Product Focus: Laboratory Power Supplies
Power supplies provide steady, precisely controlled electrical energy to electronic equipment.
Power supplies provide steady, precisely controlled electrical energy to electronic equipment. Anyone who has built or worked with a desktop computer recognizes the power supply— typically the bulkiest component— sitting behind the electrical inlet socket.
One type of supply, the uninterruptible power supply, powers electronics in the event of an emergency such as a blackout by keeping a battery constantly charged. When the main power goes out, the battery takes over before the equipment can power down.
Uninterruptible power supplies are rather uncommon in most labs, except for stand-alone computer equipment that might suffer from data loss. Academic and industrial research organizations usually have house backup generators for heavy equipment and instrumentation, and backups for both computer power and data.
Most common lab instruments have their own integrated power supplies, which enable users to plug them into a common electrical wall outlet. The power supply first uses a rectifier to convert house alternating current (AC) into direct current (DC), the only type that common instruments and devices use. It then adjusts the current and/or voltage upward or downward to meet the needs of the instrument. Throughout the device’s circuitry, additional power circuits step up or step down the current according to the various subcircuits’ needs.
One way to differentiate power supplies is to look at how they operate. Switching power supplies use a switching generator to harness efficiencies when converting electrical energy from the grid’s AC to specific DC current and voltage requirements. Switching supplies work by rapidly turning on and off. Linear power supplies operate at constant, precisely controlled voltages.
Switching power supplies are common as embedded supplies for personal computers because of their high efficiency and small footprint, but they tend to be electrically noisy and don’t regulate as well as linear power supplies, says David Pereles, marketing segment manager at Tektronix (Beaverton, OR).
Stand-alone power supplies, including uninterruptible supplies, do make sense in non-electronics labs when a lot of equipment runs on the same voltage, says Mark Swift, business development manager at Universal Electric (Canonsburg, PA). “It’s easier to provide the right electrical energy from a single piece of gear.” Other benefits are ease of connectivity and protection against heat or loss of power.
Physics and chemistry labs, particularly R&D and academic labs, use power supplies to drive basic equipment, for example in electroplating and electrochemistry work, or to supply direct current in much the same way as a battery. Biology laboratories, for example, employ power supplies to drive gel electrophoresis equipment.
Electrical/electronics lab power supplies come in three major types, depending on the work being done. Constant voltage supplies provide configurable DC voltage that is adjustable over a specific range that includes zero voltage. Constant current supplies output-regulated current independent of the voltage. Constant voltage/ constant current devices provide either voltage or current.
But by far the most common venues for power supplies are electrical testing, design, and fabrication laboratories. As they are being built, prototype circuits lack a pre-packaged power supply, so the designer will utilize one of the three power supply types according to the circuit’s specifications. Power supplies are also used to design and troubleshoot subcircuits, and to repair or diagnose problems with instruments or circuits whose power supplies have failed.
Varying voltage or current is common when prototyping an electrical circuit to do a particular task, such as timing, counting, or signal processing. “Certain circuits rely on variable voltage thresholds,” Mr. Pereles says. “Changing the voltage is useful when performing a comparison of how the circuit operates at different voltages, or in supplying accurate reference voltages or currents.”
Purchase decisions
Mr. Pereles says that choosing the right power supply should follow a “fairly predictable sequence or decision tree.” The first consideration is what voltage and current are needed. “The answers to those two questions will narrow the field quite a bit.”
Next, users should consider how many outputs they need. An uninterruptible power supply connected to a single computer might only need half a dozen, while a supply shared by two or three technicians in a repair or testing facility should have two or three times that number.
Price is often a second-tier consideration, since most laboratory bench units supplying outputs on the order of 30V and 5A sell within a narrow price range, typically $400 to $1,200.
Then come the finer points: accuracy, output noise, the ability to respond to load changes, stability when the AC voltage changes, and variable output. “Certain lab procedures may require changing the current or voltage over time,” says Mr. Pereles. Some power supplies are computer-controlled for such operations and require a separate voltage or current meter. Some have these functionalities built in, as well as microprocessor control. “Power supplies are getting smarter, displays are getting better, and the units generally can do more with each succeeding generation,” Mr. Pereles observes.