Linear Power Supply Manual

By: Eric Unterhoffer

For: Roger Hicks

Class: Industrial Techniques 243-304

Due: May 5th 2003

Table of Contents

  1. Introduction: about the Linear power supply. (Page 3)
  1. Operating instructions. (Page 4)
  1. Design specifications. (Page 5 – 11)
  1. Schematic and layout of the Linear power supply.

(Pages 12-13)

  1. PC draft design for metal casing of box. (Page 14-17)
  1. Photographs: (Interior) (Page 18)
  1. Measurements (Page 19)
  1. Part list (Page 20)
  1. Safety precautions (Page 21)
  1. Data sheets

Introduction

This Linear power supply is capable of a 0-25 volt regulation, with a 1 amp

current capacity. This particular linear power supply also has the capability to provide

two separate fixed voltages of 12 volts and 5 volts. The brains of this Linear power

supply is given through a LM723 voltage regulator, but it also has the capability to

provide two separate fixed voltages through two separate fixed regulators, the LM7805T

and the LM7812T. The LM723 is used to adjust both the voltage and the current with in your power supply. The Lm723 has a thermal capacity of -55°C to 150°C. This regulator also has a minimum 2v output, just like the 7800 series regulators. The Lm723 regulator includes, Vref, error amplification, current limitation and control element capabilities. The 7800 series regulators have a max thermal capacity of 65°C per watt, with a max of 2 watts unless properly heat sinked. (You must not forget to include room temperature to the 65°C temperature because, if the total temperature of the regulator exceeds around 120°C temperature the chip may self destruct. A key point to these regulators is that they must always stay above 2v of the output voltage, according to their data sheet. It is also necessary to provide a filter along with the regulators simply for stability. Among these aspects this linear power supply also has safety protection through not only its fuses, but also through its power transistor, which will shut down if it reaches a heat threshold of 120 degrees centigrade. Among these safety features and various capabilities, this linear power supply also has two meters and two potentiometers, one for voltages and one for currents in order to deliver precisely the desired voltages and currents needed.

Operating Instructions

First ensure that both fuses provided with this linear power supply are intact,

otherwise the system will not function. Next plug the Linear power supply into the 120

volts rms provided by hydro in order to power the device. Once power is delivered to the

system, simply press the on button or switch provided on the face of the outer casing to

activate the system. Once the system is on, retrieve two plugs, one for positive (red) and

one for ground (black), in order to complete the circuit, and plug them into the holes

located to the right side of the box. Then change the voltage using the knob on the face of

the box located to the right to receive the desired voltage level. You can also adjust the

current level using the knob on the left. If a 5 volt or 12 volt voltage supply is desired,

simply plug one cord into the center hole of three located to the left of the box to provide

ground (black), and then plug the other cord into one of the other two holes 5-volt supply

(blue) or 12 volt supply (yellow).

Design Specifications

The design for this power supply is actually made up of four sections, all of which provide equal importance to the function ability of this linear power supply. The four sections include: Transformation, rectification, filtration and regulation.

Transformation:

Transformer Wave result in secondary winding

The transformer is an amazing invention. It uses electromagnetic fields in order to reduce or increase input voltages by specific ratios in its design. Transformers can either be referred to as step up or step down transformers. In the case of this power supply, a step down transformer was used. This is because it was necessary to reduce the AC input of 120v rms coming from hydro to allow the circuit to function. In this case, a step down transformer with a 4.6:1 turns ratio was used. This essentially means that only a ¼ of the voltage is in the secondary portion of the transformer from what was originally produced. So in actuality the voltage going into this circuit was 120v AC, and after the voltage was brought to the secondary portion it was reduced to 26.09v. (This is if the transformer was 100% efficient).

