Electronic Load

What is an Electronic Load

An electronic load is an active instrument that draws a controlled current from a DC (or, in AC variants, an AC) source and converts the absorbed energy into heat or, in regenerative designs, back into the AC mains. Where a passive resistor draws whatever current Ohm's law dictates for a given voltage, an electronic load uses a closed feedback loop driving power semiconductors to hold a programmed setpoint: a fixed current, a fixed terminal voltage, a fixed equivalent resistance, or a fixed power, independent of how the source behaves. This makes it the standard tool for characterizing anything that delivers power.

Structurally, a DC electronic load has four functional blocks. First, the power stage: an array of power MOSFETs or IGBTs that act as the controllable current sink, mounted on a heatsink with forced-air or, in high-power racks, liquid cooling. Second, the control loop: a high-gain amplifier comparing the measured quantity (current, voltage, resistance, or power) against the programmed reference and adjusting the transistor gate drive thousands of times per second. Third, the measurement system: precision shunts and voltage sense, often with four-wire remote sensing to remove cable drop and a 16-bit acquisition path. Fourth, the interface: front-panel keys plus SCPI-programmable remote control over USB, LAN, GPIB, or RS-232 for automated test sequences.

The instrument exists because loading a power source realistically is hard to do with passive parts. A resistor bank cannot hold steady current as a battery discharges from 4.2 V to 2.5 V, cannot step its demand at 10 kHz to probe a supply's transient response, and cannot integrate ampere-hours to report battery capacity. An electronic load does all of these from a keypad or a script. Typical work includes verifying a power supply's load and line regulation, its current limit and overload protection, and its ripple under load; discharging and grading batteries; characterizing fuel cells and solar arrays; aging and burn-in of finished switching power supplies; and validating LED drivers and DC-DC converters.

The application scale is broad. Bench loads handle a few hundred watts at up to roughly 150 V to 500 V for R&D and production lines. Modular systems pack many independent channels into a single mainframe for multi-output supplies and chargers. High-power racks reach tens of kilowatts per cabinet and hundreds of kilowatts in parallel for electric-vehicle traction batteries, large inverters, and fuel-cell stacks. As that power grew, the energy wasted as heat became a cost and a cooling burden, which is why regenerative loads that recycle energy to the grid have become a distinct product class. No single load spans the whole range; selection is the act of mapping a device's voltage, current, power, and dynamic profile onto a specific instrument class and its safe operating area.

Four engineering attributes dominate selection: the voltage, current, and power envelope (and where inside it the load can actually operate), the regulation accuracy and measurement resolution, the dynamic performance (slew rate and transient frequency), and the energy-handling architecture (dissipative versus regenerative). The chapters below take each in turn.

Load Types and Form Factors

Electronic loads divide along two independent axes. The first is energy handling: dissipative loads burn absorbed energy as heat, while regenerative loads feed it back to the AC mains. The second is form factor and integration: benchtop, modular, and high-power rack. A fourth distinction, DC versus AC (or AC+DC) loads, separates instruments that sink direct current from those that can also sink alternating current to test variable frequency drive inverters and AC sources. The table below contrasts the main classes by typical scale and best fit.

Dissipative loads are the conventional design. Absorbed power is converted to heat in the sinking transistors and removed by fans or water, the same energy-to-heat principle as a brake resistor on a motor drive. They are simple, fast, and accurate, and dominate bench and mid-power work. Their drawbacks scale with power: the energy is wasted, the heat must be exhausted from the test area, and large fans or chillers add bulk and noise. Below roughly 1 kW the dissipative load is almost always the right economic choice.

Regenerative loads replace the dissipating transistor bank with a bidirectional power converter that returns the absorbed energy to the AC line. Chroma states its 63700 series regenerative DC loads recover energy to the grid at up to 93 percent efficiency, while its 63800R series regenerative AC loads reach up to 89 percent, with the 63800R supporting AC, DC, and AC+DC loading. The payoff appears in long, high-power tests: an EV battery being cycled or a rack of supplies under burn-in can run for days, and recycling rather than dissipating the energy cuts both the electricity bill and the cooling plant. Regenerative loads also include grid-protection logic that trips on over-voltage, under-voltage, frequency anomalies, phase imbalance, or excess current at the AC interface.

Benchtop loads integrate one or two inputs, a display, and programmability in a compact case. The Keysight EL30000 series is a representative line of three bench models, each with a built-in data logger: the EL33133A (single input, 150 V, 40 A, 250 W), the EL34143A (single input, 150 V, 60 A, 350 W), and the EL34243A (dual input, 150 V, 60 A, 600 W total). The B&K Precision 8500B series spans 150 V / 30 A / 300 W (model 8500B), 500 V / 15 A / 300 W (8502B), 120 V / 120 A / 600 W (8510B), and 120 V / 240 A / 1500 W (8514B).

