Power Supplies

Power Supplies

Power Supplies deliver electric power to a device or circuit (Electrical Load), usually by stepping down a higher voltage. Every power supply has an input or supply power and converts it by various methods to meet the demand from the electrical load. Power supplies are said to be Regulated by controlling the output voltage or current to a constant value, even if the load demand current or the supply power varies. At the heart of any power supply is a Transformer, which converts the supply power to the output power to the load.

What are AC, DC, and AC/DC Power Supplies?
AC Power Supplies step down AC Voltages. For example, 220 VAC to 24 VAC.
DC Power Supplies step down the voltage and convert it to DC. For example, 220 VAC to 12 VDC.
AC/DC Power Supplies have both AC and DC outputs for connection to different loads.

Electronic Loads. Electronic loads are instruments that simulate electronic device behavior for manufacturing and laboratory research purposes. For example, the characterizing of DC Power supplies, DC-DC Converters, batteries, fuel cells and solar cells.  Many allow high speed response and load characteristic and sequencing simulation needed for modern electronics.

Uninterruptable Power Supplies obtain their power from batteries to provide backup power to deliver it to computers and other critical equipment and use AC power to charge their batteries and power equipment when it is available.

• Voltage and current output to the electronic load
• Length of time before exhausting batteries
• Stability of the output
• Selecting a Power Supply

How to select an Adjustable Power Supply. Consider the following:

Know your input supply voltage. Check to make sure the power supply will work with the supply voltage. Some are universal, meaning they will work over a large range like 100-240 VAC, others have an external or internal switch for 110 VAC/230 VAC operation. You may need to look in the instruction manual to be sure.

Consider a multiple output power supply

• Second and third (sometimes even fourth) output options reduce bench space and cost of having additional power supplies. When selecting multiple output power supplies, read the details of the specifications. The additional outputs may be limited by the overall wattage of the power supply (i.e. you may not be able to operate all channels at maximum voltage and current simultaneously).
• Look at whether the additional channels are adjustable. Often the third and fourth outputs are fixed or limited adjustment.
• Look at the convenience of using the display. Some models will display voltage and current for all channels simultaneously.
• Series and Parallel Operation permits the output channels.
  • Tracking series operation doubles the Voltage capacity by internally connecting CH1 (Master) and CH2 (Slave) in serial and combining the output to a single channel. CH1 (Master) controls the combined Voltage output level.
  • Parallel operation, for example, doubles the current capacity by internally connecting CH1 (Master) and CH2 (Slave) in parallel and combining the output to a single channel with CH1 controlling the combined output.

Consider a Programmable Power Supply. Programmable power supplies can be remotely controlled by an input signal, trigger, or PC (by USB, RS-232, or GPIB communications) for automated use. They are often used in manufacturing and laboratory applications because of their flexibility. Some have multiple outputs.

Do you need a stepped pattern or PC communications? If you use LabVIEW, consider getting a programmable power supply with LabVIEW drivers even if not required for the immediate application. Have the capability for future applications. Some offer password protections for settings useful in manufacturing and other applications to prevent changes.

For DC Power Supplies look at whether it is a Linear or Switching Power Supply
What is the difference between DC Linear and Switch Mode Power Supplies (SMPS)?

• Linear Power Supplies are a simpler design and lower in cost and efficiency. Voltage is stepped down, then rectified and filtered. At higher currents, the step down translates to larger footprint and heavier components: transformers, heat sinks and cooling fans.
• A Switch Mode Power Supply has the AC supply rectified and filtered to obtain DC voltage. Voltage on/off switching at high frequency (10 KHz – 1 MHz) is what permits a switch mode power supply to step down the voltage transferring magnetic energy from the primary windings to secondary windings in the transformer with a smaller transformer and other components than a linear power supply.
  • It also improves efficiency by up to 40%. If the load current increases, for example, the output voltage drops and the duty cycle frequency is increased to compensate and maintain constant voltage.
  • The higher cost comes from the added complexity.
  • Another negative is the electromagnetic interference (EMI) noise.  Better switch mode power supplies do a better job in combating EMI noise.

Check the analog or digital display; display resolution; accuracy; ripple and noise; and recovery time specifications.

Look for common power supply protections in the specifications. They will protect the power supply and the load being connected.

• Over current
• Over voltage
• Over temperature protection
• Reverse polarity

Remote Sensing. Connections for one or all channels called remote sense terminals that compensate for voltage drops in the power supply leads. This helps to ensure that the correct voltage is delivered accurately to the load terminals of the DUT. Many multi-channel power supplies do not provide remote sensing, which degrades from overall system accuracy.