Power Supplies for Test and Measurement
Laboratory power supplies are fundamental test equipment that provide controlled electrical power to circuits and devices under test. Unlike production power supplies designed for specific voltage and current requirements, bench power supplies offer adjustable outputs, precise regulation, and comprehensive protection features that make them indispensable for electronics development, debugging, and validation. From simple single-output units powering hobby projects to sophisticated multi-channel programmable supplies in automated test systems, these instruments enable engineers to characterize device behavior across operating conditions.
Modern bench power supplies incorporate features far beyond basic voltage and current delivery. Programmable interfaces enable automated testing sequences, while advanced measurement capabilities provide insight into device power consumption. Protection features guard both the power supply and the device under test from fault conditions. Understanding power supply specifications, features, and operating characteristics enables effective selection and application of these essential laboratory tools.
This guide covers the types of laboratory power supplies, their key specifications and features, selection criteria for different applications, and best practices for safe and effective use in electronics testing.
Types of Laboratory Power Supplies
Laboratory power supplies are categorized by their output characteristics, regulation method, and intended application. Understanding these distinctions helps engineers select appropriate equipment for specific testing requirements.
Linear Power Supplies
Linear power supplies regulate output voltage using a series pass transistor that operates in its linear region, dropping excess voltage as heat. This straightforward approach provides excellent output characteristics including very low noise, typically below 1 mV RMS, fast transient response to load changes, low output impedance across a wide frequency range, and minimal electromagnetic interference generation.
The primary disadvantage of linear supplies is efficiency, which depends on the ratio of output voltage to input voltage. At low output settings relative to maximum capability, significant power dissipates as heat in the pass transistor. This limits practical power ratings and requires substantial heat sinking. Linear supplies remain preferred for sensitive analog circuits, precision measurements, and applications where noise performance is critical.
Switching Power Supplies
Switching power supplies use high-frequency switching regulators that chop DC into AC, transform to the required voltage level, then rectify and filter to produce DC output. This approach achieves high efficiency, typically 85-95 percent regardless of output voltage setting, enabling compact designs with high power density.
Switching supplies inherently generate electrical noise at the switching frequency and its harmonics. Modern designs minimize this through careful filtering and shielding, but noise levels typically exceed linear supplies. Transient response may also be slower as control loops accommodate switching behavior. Switching supplies suit applications where efficiency, compact size, or high power requirements take precedence over ultimate noise performance.
Single-Output and Multi-Output Supplies
Single-output supplies provide one independently adjustable voltage and current. They range from simple bench units for general-purpose use to high-precision sources for calibration and metrology. Multi-output supplies integrate two, three, or more independent outputs in a single chassis, sharing common controls, displays, and interfaces while reducing bench space requirements.
Common multi-output configurations include dual supplies providing positive and negative voltages for operational amplifier circuits, triple supplies adding a fixed or adjustable logic voltage to dual analog supplies, and quad or higher channel counts for complex systems requiring multiple supply rails. Some multi-output supplies offer tracking modes where outputs maintain fixed ratios or common adjustment.
Programmable Power Supplies
Programmable supplies accept digital control via computer interfaces including GPIB, USB, RS-232, Ethernet, or proprietary protocols. Programmability enables automated test sequences with voltage and current profiling over time, integration with test automation software and systems, remote monitoring and logging of power supply status, and sequenced startup and shutdown procedures for multi-rail systems.
Advanced programmable supplies support arbitrary waveform generation, simulating battery discharge profiles, AC ripple on DC rails, or power supply transients. These capabilities enable thorough characterization of device behavior under varied power conditions without requiring multiple specialized test instruments.
Specialized Power Supply Types
Several specialized categories address specific testing requirements. High-voltage supplies provide outputs ranging from hundreds to thousands of volts for insulation testing, tube circuits, or high-voltage component characterization. High-current supplies deliver tens to hundreds of amperes for power electronics, motor testing, or electrochemical applications. Bipolar supplies source and sink current, enabling battery simulation, regenerative load testing, or four-quadrant operation.
Source measure units (SMUs) combine precision voltage and current sourcing with measurement capability in a single instrument. SMUs characterize semiconductor devices, measure low-level signals, and provide stimulus-response testing with exceptional accuracy. While more expensive than basic supplies, SMUs offer measurement capabilities that would otherwise require separate instrumentation.
Key Specifications and Performance Characteristics
Power supply specifications define performance capabilities and limitations. Understanding these parameters enables appropriate selection and application.
Output Voltage and Current Ratings
Maximum voltage and current ratings define the operating envelope. Most supplies operate as constant-voltage sources at currents below the limit and transition to constant-current mode when the load attempts to draw excess current. The hyperbolic power curve defines the simultaneous voltage-current operating region, with maximum power available only at specific voltage-current combinations.
Minimum output voltage may be greater than zero for supplies using certain regulation topologies. Resolution of voltage and current adjustment, specified in millivolts or microamperes, determines how finely outputs can be set. Display resolution may exceed adjustment resolution, showing more digits than can be meaningfully controlled.