(This means that my circuit can vary from 0-27vrms)

Transformer equivalent circuit

Transformer Equations

1)

2)

3)

4)

Rectification:

Types of rectifiers Waves created form rectifier

The signal that first enters into the diode bridge is a full sine wave. The diodes are what divide the sine wave into a half wave so that the signal delivered is only the top portion of the wave, thus cleaning up the signal. This happens because D1 and D3 are reverse biased, this means that the voltage in the circuit is not able to flow down through the diodes. The current on the other hand is able to flow through these two diodes. D2 and D4 are what create the rectified wave necessary for the fully rectified wave. This step is done so that the AC voltage being provided by hydro will be turned to DC. This is important because it would make the power supply useless, the voltage would constantly be fluctuating while the system was in use. It’s important to remember the 0.7v diode drop across each diode in order to know your exact secondary voltage before regulation.

Rectification Equations

To find out the average value of a wave:

(The reason for the 0.636 is because only 63.6%

of the wave is used in rms).

To find out the Rms value of a wave:

Filtration:

Waves created

This diagram indicates how the capacitor smoothes out the rectified wave in order to provide a cleaner signal. This would be desirable in high quality products such as amplifiers.

This is a half rectified wave. This is a rectified wave that only involves one diode rather then four like in a fully rectified wave. The advantage of this form of rectification is that there is only a loss of one diode drop.

The whole purpose of the filter is simply to smooth out the rectified wave that was produced by the diode in the diode bridge. This filtration is all done with a simple capacitor. You can see in the diagram how at point V3 the wave turns much smoother then originally. The capacitor takes the useless parts of the wave produced from the signal and reduces it to something that is almost flat, for easier use. You can also see how greatly the current is affected by this capacitor, the current gets totally inverted in diagram (A3). Due to the capacitor. A2 is produced as a result of the diodes pulsing from the AC current. A3 gets inverted because the diode bridge has surplus current. So when the bridge shuts down the regulator still demands current. So the current comes out of the capacitor causing a current inversion. (The flat section of A3 is when the capacitor is supplying load current).

Regulation:

There are four sections, which make up the filtration portion of this power supply. These sections include: Reference voltage, error amplifier, output sampling, control element and current limiting.

1) Reference Voltage:

The reference voltage in the Lm723 regulator was 7.1v. This is important aspect of the actual chip. Vref will tell you whether your chip is functioning correctly or not. You can be certain that if your reference voltage has diminished, that your chip has been destroyed and rendered useless. The Vref is actually a fixed voltage integrated into the chip. This is used in order to vary Vout. Vref is what drives the error amplifier.

Equations

2) Error amplifier:

The error amplifier is another important part of the power supply circuit. This is pin 13 of the Lm723, but it is in fact an amplifier that is built into the chip. This is necessary because it is used in the voltage regulator. Essentially the error amplifier takes the sampling circuit and the reference voltage and uses these to allow the circuit’s voltage to be adjusted. There is a resistor and a capacitor attached to this for stability or frequency compensation. It should be noted that when testing your circuit, the error output should always remain the same or close to it. This is so because the whole purpose of the error amplifier is to replace the missing voltage form the reference voltage pin to produce the necessary 25v.

Ex. If vref were only producing 10v, then the error amplifier would produce the other 15v to bump the total voltage to 25v.

3) Output Sampling:

The output of the circuit should be able to vary from 0-25v, depending on the turn’s ratio of your transformer. This used to sample the difference between the actual voltage and the error amplifier.

4) Control element:

The control element is used to regulate the voltage to your desired level required. It can vary from 0 to vref. This is so because if you look at the circuit below, the min of the potentiometer is connected to pin6 of the chip, which is vref. Basically the way that the voltage regulator works is by adjusting the potentiometer in either a positive or negative direction to increase or decrease voltage levels. The Darlington pair is part of what allows the regulation to occur.

5) Current limiting:

The current regulator works along side with the voltage regulator. This works much differently from the voltage adjustment. The current limiter is not connected directly to the voltage output, instead it is connected to two transistors or a Darlington pair as well as the VC input of the adjustable voltage regulator[a1][a1]. This is important to have because if the current rises higher then it is supposed to then the current limiter will shut down to prevent damage.

