TECHNICAL CONSIDERATIONS IN THE SELECTION OF PROGRAMMABLE ELECTRONIC LOADS
Today, programmable electronic loads offer an extraordinarily wide variety
of features beyond just current, voltage and power. Many of these
features have no extra cost and offer substantial value for certain
applications. Clear, concise explanations of these load features are
and often omitted from manufacturer’s product literature. Our purpose for
this section is to explain them sufficiently for a user to make the
best possible decision of what features will optimize a particular
Constant-Power Operating Envelopes
The constant power operating envelope utilizes much wider range of both
voltage and current than older generation, single-maximum voltage & current-limited
envelopes.Within the very maximum voltage and current limits of the load,
maximum power is the governing limit. Accordingly, a multitude of voltage and
current combinations are provided thus expanding the usable load ranges by
as much as 50%.
One of the biggest challenges in selecting loads is guessing at how much extra
current or power to obtain beyond the immediate requirements in order
to protect against premature obsolescence when the next higher-power
product is to be
tested. Current generation loads solve this by including circuitry that
synchronously parallels loads even in dynamic modes such as slewing and
short circuit. Through
this feature, load capability can be incrementally added by either combining
like loads within a test station or by adding another small load later
in the field. In this manner, the user avoids having to replace an entire
load that falls
just short of meeting a new requirement.
Loads today are expected to measure very small currents such as leakage and Energy Star compliance along with the nominal larger currents. While a 16-bit measurement A/D starts to address this requirement, the real key is to have more current ranges, at least 3, including a special very-low current range.
Powerful Built-In Measurement instrumentation
The more advanced loads today employ embedded, high-speed digitizers to capture
waveforms from which up to a dozen or more static and dynamic measurements
are derived. In this manner, additional supporting measurement instruments
like DMMs, power meters and oscilloscopes plus the ancillary cables and multiplexers
are eliminated. Another benefit of this built-in instrumentation on loads when
testing multi-output UUTs is that channel-to-channel measurements are possible
in a single pass.
Glitchless Mode Switching
This feature assures that the UUT does not shut down due to the load resetting when changing from one mode to another. For instance, on many switching power supplies, a transition-induced-shutdown occurs when turning on into a constant-resistance load before testing in the constant current mode.
This feature allows the simultaneous setting of limits in multiple modes such as constant current and constant power that a UUT might pass through during a single test. This feature protects the UUT from run-away or shutdown conditions during multiple test mode changes in the load.
Extra Power Overhead
Another valuable feature are loads that are temperature limited so that they
will allow up to 150% of rated power at lower duty cycles. While this
safety feature should not be overused, certain applications benefit by
smaller, less-expensive load to be suitable for a momentary
Internal Pulse and Macro Modes
With both high speed pulses and complex waveforms, the ability to execute independent of a system controller is advantageous when precise execution is necessary. No longer does the control bus traffic or PC memory variables interfere with repeatable stimulus.
A typically overlooked consideration is the ease of calibrating electronic loads once they are embedded in a test system that has a near-continuous up-time requirement. Some older load designs still have internal pots that require partial disassembly of the load for access. Newer programmable electronic loads feature closed-cover calibration with non-volatile memory that stores calibration data. In this manner, loads do not have to be removed from the test system and calibration is completed in a matter of minutes.
Touch-Screen Display User Interface
The most recent feature now being introduced to the electronic load market
is a touch-screen display user interface. Considering all the features
above, it becomes clear that the decades-old load control interface
consisting of a few knobs, alpha-numeric
display and perhaps a keypad is sorely lacking. There is just
too much information for control and display. The optimum solution today
is a touch-screen display
that provides multiple graphic screens for such items as programming,
status and resulting waveform displays.