Table of Contents. 2.0 Linear Supplies 2.1 Fixed DC Regulator 2.2 Variable DC Regulator. Assignment Questions. For Further Research

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1 Table of Contents 2.0 Linear Supplies 2.1 Fixed DC Regulator 2.2 Variable DC Regulator Assignment Questions For Further Research An Introduction to Power Supplies i

3 Notes 2.0 Linear Supplies WebBench by National Semiconductor Applications Handbook by Kepko AN-1148 Linear Regulators by National Semiconductor 2.1 Fixed DC Regulator The 7800 and 7900 series three terminal regulators are very simple to use Series Positive Regulators 7900 Series Negative Regulators The 7800 series regulators are available from 5 24 volts. In order to regulate properly, the input voltage must be at least 2 volts higher than the output. An Introduction to Power Supplies 1-1

4 Notes Linear Power Supplies These regulators can dissipate approximately 2 watts without a heat sink and 15 watts if on an infinite heatsink. If the junction temperature exceeds a preset threshold, the device will automatically shut down. Floating Regulator The 7800 family can be used to create a higher output voltage than its designation suggests, if the device is floated. V out V R 1 + R = 2 ref 1 Adjustable Floating Regulator The output voltage of a floating regulator can also be made adjustable. 1-2 An Introduction to Power Supplies

5 Linear Power Supplies Notes V o = V xx V + R xx 1 + I o R Variable DC Regulator A linear regulator consists of four basic components: Series Pass (Control) element Sampling circuit (voltage divider) Reference voltage Error amplifier Simplified Linear Regulator Schematic An Introduction to Power Supplies 1-3

6 Notes Linear Power Supplies The 723 regulator is a relatively easy to use, full featured device. National Semiconductor LM723 Regulator Motorola MC1723C Regulator 723 Low Power Regulator One of the projects which the students build at Heritage, is a 0 to 25 volt linear power supply based on the 723 chip. 723 Block Diagram Internal Equivalent Circuit The 723 linear regulator contains all of the components required to make a basic variable linear regulator, except for the voltage divider. This IC also includes additional features such as current limiting and the ability to drive an external series pass transistor for high-current applications. 1-4 An Introduction to Power Supplies

7 Linear Power Supplies Notes Basic 723 Application Circuit Boosting Output Current Adding a series pass device does not affect the regulated output voltage. Since the feedback is taken from the output, the output current driver can largely be ignored. If the input voltage is derived from a potentiometer between the reference voltage and ground, the output can be varied between 0 and the reference voltage multiplied by the gain of the circuit. There is a practical limit to this circuit, which is determined by the maximum voltage rating of the IC (40 volts). An Introduction to Power Supplies 1-5

8 Notes Linear Power Supplies Boosting Output Voltage Although the 723 will tolerate a maximum input voltage of 40 volts, it can only source a high current regulated output of about 25 volts. This happens because a 40-volt peak, fully rectified sine wave, only has an average dc value of about 25 volts. In order to overcome this limitation, a pre-regulator can be used to provide a semi-regulated higher average dc voltage before it is applied to the 723 regulator. This will prevent the 723 from being exposed to the peak voltage. Project Power Supply LM317 Linear Regulator LM317 Product Folder by National Semiconductor LM317 by Fairchild LM317 Product Folder by Texas Instruments 1-6 An Introduction to Power Supplies

9 Linear Power Supplies Notes An Introduction to Power Supplies 1-7

10 Notes Linear Power Supplies Assignment Questions Quick Quiz Composition Questions To answer these questions, it will be necessary to do some research. 1. Sketch the general block diagram of a linear regulator and label its components. 2. Illustrate how to modify a three terminal regulator to make it a floating regulator. 3. Design a circuit to modify a 7815 regulator to produce a 30 volt output. 4. What is the maximum input voltage to a 723 regulator? 5. What is the maximum output voltage of a 723 regulator and why? 1-8 An Introduction to Power Supplies

11 Linear Power Supplies Notes For Further Research Heat Sinks An Introduction to Power Supplies 1-9

Block diagram of Basic Three Terminal IC Regulator The figure shows the functional block diagram of basic three terminal IC regulator.

Block diagram of Basic Three Terminal IC Regulator The figure shows the functional block diagram of basic three terminal IC regulator. Three Terminal Fixed Voltage Regulators As the name suggests, three terminal voltage regulators have three terminals namely input which is unregulated (V in ), regulated output (V o ) and common or a ground