A DIY constant current dummy load is an adjustable current sink used to evaluate a power supply or discharge a battery under defined conditions. Its usefulness depends on more than holding one current reading: the circuit must remain controllable at low voltage, survive its thermal operating points and produce measurements with known uncertainty.
This guide develops the design and validation decisions for a low-voltage bench load. It is not a complete construction schematic or a tested bill of materials. Retain the current-setting, assembly and testing steps below, then use a reviewed circuit with component-specific compensation and protection before building hardware. The worked 12 V, 2 A example is illustrative, not a rating for a 吃瓜tv product.
Define the useful load envelope before choosing components
Write down the minimum and maximum input voltage, continuous current, continuous power, pulse duration, duty cycle and ambient temperature. These limits form an operating envelope. A unit capable of 2 A at 12 V is not necessarily capable of 2 A at 30 V: the absorbed power changes from 24 W to 60 W. Conversely, low-voltage operation may be limited by available control headroom even when dissipation is small.
Constant-current mode holds current approximately constant while voltage changes. Constant-resistance mode changes current with voltage, and constant-power mode increases current as voltage falls until another limit intervenes. Specify which mode the test requires. A battery capacity test, a converter startup test and a transient-response test can demand different behavior.
For battery testing, define the cell chemistry, permitted discharge current, cutoff voltage and temperature limits from the battery supplier. Stop the discharge automatically at the chosen limits; do not depend on an operator watching a meter. Use suitable protection and connections for the available fault energy. The load is not inherently safe simply because it replaces the normal product.
Under truly constant current, capacity in ampere-hours is I 脳 t in hours. If current varies, integrate the measured current over time. Energy in watt-hours requires integrating voltage multiplied by current, so it cannot generally be calculated from nominal battery voltage alone. Record the cutoff rule, starting condition and temperature beside any capacity result.
The underlying feedback behavior is explained in the constant-current circuit guide. Keep this page focused on the load as a measurement fixture, rather than treating a current-source circuit and a complete electronic load as equivalent.
Design the feedback loop and distribute the absorbed power
A common arrangement uses an op-amp to drive an N-channel MOSFET, with a shunt resistor in the source return. The amplifier compares the sensed shunt voltage with a setpoint and adjusts the gate. Ideally, I = Vset/Rshunt. The device under test provides the load energy; a separate control supply can keep the amplifier operating as the tested voltage falls.
For a 2 A example with a 0.1 惟 shunt, setpoint voltage is 0.2 V and shunt dissipation is 0.4 W. With 12 V at the load input and negligible other losses, the transistor dissipates approximately (12 鈭� 0.2) 脳 2 = 23.6 W. The shunt is not the only component absorbing power. A resistor wattage label by itself therefore cannot establish the load’s continuous rating.
Allow for the shunt’s specified temperature derating, temperature coefficient and pulse capability. A 1 mV input-referred error across a 0.1 惟 shunt corresponds to 10 mA; at a 100 mA setpoint, that is a substantial fraction of the reading. Include reference error, amplifier offset, shunt tolerance, wiring effects and measurement uncertainty in an error budget.
The MOSFET operates as a controlled dissipative element, not as a fully enhanced switch. Review its for the actual voltage, current, time and case temperature. A large drain-current rating or low on-resistance is not sufficient. Do not extend a short-pulse SOA curve into continuous operation, and do not assume two parallel devices share current evenly.
Analog Devices’ highlights the combined headroom and device limitations. Select the amplifier for its input range, output swing and ability to drive the gate network. Gate capacitance and wiring affect stability; compensation must be designed and checked for the actual circuit, rather than copied as an arbitrary capacitor value.
Include an off state that actually removes gate drive, a controlled startup, and defined behavior if the control supply or setpoint disappears. Decide how the system detects overtemperature, reverse connection and loss of cooling. These are circuit requirements, not features that a firmware display alone can provide.
Build the PCB, thermal path and measurement connections together
Place the shunt so the high-current input and return paths are short and clearly separated from its sensing connections. Take a Kelvin pair from the resistor’s intended sensing points to the amplifier. Keep that pair away from the gate-drive path and any switching converter supplying the control electronics. A convenient ground connection at an input connector may include enough copper drop to distort the feedback measurement.
Review the complete power PCB layout, including connectors, vias, copper neck-downs and mechanical attachments. Heavy-copper construction can be relevant to a current path, but it does not replace a heatsink or improve the transistor’s intrinsic SOA. Give the fabricator the intended finished copper and stackup requirements instead of relying on a vague high-current label.
For the 23.6 W transistor example, a hypothetical 40 掳C ambient and a chosen 125 掳C junction design ceiling allow a total junction-to-ambient thermal resistance of about 3.6 掳C/W. This is an initial steady-state budget: subtract the device’s junction-to-case contribution and interface contribution before assigning a heatsink target. Use actual data and margin; the chosen ceiling is not a device specification. Case-temperature measurements and SOA derating still need review.
Mounting hardware, interface thickness, airflow obstruction and enclosure orientation can change the result. Check whether the transistor tab is electrically connected to the drain before attaching it to shared metalwork. Choose any electrical insulation and thermal interface as part of the design, with the required clearances and mounting method. Do not assume thermal adhesive or a metal enclosure automatically provides a suitable mounting solution.
Add accessible points for the setpoint, shunt sense voltage, gate drive, control supply and input voltage. Provide a low-current first-power-up procedure. For a bare PCB prototype, include the mechanical holes and critical footprints in the review. If requesting prototype assembly, specify shunt orientation, approved transistor alternatives, thermal hardware and any parts installed after reflow.
Validate accuracy, stability and fault response before relying on results
First inspect polarity, insulation, continuity and control-supply operation with the power stage disabled. Start from a current-limited bench source at a low setpoint, then compare actual current with an appropriate independently checked instrument. Record zero offset and several points across the range. Adjusting one potentiometer at one operating point is not a complete calibration.
Sweep input voltage at a fixed current command to find dropout, and repeat at several currents. Observe the shunt waveform and input voltage with an appropriate oscilloscope setup. Look for ringing, oscillation and overshoot when the setpoint changes or the input connects. Distinguish the power supply’s response from ringing caused by fixture leads and the load’s own loop. A stable meter average can conceal an oscillating current.
Run thermal tests long enough to observe the relevant settling behavior, recording ambient, airflow and case temperatures. Test the intended protective response to loss of airflow without allowing parts to exceed their limits. Verify shutdown and restart behavior, including what happens if the controller resets while the input remains connected. An electronic load should not be used as a calibration reference until its accuracy and uncertainty have been established.
For battery tests, log voltage, current and time at known intervals, verify cutoff independently, and retain the raw data. For power-supply tests, record lead length, connection arrangement, output capacitance and measurement bandwidth. This makes a result reproducible and helps distinguish a board defect from a test-fixture limitation.
Related demonstration: the EEVblog video below discusses a DIY constant-current dummy load. It is third-party educational material, not evidence of 吃瓜tv manufacturing or a substitute for checking your own component limits.
For PCB manufacturing, prepare the schematic, fabrication files, BOM, quantity, mechanical drawing and test limits. Identify which measurements are required on every unit and which belong to engineering qualification. Request a PCB or assembly review for the load board with the voltage/current envelope and thermal construction. A reviewed board build and a qualified measurement instrument are separate deliverables.