Regulation Specifications
Load regulation specifies output voltage change as load current varies from zero to maximum at constant input voltage and temperature. Line regulation specifies output change as input AC line voltage varies across its specified range. Both are typically specified as percentages or absolute voltage values, with high-quality supplies achieving better than 0.01 percent regulation.
Programming accuracy defines how closely the actual output matches the programmed value. This specification accounts for calibration drift, temperature effects, and digital-to-analog conversion errors in programmable supplies. Readback accuracy separately specifies measurement precision for displays or digital interfaces.
Noise and Ripple
Output noise and ripple specifications describe AC components superimposed on the DC output. Ripple refers to periodic components at line frequency (50/60 Hz) and switching frequency, while noise encompasses broadband random variations. Specifications may state RMS values, peak-to-peak values, or both, measured under specific bandwidth conditions.
Common-mode noise specifications address high-frequency noise between outputs and earth ground, relevant for sensitive measurements where ground loops or radiated interference are concerns. Low common-mode noise is essential for instrumentation amplifier applications and precision measurements.
Transient Response
Transient response characterizes output behavior during load steps. Specifications include recovery time to return within a defined voltage band following a step load change, voltage deviation amplitude during the transient, and current slew rate limiting to prevent damage to sensitive loads. Fast transient response requires low output capacitance and rapid control loop response but may compromise noise performance.
Protection Features
Protection specifications define how supplies respond to fault conditions. Overvoltage protection (OVP) trips when output exceeds a set threshold, protecting loads from excessive voltage. Overcurrent protection (OCP) limits or shuts down output when current exceeds the set limit. Overtemperature protection (OTP) shuts down when internal temperatures exceed safe limits.
Protection response behaviors include constant-current limiting that reduces voltage to maintain set current, foldback limiting that reduces both voltage and current under overload, shutdown that latches output off until manually reset, and hiccup mode that repeatedly attempts restart following fault clearance.
Advanced Features and Capabilities
Modern laboratory power supplies incorporate features that extend functionality beyond basic voltage and current delivery.
Output Sequencing
Multi-output supplies often require specific power-up and power-down sequences to prevent damage to devices under test. Sequencing features enable programming delays between channel activations, defining dependency relationships between outputs, establishing ramp rates for gradual voltage application, and setting timeout and fault conditions for sequence steps.
Proper sequencing is essential when testing devices with multiple supply rails where incorrect power application order can cause latch-up, excessive current draw, or permanent damage.
Measurement and Monitoring
Integrated measurement capabilities provide insight into device power consumption without external instrumentation. Features include simultaneous voltage and current measurement on all channels, power computation from voltage and current products, data logging of parameters over time, and triggered acquisition synchronized to external events or programmed conditions.
Resolution and accuracy of built-in measurements vary significantly across supply models. High-precision applications may require external meters even when supplies include measurement displays.
List Mode and Waveform Generation
List mode enables programming sequences of voltage, current, and timing values that execute automatically. Applications include battery simulation with discharge curves, power supply ripple injection for immunity testing, dynamic voltage scaling for processor power management testing, and stress testing with voltage excursions.
Some supplies generate arbitrary waveforms on their outputs, enabling simulation of complex power conditions including startup transients, brownouts, and power line disturbances.
Remote Sensing
Remote sensing compensates for voltage drop in cables connecting the supply to the load. Separate sense leads measure voltage directly at the load rather than at the supply output terminals. The supply control loop then adjusts output to maintain the programmed voltage at the sense point despite cable resistance.
Remote sensing is essential when load currents and cable resistances would otherwise cause unacceptable voltage drop. Four-wire connections (force and sense for each polarity) are standard for precision applications. Sense lead resistance specifications and maximum cable drop compensation capabilities limit remote sensing effectiveness.
Parallel and Series Operation
Multiple supplies can be combined for higher current (parallel) or higher voltage (series) than single units provide. Auto-parallel features enable automatic current sharing among paralleled supplies. Master-slave configurations designate one supply to control output while slaves follow.
Series operation requires supplies rated for the resulting voltage stress to chassis ground. Not all supplies support series operation, and those that do specify maximum floating voltage between output and ground.
Interface and Control Options
Interface capabilities determine how power supplies integrate with test systems and automation environments.
Front Panel Controls
Manual controls range from rotary encoders with numeric keypads to touchscreen interfaces with graphical displays. Key usability considerations include adjustment resolution and responsiveness, display visibility of all relevant parameters, logical menu structure for configuration access, and quick access to common functions like output enable and protection reset.
Digital Communication Interfaces
Common interfaces include GPIB (IEEE-488) which remains standard in automated test equipment, providing reliable communication in electrically noisy environments. USB offers convenient connection to computers without special interface cards. Ethernet enables network-connected operation with multiple simultaneous connections and web-based control. RS-232 provides simple serial communication for basic remote control needs.
Command sets may follow SCPI (Standard Commands for Programmable Instruments) conventions for interoperability or use proprietary protocols requiring manufacturer-specific drivers. SCPI compliance enables easier substitution of equivalent instruments and integration with standard test software.
Analog Programming
Some supplies accept analog control signals (0-5V or 0-10V) for voltage and current programming. Analog programming provides fast response for closed-loop control applications, enables simple interfacing with external controllers, and supports proportional control from sensors or other analog sources. However, accuracy and noise immunity are typically inferior to digital programming.
