Download 02 Linear Supplies

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
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_____ Notes _____
2.0 Linear Supplies
WebBench by National Semiconductor
http://www.national.com/appinfo/power/webench/help/index.html
Applications Handbook by Kepko http://www.kepcopower.com/hbk.htm
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.
7800 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
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Linear Power Supplies
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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.
 R 
Vout  Vref 1  2 
R1 

Adjustable Floating Regulator
The output voltage of a floating regulator can also be made adjustable.
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An Introduction to Power Supplies
Linear Power Supplies
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V

Vo  Vxx   xx  I o  R2
 R1

2.2
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
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Linear Power Supplies
_____ Notes _____
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.
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An Introduction to Power Supplies
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
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Linear Power Supplies
_____ Notes _____
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
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An Introduction to Power Supplies
Linear Power Supplies
_____ Notes _____
An Introduction to Power Supplies
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Linear Power Supplies
_____ Notes _____
Assignment Questions
Quick Quiz
1.
2.
Composition Questions
To answer these questions, it will be necessary to do some research.
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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?
An Introduction to Power Supplies
Linear Power Supplies
_____ Notes _____
For Further Research
http://www.national.com/appinfo/power/
http://www.kepcopower.com/fowler.htm
http://my.integritynet.com.au/purdic/
http://www.marlow.com/design.htm
http://www.epanorama.net/psu.html
http://sound.westhost.com/power-supplies.htm
Heat Sinks
http://archives.e-insite.net/archives/ednmag/reg/1995/101295/21df3.htm
An Introduction to Power Supplies
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