Modular loads place several independent load channels into one mainframe so that a multi-output supply or a multi-cell charger can be tested in parallel. The Chroma 63600 series provides up to ten channels within a 4U mainframe from five dual-channel 100 W modules, reaching up to 2 kW in a single mainframe when high-power modules are paralleled. High-power rack loads serve battery, inverter, and fuel-cell work: the Chroma 63200A series offers 60, 150, 600, and 1200 V models from 2 kW to 24 kW per unit and up to 2000 A in a single chassis, scalable to far higher power when units are paralleled.

Regulation Modes: CC, CV, CR, CP

The defining capability of an electronic load is its set of regulation modes. Four are standard across the industry: constant current (CC), constant voltage (CV), constant resistance (CR), and constant power (CP, sometimes labeled CW for constant wattage). Each holds one electrical quantity fixed while the others move, and each suits a different test question. The table below summarizes what each mode regulates, how current responds to a voltage change, and where it is used.

Constant current is the workhorse. The load sinks the programmed current regardless of input voltage, up to its maximum rating, by continuously adjusting its internal equivalent resistance. This is the mode for confirming a power supply can hold its rated current, for driving an LED string at a fixed current, and for discharging a battery at a controlled rate. Because the current stays put while the source voltage may sag, CC mode directly exposes a supply's load regulation and its current-limit knee.

Constant voltage turns the question around: the load draws whatever current is necessary to clamp its own terminals at a programmed voltage, limited by its maximum current. It is used to test current sources and battery chargers, where you want to set the bus voltage and observe the current the source pushes. CV mode emulates a battery sitting at a fixed state-of-charge voltage while a charger tries to drive current into it.

Constant resistance makes the load behave like a fixed resistor: it senses its input voltage and sinks a current linearly proportional to that voltage, so the ratio stays constant. Because the current naturally rises as the source comes up, CR mode is well suited to observing inrush, soft-start, and start-up sequencing, where a resistive characteristic is the realistic load.

Constant power holds the product of voltage and current fixed: as input voltage falls, the load increases current so that absorbed watts stay constant. This is the realistic emulation of a downstream DC-DC converter, which draws constant power and therefore more input current as its supply voltage drops. CP mode is the right choice for confirming a source delivers rated watts across its full voltage range, and for battery and fuel-cell runtime tests where the downstream system is a constant-power consumer. On top of these static modes, loads add dynamic (transient) operation that toggles between two CC levels at a programmable frequency, plus list and sequencing functions that step through a stored profile, and a short-circuit test that commands maximum current to emulate a fault.

Sink Principle and Safe Operating Area

Inside a dissipative DC electronic load, the current sink is one or more power MOSFETs (or IGBTs in higher-power designs) operated in the linear, or active, region rather than as on-off switches. A control amplifier sets each transistor's gate voltage so that its drain-to-source channel presents exactly the resistance needed to draw the programmed current at the present input voltage. The transistor and its sense shunt sit in series across the input terminals; the entire absorbed power, the product of the terminal voltage and the sink current, is converted to heat in the transistor die and carried away by the heatsink.

This linear-mode operation is the harshest duty a power MOSFET can face, which is why electronic loads use devices specifically rated for it. In switching applications a MOSFET spends almost no time in its linear region, so it dissipates little. In a load it lives there permanently, dropping voltage while passing current. The critical hazard is thermal runaway: at high drain current the device's gate-threshold voltage decreases as the die heats, which tends to increase current further, which heats it more. Manufacturers such as Infineon and Nexperia publish dedicated linear-mode safe operating area curves precisely because a transistor can self-destruct inside its nominal voltage, current, and power ratings if operated linearly at the wrong point.

The safe operating area (SOA) is therefore the single most important concept behind a load's headline numbers. SOA is the set of simultaneous voltage and current combinations the sink can sustain without exceeding its current, power, or junction-temperature limits. A load rated, for example, 150 V and 40 A cannot do both at once, because 150 V times 40 A is 6 kW, far above a 250 W bench rating. The usable envelope is bounded by three lines: maximum current along the low-voltage edge, a constant-power diagonal across the middle, and maximum voltage along the high-current-limited edge. Any operating point must sit inside that envelope, not merely under the maximum voltage and maximum current taken separately.

The table below shows why the SOA matters in practice, using a representative bench load with separate maximum voltage, current, and power ratings. The achievable current is whichever of the current limit and the power limit is lower at a given voltage.

The other consequence of the sink principle is the minimum operating voltage. To push rated current through the channel, the transistor needs a minimum drain-to-source voltage; below it the device cannot reach full current. Most bench loads consequently reach full current only above a stated minimum terminal voltage and offer reduced current beneath it. This is decisive for low-voltage, high-current sources: single-cell lithium batteries ending discharge near 2.5 to 3.0 V, hydrogen fuel cells around 0.6 to 1.0 V per cell, and 1 V point-of-load rails can all sit below where a general-purpose load reaches full current. Such work needs a load designed for low-voltage operation, or a series boost element, otherwise the programmed sink current is simply unreachable.

