Javascript Electronic Notebook
Un-Regulated Power Supply
by Martin E. Meserve

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Introduction

Description

Initial Calculations

Secondary Voltage

Load Currents

Final Design

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 the diagram below, is kind of like a pre-conditioner for the Regulator. AC Input voltage is converted from the power mains (117 VAC) using Transformer (T1), converted to DC with the Rectifiers Diodes (D1-D4), and then filtered by the Electrolytic Capacitor (C2).

This web page takes you through the steps of designing the AC to DC Rectifier/Filter part of a power supply. It allows you to define some initial information, like current and voltage required, and then calculates the necesary transformer specifications, rectifier diode specs, and filter capacitor requirements. The program basically follows the same structure as similar to a program in Ham Calc, written by George Murphy, VE3ERP.

It is assumed that, power supplies designed with the aid of this web page will plug into 117 VAC. If you are designing for another primary voltage you will need to adjust the specifications accordingly.

In many cases you may wish to convert to voltages higher than the input voltage, but this web page is really intended for working with power supplies that convert to lower voltages. You can still use this web page for making high voltage power supplies but you won't be able to use the LM317 Regulator for anything over 37 Volts.

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Introduction

Description

Initial Calculations

Secondary Voltage

Load Currents

Final Design

Program Description

This web page is split into several parts.....

In the Initial Transformer Calculations section you are asked to supply an output voltage which is then used to calculate the optimum transformer for the job. It is assumed that the output voltage, under no load, will be 1.41 times the full load output voltage. This factor is used to calculate the regulation percentage and is due to a Full-Wave Bridge Rectifier being used.

The previous step is likely to compute a non-standard transformer voltage rating. In the Transformer Secondary Voltage section you are asked to supply a more standard output voltage. The voltage specified should be greater than, or equal to, the calculated value, from the previous section. A new set of power supply output voltage specifications are then computed. The output might be higher than originally intended but you can follow this pre-conditioner with a LM317 Voltage Regulator to obtain an more exact voltage. Just make note of the Full Load Output Voltage and the DC Output Voltage and transfer them to the regulator design.

Then, in the section on Load Currents you are asked to supply your expected maximum current drain and your expected DC ripple percentage. 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. This data was extracted from a program in Ham Calc, written by George Murphy, VE3ERP.

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%

The value of a filter capacitor to achieve these specifications will be calculated. Almost invariably, this is going to be a non-standard value. Following the calculations, you are asked to supply a more standard value and new ripple percentages will be calculated.

If you don't have a single capacitor near the calculated value, one or more capacitors can be placed in parallel to approximate the calculated value. For example, if 10,500 uF is the calculated value, two 5,000 uF capacitors can be connected in parallel to make 10,000 uF. This value would then be entered for the standard value.

The Final Design section presents you with complete design information. A fuse and bleeder resistor are included for safety purposes and a LED/Resistor combination is included as a visual power-on indicator.

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Introduction

Description

Initial Calculations

Secondary Voltage

Load Currents

Final Design

Initial Transformer/Load Calculations

In the text box below, enter the required power supply DC output voltage. You do this so that the web page can calculate the necessary transformer and load specifications.

Required DC Output Voltage (Volts)

The chart on the right lists the Primary/Secondary transformer voltages based on the specified DC Output Voltage. Then, using your specified DC Output Voltage, the No Load DC Output Voltage is calculated along with the Load Percentage. If you already have a particular transformer output voltage in mind, than skip over this section and go to the section for specifying the Transformer Secondary Voltage.

Transformer Specifications for a Un-Regulated Power Supply with x Volts DC Output

Primary Voltage -

x Volts AC

Secondary Voltage -

x Volts AC

No Load DC Output -

x Volts DC

Full Load DC Output -

x Volts DC

Load Regulation -

x %

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Introduction

Description

Initial Calculations

Secondary Voltage

Load Currents

Final Design

Standard Transformer Secondary Output Voltage

In the text box below, enter the Transformer Secondary Voltage for the transformer that you wish to use. The voltage entered should be greater than, or equal to, the voltage calculated in the previous section.

Transformer Secondary AC Voltage

The chart on the right lists the power supply DC output voltages based on the AC secondary voltage that you specified. Further transformer specifications will be in the Final Design section.

Un-Regulated Power Supply Specifications using a Transformer with x Volts AC Output

Primary Voltage -

x Volts AC

Secondary Voltage -

x Volts AC

No Load DC Output -

x Volts DC

Full Load DC Output -

x Volts DC

Load Regulation -

x %

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Introduction

Description

Initial Calculations

Secondary Voltage

Load Currents

Final Design

Load Current Calculations and Standard Value Capacitor

In this section you are asked to define your Maximum DC Current requirements and the Maximum Ripple Voltage that you require. Not that, the ripple percentage, presented in this section, is not the same thing a the load regulation percentage mentioned earlier.

The output of the bridge rectifier is a pulsating DC voltage and the capacitor on the output 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 capacitors 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.

Use the text boxes, on the right, to enter your Maximum DC Current and Maximum Ripple Voltage requirements. The text below these boxes will then reitterate your requirements and the value of the capacitance required to meet these requirements. The Maximum Ripple Voltage should be expressed in a percentage. 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.

DC Output Current (Amps)

Maximum Ripple Percentage (0-100 %)

A DC Output Current of x and a Maximum Ripple Percentage of x % would require a Filter Capacitor, C1, of x uF @ x WVDC.

Now, in the text box below, enter the value of the capacitor you will actually be using. As previously mentioned, one or more standard capacitors can be placed in parallel to approximate the calculated value. Entering a value less than the calculated value will cause a ripple percentage that is higher than previously specified, at the specified current.

Standard Value Capacitor (uF)

(No commas please)

Note that the filter capacitor will not have much effect on the load regulation. To get better load regulation use the power supply designed here and follow it with a LM317 Voltage Regulator. Make sure the the output voltage, from the power supply on this page is at least 2 volts higher than final required output voltage.

Top

Introduction

Description

Initial Calculations

Secondary Voltage

Load Currents

Final Design

Final Power Supply Design Data

Listed below is the final power supply design data based on the information you supplied above. Any changes in the design requirements will be automatically reflected in the output below. The parts list below is for a power supply with a no-load output of x, a Full Load Output of x and a Ripple Percentage of x.

Final Parts List

Designator

Description

Designator

Description

T1

x

C1

x

R1

x

D1-D4

x

R2

x

D5

x

F1

x