Introduction to Bench Power Supplies

Basic Introduction to: Bench Top Power Supplies

Basic Information about Bench Top Power Supplies

Greg Von Rehder - Product Marketing Director

B&K Precision Corporation

 

Types of Power Supplies

AC Power Supply – Alternating current power supply.

DC Power Supply – Direct current power supply.

Fixed Power Supply – The output voltage and current is fixed at a specified voltage.

Linear DC Power Supply – Use heavy iron core transformers. They provide very clean DC power and usually cost more than a switching power supply with the same power rating.

Switching DC Power Supply – Use electronic circuits to convert AC to DC power. They are light weight and can produce large amounts of power in a small form factor. They are usually less expensive then linear power supplies with the same power rating. One downside to switching power is they produce more ripple and noise on the output when compared to a linear supply.

Variable Power Supply – The output voltage and/or current is adjustable or settable.

Theory of Operation

AC Power Supplies: An AC power supply essentially isolates the power from the wall outlet to provide safe servicing of "hot chassis" (transformerless) AC-powered equipment. This "isolation" provides protection for the person servicing the equipment and protection to the test equip­ment used in servicing the equipment.

DC Power Supplies: Basically a DC Power Supply converts AC electricity obtained from the wall socket to direct current power. This is done by reducing the AC voltage via a step-down transformer, rectifying the voltage to pulsating DC, and then filtering the pulsating DC voltage to a pure DC voltage. DC Power Supplies also contain a voltage regulator that maintains a constant voltage at the load, even when the amount of load varies.

Function

A power supply provides a primary source of electrical energy (volts & amps) to a circuit or to an entire electronic device. Electrical energy may be in the form of DC (direct current) or AC (alternating current).

Limitations

Power supplies are limited by their power rating stated by the maximum voltage and current the unit can provide.

Applications

Power supplies are used for:

- Circuit Design

- Troubleshooting

- Circuit Repair

- Electroplating

Key Terms

CONSTANT CURRENT SOURCE - A regulated power supply that delivers a constant current to a load, even when the load resistance changes.

CONSTANT VOLTAGE SOURCE - A regulated power supply that delivers a constant voltage to a load even when the load resistance changes.

CURRENT LIMITING - Ability to limit maximum current output at a preset value. This feature helps protect the load from over-current damage.

ISOLATION - Removed from earth ground.

LEAKAGE - Undesirable current flow from the AC line to exposed metal parts of an electrical appliance (such as televisions, radios, etc.). Expressed in microamperes (uA). Authorities generally agree that leakage current greater than 500uA (or 1/2000 A) represents a safety hazard.

LINE REGULATION - Specification that states how much the load voltage or current changes when the power supply is operated at varying line voltages throughout a given range. The specifica­tion is typically stated as a percentage of the total voltage or current available from the supply. A rating of ^"0%^" would mean perfect regulation.

LINE VOLTAGE - Line voltage is the AC power source used to power the power supply. Usually it refers to the voltage from a wall socket.

LOAD - Anything connected to the output of the power supply.

LOAD REGULATION- Specification that states how much the load voltage or current changes between operating the power supply at no-load and full-load conditions. The specification is typically stated as a percentage of the total voltage or current available from the supply. A rating of "0%" would mean perfect regulation.

OVERLOAD PROTECTION - Means by which a power supply is protected from permanent damage due to short circuits, excessive loads, or reverse polarities connected across the load terminals. Protection may be as simple as a fuse (which can be economically replaced), or may be electronic protection circuitry which automatically monitors load conditions as well as power supply component temperatures. Electrically protected power supplies automatically reset to normal operation once the offending condition is removed from the load terminals, requiring no replacement of blown fuses.

POWER REQUIREMENTS - The line voltage that the power supply requires to operate. High quality power supplies have a selector switch that per­mits operation from 110, 120, 220, and 240 VAC sources. Some switching power supplies have the ability to accept 90 to 240VAC line voltages with out having to use a selector switch.

REGULATION - The ability to maintain a cons­tant voltage or current at the load despite changes in line voltage or load resistance.

RIPPLE VOLTAGE - The portion of unfiltered AC voltage and noise present at the output of a filtered power supply. When stated as a specifica­tion, the power supply is operated at full load and the remaining impure portion of the DC voltage is measured. Specification is typically stated as an rms and peak-to-peak AC voltage (with zero ripple voltage representing a perfect DC power supply).

VA - Abbreviation for volt-ampere. Unit of input power delivered to a load. For electronic equip­ment, the "VA" load imposed on the isolation transformer or AC power supply is simply the load voltage multiplied by the load current, or the wat­tage rating of the load.