Understanding the Basics of DC Power Supplies

BAS interface devices (such as output transducers, multiplexers and transmitters) typically require 24 VDC power. Since building power is 120 volts or greater and of the AC variety, a power supply is required to obtain the 24 volts DC necessary to operate these devices. Accurate selection and sizing of the DC power supply are very important to ensure proper operation of the powered devices. However, this procedure is sometimes neglected. Power supplies used for BAS control devices perform several important functions including: voltage transformation, rectification, filtering, and regulation.

1. Voltage Transformation
   The first step is to convert the building AC line voltage into another more suitable voltage. A transformer is used to step down the AC line voltage to 24 VAC. The transformer also provides some isolation by electrically separating the AC neutral or ground and the power supply output common.
2. Rectification
   The stepped down 24 VAC voltage is then rectified or converted into a pulsating DC voltage. A common method of making this conversion is to use a bridge rectifier. The bridge rectifier consists of four diodes arranged as shown in Figure 1 and produces a voltage wave form as shown in Figure 2.
3. Filtering
   The pulsating full wave rectified DC voltage is smoothed by a filter capacitor. This capacitor is a large-value electrolytic capacitor which holds its charge between half cycles of the rectified DC voltage and produces the voltage shown in Figure 2.
4. Regulation
   The filtered DC voltage is controlled by a linear regulator to give a constant output voltage, even with variations of the input line voltage, the output load and temperature. The linear regulator is usually an IC regulator but may also be a discrete circuit when certain performance characteristics must be enhanced. The regulator also helps to suppress any output ripple voltage by means of its regulating action. Ripple is the magnitude (usually measured in millivolts peak-to-peak) of AC voltage on the output of a power supply as the result of feed through of the rectified line frequency.

A regulated power supply is best suited to meet the power requirements of electronic circuits found on BAS interface devices. On a supply that is rectified and filtered but unregulated, the output voltage of the supply will vary with the load. A lightly loaded, unregulated power supply will cause the output voltage to increase. A heavily loaded, unregulated supply will cause a decrease in voltage output. Fluctuations in the input voltage will also directly affect the output of an unregulated power supply. Because many electronic devices cannot tolerate these variations in voltage, it is best to use only regulated supplies. The voltage produced by rectifying the AC voltage with a bridge rectifier (but without any filtering or regulation) is not suitable for powering electronic circuits.

When selecting a DC power supply, determine the input voltage that is available. If this is line voltage, a power supply with a built-in transformer can be used. A power supply without a built-in transformer (e.g., Kele model DCP-1.5-W) can also be used by providing a separate transformer. As a rule of thumb, this input transformer’s VA rating at 24 VAC should be 43.2 multiplied by the desired DC output amps. To determine the DC output current, add the current draw of all devices to be powered from the supply. The DC power supply should then be selected with an output current rating equal to or greater than the sum of the current requirements of all the devices powered from it.