Introduction

When you have a project, you usually need a power supply. Sometimes you have a general purpose supply that allows you to vary the voltage/current. But sometimes you need to build the supply into your project. I look at power power supplies as two separate sections (P1 and P2 in the drawing). The AC to DC Rectifier/Filter section and the Regulator section. While this page only deals with the low voltage (5-24V) AC to DC Rectifier/Filter, some of the same principals apply to AC to DC Rectifier/Filters for high voltages.

So this page provides an analysis of various rectifier configurations. The user can specify transformer and load characteristics and visualize what can be expected. It is assumed that these rectifier configurations are to be used with external regulators. So the specified ripple is a trade off between very large capacitors and dropout from the regulators.

There are effectively three AC to DC Rectifier/Filter configurations. Half-Wave, Full-Wave Center Tapped, and Full-Wave Bridge. This page presents all three of them along with some variations, like the Double Bridge configuration.

Terms

The drawing on the right shows a single cycle of a pure sine wave, as you might see it with an oscilloscope. The drawing also shows the relationship of the various terms used to describe AC voltages. Where:

• V_P-P = AC Voltage Peak-to-Peak
• V_P = AC Voltage Peak
• V_RMS = AC Voltage Root-Mean-Square
• V_AV = AC Voltage Average

If you view an AC Sine Wave with a oscilloscope, you will see the Peak-to-Peak value V_P-P. But, if you read the AC voltage with a meter, digital or analog, you will see the Root Mean Square value, V_RMS. Mathematically, V_RMS is equal to (V_P-P / 2) * 0.707. Or, the other way around, V_P-P is equal to (V_RMS * 1.414) * 2. Some meters may have the capability to measure other AC voltage parameters, but that is for another discussion.

Transformer

Winding	Resistance
Primary	4 - 8 Ω
High-voltage	200-400 Ω
H-V center tap	Half the resistance of the two outer leads
5-volt rectifier	less than 1 Ω
6.3 volt filament	less than 1 Ω

Each of the power supply analyzers below has space for entering the transformer specification. If you know that information, then enter it in the space provided. However, if you don't know the exact voltage and resistance, you can estimate it using the chart on the right.

The power supply analyzers use the initial assumptions the use of a Filament Transformer with a Primary of 117 VAC at 4 Ω and a Secondry of 12.6 VAC at 0.1 Ω. These type of transformers will be able to supply about 2-3 Amps. Generally, higher current transformers will have lower primary and secondary resistances.

Note: If you attempt to determine the winding resistances using and ohm meter, be very careful. Initially connecting the meter will induce a voltage, which will be transformed by the other windings. This means that when you are measuring low voltage windings, higher voltage windings will have a high voltage. It probably won't kill you, but it will make you take notice.

Load Configurations

All of the analyzers below, except for the Double Bridge Analyzer, have 5 possible load configuration. The configuration are listed below.

• Res. - Rectifiers driving a resistive load with pulsating DC.
• Cap./Res. - Rectifiers driving a capacitor and resistive load.
• Cap./Reg. - Rectifiers driving a capacitor and regulator load. Only LM78xx and LM78Lxx series of regulators are implemented.
• Cap./Bat - Rectifiers driving a capacitor with series current limiting resistor and battery.
• Bat. - Rectifiers driving a series current limiting resistor and battery.

Capacitance

In configurations that use a capacitor (Cap./Res., Cap./Reg. and Cap./Bat.), the Capacitor Voltage rating listed on the schematic is calculated at roughly twice VCMAX. Anything above VCMAX is acceptible.

The default capacitance is 1,000 µF. Note that, as the load current requirement increases (R_LOAD decreases), a higher capacitance will be needed to keep the Ripple Voltage small.

In configurations that "do not" use a capacitor (Res. and Bat.), the Steps and Iterations entry areas will be ignored. Internally, Steps will be fixed at 360 and Iterations will be fixed at 1. It is usually not necessary to change these settings as the defaults are sufficient.

While not shown, the Cap./Reg. configuration should include a 0.33 uF capacitor on the regulator input and a 0.1 uF capacitor on the regulator output.

Load/Charging Resistor

The Res., Cap./Res. and Cap./Reg. configurations show a load resistor. This resistor is not a real resistor, but is there to represent your actual current requirements. To work with a higher current, lower the value of R_LOAD.

The Cap./Bat. and Bat. configurations will have a R_CHARGE resistor. This is a series resistor for limiting the battery charging current. Do not exceed the charging recomendations for the battery in use.

Steps/Iterations

For analysys, Steps, specifies the number of pieces that each AC cycle is divided by. For example, setting Steps to 180 will analyze the circuit every "2" degrees of each AC cycle. Iterations then determines the number of AC cycles. Configurations with capacitances need multiple AC cycles for the capacitors to charge to a steady value. For the Res., Cap./Res. and Cap./Reg. configurations, IAVE, the average diode current, and ICHECK, should be equal. If not, increase number of iterations. The default values should be sufficient for any of the analysys.