Trigger and Synchronization
Trigger inputs and outputs enable synchronization with other test equipment. Applications include starting transient tests synchronized with oscilloscope captures, coordinating multi-supply sequences in complex test systems, and triggering data acquisition on power supply events. Both hardware trigger lines and software triggers via digital interfaces are common.
Selection Criteria and Applications
Choosing appropriate power supplies requires matching instrument capabilities to application requirements.
General Bench Use
General-purpose bench supplies serve diverse debugging and development needs. Key selection factors include sufficient voltage and current range for anticipated projects, multiple outputs for common multi-rail requirements, reasonable accuracy and noise for typical analog and digital circuits, intuitive controls for frequent manual adjustment, and moderate cost for budget-conscious laboratories.
Precision and Calibration Applications
High-precision applications demand supplies with exceptional accuracy and stability. Requirements include traceable calibration with documented uncertainty, very low noise for sensitive analog measurements, excellent temperature stability for consistent operation, four-wire sensing for accurate voltage at the load, and fine resolution for incremental testing and characterization.
Automated Test Systems
Production test and automated characterization systems require supplies with comprehensive programmability, fast settling for throughput optimization, reliable protection to prevent device damage during unattended operation, standard interfaces for software integration, and proven reliability for continuous operation.
High-Power Applications
Testing power electronics, motors, or high-current devices requires supplies delivering hundreds of watts to kilowatts. Considerations include efficiency to minimize heat generation and operating costs, adequate cooling for the installation environment, appropriate protection response for high-current faults, and safe interconnection methods for high currents.
Battery Simulation
Battery-powered device testing benefits from supplies that simulate battery characteristics including internal resistance simulation matching battery impedance, discharge curve programming for realistic battery behavior, fast response to dynamic load changes, and bidirectional capability for charging circuit tests.
Safe Operation and Best Practices
Safe and effective power supply use requires attention to proper procedures and potential hazards.
Initial Setup and Verification
Before connecting loads, verify supply output with a multimeter to confirm expected voltage, check current limit operation by briefly shorting output, set protection thresholds appropriate for the device under test, and ensure proper grounding of supply chassis. These precautions prevent damage to devices under test from configuration errors or supply malfunctions.
Connection Practices
Proper interconnection ensures safe operation and accurate power delivery. Use wire gauges appropriate for expected currents to minimize voltage drop. Secure all connections to prevent intermittent contact that could cause arcing or noise. Implement remote sensing when cable drop would affect results. Verify polarity before energizing connections.
Protection Configuration
Set protection thresholds based on device ratings rather than supply maximums. Allow appropriate margin for transient conditions while providing meaningful protection. Understand whether protection latches off or auto-recovers, as each behavior suits different testing scenarios.
Thermal Considerations
Power supplies generate heat that requires adequate ventilation. Ensure cooling air paths are unobstructed, maintain clearance from adjacent equipment, monitor ambient temperatures in enclosed installations, and consider duty cycle effects for supplies operating near thermal limits.
Maintenance and Calibration
Regular calibration maintains measurement accuracy essential for valid test results. Calibration intervals depend on accuracy requirements and supply stability specifications. Performance verification between calibrations confirms continued operation within specifications.
Troubleshooting Common Issues
Understanding common power supply problems aids efficient diagnosis and resolution.
Output Regulation Problems
Poor regulation may indicate overloaded conditions exceeding supply capacity, failed regulation circuitry requiring service, high-impedance connections introducing resistance, or insufficient input voltage for linear supplies. Comparing actual output to specifications under known loads helps isolate causes.
Noise Issues
Excessive output noise may originate from normal switching noise exceeding application requirements, ground loops in the measurement or load circuit, degraded filter capacitors requiring replacement, or external interference coupled into supply or load. Measurement technique affects noise observations, and proper probing practices are essential for valid characterization.
Protection Tripping
Frequent protection activation suggests load faults drawing excessive current, protection thresholds set below normal operating requirements, inrush currents exceeding momentary capability, or supply fault requiring service. Monitoring current waveforms during operation helps identify whether load behavior or supply problems cause tripping.
Conclusion
Laboratory power supplies are essential test equipment enabling controlled power delivery for electronics development, testing, and manufacturing. The range of available types, from simple linear bench supplies to sophisticated programmable sources, addresses diverse application requirements. Understanding specifications, features, and operating characteristics enables effective selection and application of these fundamental instruments.
Key considerations in power supply selection include matching voltage, current, and power capabilities to application needs, selecting regulation type appropriate for noise and efficiency requirements, ensuring adequate protection features for the devices under test, and choosing interfaces that enable required automation and integration. Following best practices for setup, connection, and operation ensures safe use while maintaining the measurement integrity essential for valid test results.
As electronic devices continue advancing in complexity and power requirements, laboratory power supplies evolve correspondingly. Enhanced programming capabilities, improved measurement functions, and sophisticated protection features expand the role of power supplies from simple energy sources to intelligent test instruments that contribute directly to device characterization and validation.