A programmable load is a type of test equipment or instrument which emulates DC or AC resistance loads normally required to perform functional tests of batteries, power supplies or solar cells. By virtue of being programmable, tests like load regulation, battery discharge curve measurement and transient tests can be fully automated and load changes for these tests can be made without introducing switching transient that might change the measurement or operation of the power source under test.
Implementation
In a sense, a fully programmable load is simply a programmable power supply that applies a voltage and sinks current (absorbing power) or sources current (transferring power). This is necessary to simulate real loads where there are moments when current and voltage have polarity which makes the load appear to act as a source of power; for example, circuits with capacitive or inductive elements when an AC or varying DC is applied; see power factor. By being able to change the current direction, the programmable load schematically will still "look like" a programmable resistor while still being a programmable (Thévenin) voltage source.
However, most commercially available "electronic DC loads" or "programmable DC loads" on the market cannot source power, since by being DC loads, by strict definition, they do not need to. (They are not power supplies), being only able to absorb power (sinking current from a positive voltage, or sourcing to a negative voltage). These most commonly use one transistor/FET, or an array of parallel connected transistors/FETs for more current handling, to act as a variable resistor. Internal circuitry in the equipment monitors the actual current through the transistor/FET, compares it to a user-programmed desired current, and through an error amplifier changes the drive voltage to the transistor/FET to dynamically change its resistance. This 'negative feedback' results in the actual current always matching the programmed desired current, regardless of other changes in the supplied voltage or other variables. Of course, if the power source is not able to supply the desired amount of current, the DC load equipment cannot furnish the difference; it can restrict current to a level, but it cannot boost current to a higher level. Most commercial DC loads are equipped with microprocessor front end circuits that allow the user to not only program a desired current through the load ('constant current' or CC), but the user can alternatively program the load to have a constant resistance (CR) or constant power dissipation (CP).
Advanced DC Programmable loads offer additional test capabilities with internal controls that can detect oscillations within a fraction of a second and immediately stabilize the load profile. By eliminating the effects of oscillation and current spikes a programmable load can protect against damage to your mission critical electronics.
Electronic load testing systems for defense, aerospace, commercial electronics and utilities industries, as well as for mega data center performance often require special modes of operation such as constant voltage, constant current, constant resistance and constant power. For some demanding applications, these electronic load banks may offer fast slew rates and pulsed load profiles such as sine, square, triangle and ramp as well as internal capacitance, inductance, and time delay modes of operation. Today's most advanced programmable loads typically offer 16bit measurement and drive accuracy for superior performance.