Electronic load attributes

The single greatest problem with electronic loads is leakage current, or idle current; it is always present to some degree. It can vary from a few milliamperes (mA) to hundreds of mA, and most manufacturers will usually not specify the amount in their devices. Even for cases where leakage may not seem to be a critical factor, it is always prudent to seek measured data for this parameter from the manufacturer.

In reality, the amount of leakage can change as test conditions change. For instance, the leakage can fluctuate as the applied voltage fluctuates, causing the leakage current to become non-linear. In some cases, the current contributed by leakage paths in the electronic load can even fluctuate with the change in the amount of current being drawn through the main current path.

Temperature instability can be another source of problems during testing. Good engineering practices can supplement common sense in order to avoid entry into thermally induced instability or failure modes.

More power devices create more leakage current, as well as greater temperature instability. Therefore, the power rating of the electronic load must be carefully selected to provide the optimal range for best stability for a given power-system testing platform. The generally accepted rule of thumb is to use loads that have a power capacity ranging from 20% to 50% more power than will be needed to fully characterize the UUT.

Electronic loads using older load designs can often be another source of trouble when testing a power system. Like their more modern counterparts, these units generally dissipate heat through power transistors. They also give good stability for long wire connections to the device being tested. However, the slew rate for this type of load is slow, limiting the minimum voltage supported by these loads to approximately 2.5 V.

Any voltage applied across the load below this value usually causes problems with the load (or, at the very least, reduces the control accuracy of the current channeled through the load). Therefore, the actual load current should always be independently checked with a separate instrument to ensure that the electronic load is working correctly.

Another type of electronic load, the field-effect-transistor (FET) type, has faster slew rates and can operate at much lower voltages (including the 0.3 V regime needed for testing fuel cells). FET-type loads also have lower current leakage and higher operational temperature stability, but their major drawback is that long leads to the load can cause instability or oscillation of the FETs in the power stage. Even though the FETs usually contribute capacitance to the output of the circuit, many manufacturers of FETs will place additional output capacitors within the loads to improve stability.

Additional problems, many also be related to stability, can occur when attempting to remotely control the electronic load with software or hardware. Ground loops from the load to the control card are likely to introduce instability into the test circuit. To guard against this effect, the best electronic loads offer electrically isolated stages (or other options) for remote control of the electronic load.

For analog control schemes, it is also important to verify that the slew rate of the electronic load tracks with the slew rate of the control voltage. While typical control ranges for analog control schemes are 0 V to 5 V, or 0 V to 10 V, the input voltage may not control the slew rate of the electronic load in some instances. For these units, the voltage-to-current relationship between the input and the output (which is almost always linear) only applies to the steady-state condition of the load.

In these instances, the electronic load will usually be working in constant-current mode. If a constant voltage or constant power is desired for the output of the power system under test, then the software or firmware controlling the test platform must then adjust the current accordingly. This, in turn, may require a multifunction data acquisition control card to adequately monitor the appropriate nodes in the electronic load's circuitry.

The algorithms needed to perform this monitoring and control can be quite complex. Furthermore, if an IEEE-488 bus is used to remotely control an electronic load, the send and receive latencies of this interface can make it virtually useless for any type of high-speed testing.

Additional load characterization

While voltage and current are the fundamental parameters needed to calculate power dissipation in electronic loads, it is often necessary to test some of their other parameters. For example, it is usually quite beneficial to know the minimum on resistance of an electronic load. This parameter reveals the lowest possible voltage across the load, as well as the highest possible current supported by the load.

On resistance also reveals something about the circuit impedance, which can be used to predict how the load will react with a particular power system, such as a basic fuel-cell stack (in general, the on resistance of the load should be approximately 10% of the impedance of a fuel cell). One additional insight provided by an electronic load's on resistance is its susceptibility to thermal drift. Thermal drift tends to increase with on resistance.

Given these many challenges, it is clear that for most power systems, especially portable military systems based on fuel cells or batteries, a custom-designed electronic load would be the best option. However, engineers should not expect to find ready-made loads specifically designed for the comprehensive testing of fuel cells or batteries. Rather, it is often the responsibility of the military design engineer to develop multiple testing platforms for a single power system. To meet this challenge, the availability of customized modules, each having the necessary voltage, current, resistance, and thermal parameters, would be highly beneficial.

Executive Engineering makes electronic load blocks specifically for this purpose. Each block is a miniature module that can be connected in different ways to produce an electronic load that is tailor made to fit the needs of any project.

ABOUT THE AUTHOR

David Weber is the founder and president of Executive Engineering, Ft. Lauderdale, Fla., which has been in business for about 12 years. Prior experience includes design engineer, IRD Mechanalysis in Columbus, Ohio, R&D engineering specialist for Allied Signal Corporation, director of engineering for National Avionics, and test engineering manager for Unipower Corporation.

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