Half-Wave Power Supply Analyzer

This calculator analyzes the performance a Half-Wave power supply with several load configurations, using the Time Step Method. The analyzer accounts for dynamic diode drop and transformer winding resistance.

Initially, select the Load Configuration. Then enter the Primary and Secondary information for the transformer. Finally, select the Diode Type and enter the Load Resistance, R_LOAD. For the Battery Charger configurations, the Charging Resistance will be R_CHARGE.

If a configuration does not require a particular input, the input data is simply ignored. Changing one of those ignored inputs will not affect the calculation. For example, the Res. configuration (Resistor Load) will ignore the Regulator, Battery V, and C1 input selections.

For the Cap./Res., Cap./Reg. and Cap./Bat. configurations, if I_AVE, the average diode current, does not equal I_CHECK, increase number of iterations.

Full-Wave Center Tapped Power Supply Analyzer

This calculator analyzes the performance a Full-Wave Center Tapped power supply with several load configurations, using the Time Step Method. The analyzer accounts for dynamic diode drop and transformer winding resistance.

Initially, select the Load Configuration. Then enter the Primary and Secondary information for the transformer. Finally, select the Diode Type and enter the Load Resistance, R_LOAD. For the Battery Charger configurations, the Charging Resistance will be R_CHARGE.

If a configuration does not require a particular input, the input data is simply ignored. Changing one of those ignored inputs will not affect the calculation. For example, the Res. configuration (Resistor Load) will ignore the Regulator, Battery V, and C1 input selections.

For the Cap./Res., Cap./Reg. and Cap./Bat. configurations, if I_AVE, the average diode current, does not equal I_CHECK, increase number of iterations.

Full-Wave Bridge Power Supply Analyzer

This calculator analyzes the performance a Full-Wave Bridge power supply with several load configurations, using the Time Step Method. The analyzer accounts for dynamic diode drop and transformer winding resistance.

Initially, select the Load Configuration. Then enter the Primary and Secondary information for the transformer. Finally, select the Diode Type and enter the Load Resistance, R_LOAD. For the Battery Charger configurations, the Charging Resistance will be R_CHARGE.

If a configuration does not require a particular input, the input data is simply ignored. Changing one of those ignored inputs will not affect the calculation. For example, the Res. configuration (Resistor Load) will ignore the Regulator, Battery V, and C1 input selections.

For the Cap./Res., Cap./Reg. and Cap./Bat. configurations, if I_AVE, the average diode current, does not equal I_CHECK, increase number of iterations.

Double Bridge Power Supply Analyzer

This section presents a method of obtaining a power supply with dual voltages (equal + and -) using a transformer that does not have a center tap.

This analyzer is similar to the others on this page in that, the user needs to supply the transformer information and basic requirements. Enter the Primary and Secondary information for the transformer, and enter the Load Resistances, RL₁ and RL₂.

Note that the capacitors values are a direct product of the Ripple value. Small ripple requirements, require very large capacitors. It would be best to lower your ripple requirements and follow this supply with a regulator. Then you just have to keep the ripple requirements low enough so that the regulator does not experience input dropout.

As an example, consider using this configuration with a LM7812 and LM7912 regulator. Each regulator would require 14 volts input, to prevent the output from falling out of regulation. With the default values provided (5% Ripple), the positive voltage is barely sufficient and the negative voltage may be too low to sustain regulation. However reducing the ripple requirement to 4%, and increasing the capacitor values (C1 & C4 = 12,000 µF, C2 & C3 = 36,000 µF) the voltages will be sufficient. That is, of course, as long as the current requirements do not exceed that specified by the load resistors.

Also note that, for dual voltage power supply, for say driving OP-Amps, the negative side of RL₁ and the positive side of RL₂ should be tied together to provide a common ground.

Configurations with a Center-Tapped Transformer

The Full-Wave rectifier, shown above, uses a center tapped transformer. The center tap is connected to ground and used as a reference for the output voltage. This section shows a couple of other power supply configurations using a diode bridge and the addition of a center tapped transformer. The circuits shown employ a combination of Full-Wave and Bridge rectifier circuits.

In the Dual V configuration, the voltage at +V1 is obtained from the Bridge circuit (D1 thru D4). The voltage at +V2 is the normal voltage obtained by the Full-Wave circuit (D2 and D4).

The +/- V configuration uses two Full-Wave circuits to generate two equal, and opposite voltage. That is, assuming the transformer center tap is used as a ground reference. And that, there is equal voltage on both sides of the center tap.

The only real difference between them is which output is considered ground. The Dual V configuration is a combination Full Wave and Full Wave Bridge and generates two voltages.