Photographs

Interior View

Measurements

Transistors
Q1 / Q2 / Q3
B / 42.8 V / B / 41.9 V / B / 29.3 V
C / 42 V / C / 29.4 V / C / 43 V
E / 43.3 V / E / 42.5 V / E / 29.1 V
LM723 V Regulator
Pin / Voltage
4 / 6.2 V
5 / 6.1 V
6 / 7.9 V
7 / 0 V
9 / 0 V
10 / 6.9 V
11 / 31.4 V
12 / 42.8 V
13 / 8.2 V
Regulators
7815 IN / 21.5 V
7815 OUT / 12.5 V
7805 IN / 13.3 V
7805 OUT / 5.4 V
Transformer
Primary / 123.2 V
Secondary / 21.7 V
Rectification
Power / 43.6 V
Ground / 0 V
Diodes/LED
LED A / 2.3 V
LED K / 0 V
1N4003 A / 43.3 V
1N4003 K / 42.5 V

Parts list

Component Part Number Description

T1 166L25 Transformer – 115V to 25V CT @ 2 A

Q1, Q3 2N4403 T0-92 PNP small signal transistor

Q2 2N3055 T0-3 NPN power transistor

D1-D4 1N5402 Rectifier diodes 3A 200 PIV

D5 1N4003 Rectifier diode 1A 200 PIV

U1 LM7812T Fixed voltage regulator (+12V, 1A)

U2 LM7805T Fixed voltage regulator (+5V, 1A)

U3 LM723 Adjustable voltage regulator (DIP 14L)

R1 3K9 ½ W 5% Carbon resistor

R2 3K3 ¼ W 5% carbon resistor

R3 0.56  1 W Carbon resistor

R4 100K ¼ W 5% carbon resistor

R5 3K9 ¼ W 5% carbon resistor

R6, R9 22K ¼ W 5% carbon resistor

R7, R10 82K ¼ W 5% carbon resistor

R8 4K7 ¼ W 5% carbon resistor

R11 47 ¼ W wire wound resistor

R12 1K ¼ W 5% carbon resistor

VR1 500R Linear potentiometer (Panel mount)

VR2 5K Linear potentiometer (Panel mount)

C1 2200uf 35V Electrolytic capacitor (PC mount)

C2 10uf 16V Electrolytic capacitor (PC mount)

C3 10uf 10V Electrolytic capacitor (PC mount)

C4 2200uf 50V Electrolytic capacitor (PC mount)

C5, C6 0.1uf Ceramic capacitors (PC mount)

C7 10uf 16V Electrolytic capacitor (PC mount)

S1 125 VAC Switch (SPDT panel mount)

F1 1A Fuse

F2 0.25A Fuse

J1-J6 Panel mounted banana jacks

J7 14 DIP IC Socket

J8, J9 Fuse holders (Panel mount)

P1 Power cord

SR1 Strain relief bushing for cord

M1 LED mounting kit

M2 T0-3 transistor mounting kit

M3, M4 T0-220 transistor mounting kit

VM Meter 0-25V panel mount volt meter

AM Meter 0-3A panel mount Amp meter

Safety Precautions

This Linear power supply is designed relatively safely. If at any time you get shocked by the metal casing or in any other fashion, there may be a ground problem. Be aware of this because there is a lot of power involved with this power supply. If you do get shocked, unplug the box immediately and open the top of the box, and make sure that the neutral wire from hydro is properly grounded, as well as the green terminal output. Never touch the interior of this power supply if ON or plugged in. Also, if testing this power supply while live, be sure not to allow the heat sink of the regulators touch the casing of the power supply because this will cause arcing. Also be sure to not touch two uncommon tracks at the same time. When using this power supply, always be sure that your current and voltage is at the proper level for your use. If it is too high it could cause components to burn out, or even explode.

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