW is inadequate and, in such cases, many DC (and AC) power supplies offer a feature that allows multiple units to be connected in a master/slave configuration so that the outputs of the separate units can be added together. In the case of the Chroma 62000H Series DC supplies, as many as 10 units can be connected in parallel for as much as 150 kW power.
All power supplies have limits to their power density—the amount of power available from a given sized package. In some cases, the flexibility of a smaller package may be more important than higher output power levels, particularly for general-purpose testing. The California Instruments RP Series of AC power supply units 2001RP and 2003RP both pack 2000 VA output power into light-weight, rack-mountable units only 3.5 in. high. Available in single- and three-phase versions, these supplies have voltage ranges of 0 to 150 V and 0 to 300 V with as much as 6.7 A current in the higher voltage range and as much as 13.4 A current in the lower voltage range. As much as 44.5 A peak current is available in the lower voltage range. The AC frequency can be set from 16 to 5000 Hz.
These AC and DC power supplies only scratch the surface of some of the performance specifications of operating functions available from modern power sources, many of which offer a great deal of programmability without need of an external computer. Functions such as programmable output impedance, slew rates, voltage ramps over time, even simulations of power-line disturbances and harmonics using an internal synthesizer can add to the versatility of a particular AC or DC power supply, although while also typically adding to the size, weight, and final cost.
Weighing Electronic Loads
Electronic loads can also be specified for AC or DC use. These instruments are designed to be adjustable loads connected to an AC or DC power supply for testing, to evaluate how the supply responds to changes in load conditions. Electronic loads provide four operating modes: constant voltage, constant current, constant resistance, and constant power. By automatically adjusting the related electrical parameters, the parameter of interest can be held constant. For example, for constant voltage, a load will sink the current required to keep the voltage at a set level. At constant power, the load will adjust the current draw to compensate for changes in voltage and maintain the power level.
When reviewing different AC and DC electronic load performance levels, considerations include how low a load’s voltage can be set while still drawing rated current? This can be critical when testing a low-voltage supply at higher power levels. A load’s transient response is important when testing the transient response of a power supply, since the load’s rise/fall times must be much faster (typically five times faster) than the rise and fall times of the power supply it is evaluating. For such testing, an additional analysis instrument, such as a digital storage oscilloscope (DSO), is needed to capture and display the transient responses.
As with power supplies, electronic loads can be categorized by their maximum power ratings, voltage ranges, and current ranges. Programmable AC/DC loads in the Chroma 63800 Series provide AC and DC sections for testing with both types of supplies, with parallel and three-phase operation. Individual models are rated for maximum power of 1800, 3600, and 4500 W with all offering adjustable voltage ranges of 50 to 350 V (rms) and respective current ranges of 0 to 18 A (rms), 0 to 36 A (rms), and 0 to 45 A (rms). Frequencies can be set at DC or from 45 to 440 Hz.
Some electronic AC/DC loads provide advanced waveform capabilities, allowing a user to emulate linear and nonlinear modes of operation or even short-circuit conditions. The NH Research 4600 Series of AC electronic loads support standard emulation modes, such as constant voltage, constant current, constant resistance, and constant power, but also provide 12 internal measurement functions and allow an operator to define specific waveforms for testing.
Of course, these are very basic descriptions of the specifications and functions that apply to AC/DC power supplies and loads and just a handful of example instruments on the market. More information on these and other power supplies and loads, including downloadable data sheets, is available by contacting Axiom Test Equipment’s sales department at email@example.com or by calling 760-806-6600.