Key Specification Parameters

A load datasheet may list dozens of numbers, but eight decide most selections: the voltage, current, and power envelope; the minimum operating voltage; regulation accuracy in each mode; measurement resolution and accuracy; slew rate; transient (dynamic) frequency; remote sense; and the programming interface. Each is decoded below.

Voltage, current, and power envelope is the SOA corner described in Chapter 4. Read all three ratings together and confirm your worst-case operating point lies inside the published curve. A 1500 W load rated 120 V and 240 A, such as the B&K 8514B, can supply 240 A only at low voltage and is power-limited above 6.25 V.

Minimum operating voltage is the lowest terminal voltage at which the load still reaches full rated current. It governs whether the instrument can fully discharge a low-voltage cell or load a sub-1 V rail. If the datasheet quotes a derated current below a threshold, plan the test around that derating.

Regulation accuracy is how closely the load holds the setpoint in each mode, typically stated as a percentage of setting plus a percentage of range or a fixed offset (for example a CC accuracy expressed as a fraction of setting plus a few milliamps). Measurement resolution and accuracy describe the readback path: the B&K 8500B series uses a 16-bit voltage and current measurement system with resolution down to 0.1 mV and 0.1 mA. Resolution is not accuracy; a high-resolution readback can still carry a larger accuracy band, so read both.

Slew rate and transient frequency govern dynamic testing:

• Slew rate: the programmed rate of current change in amperes per microsecond. Higher slew emulates faster load steps and stresses a supply's transient response harder. The achievable slew rate often falls at low input voltage.
• Transient (dynamic) frequency: the maximum rate at which the load toggles between two CC levels. The B&K 8500B series specifies transient operation up to 10 kHz in CC mode; the B&K 8600 series reaches up to 25 kHz.
• List and sequencing: a stored profile of steps the load executes automatically, used for automated production sequences and for replaying a real-world load pattern.

Remote (four-wire) sense routes a separate pair of wires to measure voltage at the DUT terminals rather than at the load input, removing the IR drop in the load cables. This is essential at high current and low voltage, where a few milliohms of cable resistance otherwise corrupts both the regulation and the measurement, and it is required for trustworthy battery-capacity results. The programming interface determines automation: modern loads are SCPI-programmable over USB, LAN, and optional GPIB (the Keysight EL30000 series, for example), while many bench loads also offer RS-232. Built-in functions such as battery test (capacity integration with voltage, capacity, or timer stop conditions, as on the B&K 8500B series), short-circuit test, and data logging reduce the external code you must write.

Selection Decision Factors

To turn the preceding chapters into a specific model, follow the ordered sequence below. Most selection errors come not from a single wrong number but from deciding power before checking the safe operating area, or choosing a form factor before counting channels. These eight steps double as an RFQ template.

1. Define the electrical envelope and SOA point: List the device's maximum voltage, maximum current, and the worst-case simultaneous voltage-and-current point. Confirm that point lies inside the candidate load's published SOA curve, not merely under its headline voltage and current. Add margin for transients.
2. Check the minimum operating voltage: For batteries, fuel cells, and low-voltage rails, verify the load reaches full current down to the device's lowest working voltage. If not, choose a low-voltage load design or plan for current derating.
3. Pick the required modes: CC for current-limit and discharge, CV for charger and source tests, CR for inrush and start-up, CP for rated-power and runtime work. Confirm the load supports every mode your test plan needs, plus dynamic mode if you test transient response.
4. Set the dynamic requirements: Specify the slew rate (A/us) and transient frequency (kHz) your transient-response and processor-rail tests demand, and confirm them at your actual operating voltage, not just at the rated maximum.
5. Choose the form factor: Count channels and per-channel power first. Single DUT means benchtop; many outputs mean modular; tens of kilowatts mean a high-power rack. Decide dissipative versus regenerative on test duration and energy cost.
6. Specify measurement and sense: Required readback resolution and accuracy, four-wire remote sense for high-current or low-voltage work, and any built-in battery test, short-circuit test, or data logging you would otherwise have to code.
7. Define interface and automation: SCPI over USB / LAN / GPIB / RS-232, driver availability for your test framework, and synchronization if multiple channels or units must step together.
8. Account for total cost of ownership: Purchase price plus electricity and cooling for dissipative high-power work, calibration interval, spare parts, and rack space. For long, high-power burn-in, a regenerative load's energy recovery (Chroma cites up to 93 percent on its 63700 DC series) can repay its premium.

One dimension that is easy to overlook is serviceability and support: local calibration capability, spare-module availability for modular and rack systems, firmware updates, and the quality of the SCPI command set and instrument drivers, which determine how cleanly the load integrates into an automated test station years after purchase. Chroma, Keysight, and B&K Precision all maintain established service and documentation networks, which makes them dependable choices for production-line deployments where downtime is costly.