Introduction

Power Supplies generally consist of two parts, the AC to DC Rectifier/Filter followed by the Regulator. The AC to DC Rectifier/Filter, described here and shown in Figure 1, is kind of like a pre-conditioner for the Regulator.

AC Input voltage is converted from the power mains (110-120 VAC) using Transformer (T1), converted to DC using a Full-Wave Bridge Rectifier made up of Diodes (D1-D4), and then filtered by the Electrolytic Capacitor (C1).

The output of the Full-Wave Bridge Rectifier (D1-D4) is a pulsating DC voltage and the Electrolytic Capacitor (C1) is used as a storage device. During positive transitions of the DC voltage, some of the current goes directly to the load and some gets stored in the capacitor. In between these positive transitions, the capacitor provides the current to the load. This effect smooths the DC voltage waveform. In general, the higher the value of capacitance, the more energy the capacitor can store and thus, the less ripple on the output DC voltage waveform.

CW transmitter multipliers & amplifiers	5% max.
Linear amplifier plate voltage	3% max.
Linear amplifier bias supply	1% max.
VFOs, speech amplifiers and receivers	0.01% - 0.1%
Non-critical audio devices	1% - 10%
Devices not requiring DC smoothing	10% - 100%

R1 is a bleeder resistor to insure that the capacitor does not store a charge for long after the main power is removed. R2 and D5 (Light Emitting Diode)are used as a "Power On" indicator.

The data in the table can be used as as a rough guide, when specifying the required ripple percentages. It gives you a general idea of the ripple percentages that can be tolerated by certain pieces of equipment.

It is assumed that, power supplies designed with the aid of this web page will plug into 110-120 VAC. If you are designing for another primary voltage you will need to adjust the specifications accordingly.

Un-Regulated Power Supply Design

The table below is for describing your initial load conditions. The first two input areas help define the power transformer and rectifiers. The other areas help to further define the filter requirements. As you change the requirements, the annotation on the schematic will be updated.

For the Full Load DC Output Voltage of 13.8 V, the Transformer's Minimum Secondary Voltage is 10.78 VAC. Enter a Transformer Secondary Voltage that is greater than, or equal to that voltage. Note that, entering a secondary voltage that is greater than the minimum will increase the No Load and Full Load DC Output Voltages. The transformer secondary should be capabale of 1.03 A, 13 VA.

With a DC Output Load Current of 1.00 A and a Maximum DC Ripple of 5%, a minimum Filter Capacitor value of 4,724 uF @ 18 VDC is recommended. Enter a capacitor value near the specified value.

Note: Using a transformer that has a output voltage greater than the specified minimum voltage will increase the full load and no load output voltages. This may drive a requirement for a regulator circuit on the output. If the current requirements are low (<100 ma), a simple Zener diode regulator may be all that you need. However, for higher currents you will probably need a pass transistor. See my web page on Zener Regulation.

Note: If this power supply section is going to drive a regulator, make sure you account for the offset voltage (~2.0 Volts) required by the regulator.

Notes

Full Load DC Output Voltage is the minimum DC output voltage required, under full load. The minimum secondary voltage is calculated and displayed in the description above.

Transformer Secondary Voltage is the actual secondary voltage for the transformer that you will be using.

DC Output Load Current is the maximum DC current that you expect to draw from the power supply. This value is used to calculate the transformer's current and Volt-Amp (VA) ratings, plus the fusing requirements. A Full-Wave Bridge arrangement is assumed for the rectifier, which is made up of Diodes (D1-D4).

Max DC Ripple is the percentage of AC ripple riding on the DC Output Voltage. This is not the same thing as the Load Regulation percentage. The DC Ripple will be the same for all load conditions, whereas the output voltage will change as the load is changed from minimum to maximum. These two values drives the capacitor value calculation. High DC output current and low ripple percentages mean that the required capacitance is going to be big.

Filter Capacitor is the capacitance value for the capacitor (C1) that you will be using. Make sure that the capacitor meets or exceeds the listed WVDC (Working Voltage, DC). If you are using multiple capacitors in parallel to make up the required capacitance, enter the total value of all capacitors.