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Thursday, 16 March 2017

+15V & -15V Regulated Dual Polarity Power Supply Circuit

Admin 09:20 0

Introduction

Power supplies are important part of electronic circuits. Many electronic circuits and components require negative voltage in addition to positive voltage. An example can be operational amplifiers. They require +15 and -15 for biasing. Similarly other components also require negative voltage. Providing negative voltage is always been a challenge. If a circuit requires both positive and negative voltage than providing two power supplies for a single circuit increases cost and size of electronic circuit. Due to this reason lots of useful circuits and components are regarded as impractical because of dual polarity voltage requirement. The solution to this problem is given in this article by presenting a regulated dual polarity power supply circuit. The design uses single power supply circuit to supply voltage of dual polarity. Its negative voltage power supply as well as positive voltage power supply in one circuit.

It must be noted that this article is about dual polarity power supply. Same configuration can be used to build dual voltage power supply which is explained in this article Dual voltage power supply

Components and ratings of dual regulated power supply

Following components are required for dual polarity power supply.

SPST Switch
Fuse 1A
4 Diodes 1N4007
Transformer 220/25V
Capacitors 2200uF, 0.33uF and 0.1uF (2 each)
Voltage regulator IC LM7815 & LM7915
Veropins (Optional)

1A fuse is used to provide protection against high current currents. The rating of fuse depends upon the rating of other electronic components. A center tap transformer capable of reducing 220V to 25V is need in dual polarity power supply. Diodes are used to construct bridge rectifier. Since voltage is step down to 25V, PIV (peak inverse rating) of each diode is 50V since double voltage appears across each diode in rectification process.

Circuit diagram: Positive & Negative voltage power supply circuit

Following figure shows circuit diagram for dual regulated power supply.

connection diagram for dual polarity power supply circuit

Transformer steps down 220V 50Hz voltage to 25V and gives it to the bridge configuration for rectification. Bridge circuit converts AC to pulsating DC. In order acquire smooth constant DC voltage, smoothing capacitor of 2200 uF is installed next to the bridge. Till this point a 25V DC unregulated voltage is achieved. To get regulated 15V supply, voltage regulator IC 7815 is used. In order to understand more about LM7815 click here.

On both sides of voltage regulator IC, bypass capacitors (0.33uF and 0.1uF) are connected to provide optimum stability and helps in correct transient response. The values of bypass capacitors are provided in voltage regulator datasheet. The output of LM7815 gives regulat
ed 15V. For negative voltage, the same configuration as for positive voltage is connected with back end of diode bridge configuration. So LM7915 along with its parallel capacitors captures negative cycle of AC supply and converts into negative DC voltage.

Voltage regulator IC can also be used to make constant current source which is explained in this articleConstant current source using LM317

Simulation of Dual polarity power supply

Above explained circuit is simulated in proteus ISIS and the result is shown

simulation Positive and negative voltage power supply

simulation of dual regulated power supply

It can be seen that one side porivdes +15V and second terminal provides -15V.

Simulation Video

PLC Tutorial: Star delta starter for induction motors

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Admin 09:16 0

Introduction to star delta motor starter

When electric motor is started, it draws a high current typical 5-6 times greater than normal current. In DC motors there is no back emf at starting therefore initial current is very high as compared to the normal current. This concept is explained in detail in my article

Three point starter for DC motor.

In induction motor, the winding or rotor are normally shorted. Also there is no opposing flux against the actual flux. As a result motor draws high current in starting. High starting current of Induction motor is explained in

Starting of induction motors

Working principle of star delta starter

The following figure shows the star delta configuration

Figure A shows the star configuration. If the motor windings are connected in configuration, the total voltage applied to each winding will be

$V_{ph}=\ \frac{1}{\sqrt{3\ }}\ V_{Line}$

So the total applied voltage will be reduced to 58% of total line voltage. For instance of total or line voltage is 400V, than in star configuration the total voltage applied to each winding is 220V.

On the other hand, the total voltage applied to each winding in delta configuration will be full line voltage.

Due to the reduced applied voltage in star configuration, the total starting current will be 1/sqrt(3) or 58% of the total current in delta configuration.

Star Delta starter diagram

The following figure shows the winding connections in star and delta configuration one by one.

windings in star and delta configuration

It can be seen that in star connection, one end of all three windings are shorted to make star point while other end of each winding is connected to power supply. In delta configuration, the windings are connected such that to make a close loop. The connection of each winding is shown in above figure. In actual motor the three phase connections are provided in the following order as shown

So in order to make winding connection in star and delta style in practical motor, the connection is shown below.

star and delta connections of motor winding for star delta starter

To change the motor connections from star to delta, contactors are employed. To practically shift connection of motor from star to delta in running posito, three contactors are required. It is shown in the following star delta starter connection diagram

Main contractor is used to supply power to the windings. It must be turned on all the time. Initially the star contactor is closed while delta contactor is open It makes the motor windings in star configuration. When the motor gains speed, the star contactor is opened while delta contactor is closed turning the motor windings into delta configuration.

The contactors are controlled by using PLC. The following section of PLC tutorial will explain the ladder programming for star delta motor starter.

PLC Tutorial program for star delta motor starter

The ladder program for the star delta starter is written in LOGOSOFT PLC software as shown in following figure

plc tutorial for star delta motor starter

Rung 1 Main contactor : The main contactor depends upon the normally open input start push button (I1), normally closed stop button (I2) and normally closed overload relay. It means that Main contactor will only be energized if start button is pressed, while stop is not pressed and overload relay is not activated. A normally open input named (Q1) is added in parallel to the start button I1. By doing so, a push button is created which means that once motor is started, it will be kept started even if start button is released

Programing of push button and other requirements for simple motor starter is explained in PLC Tutorial: Motor starter

Rung 2 Star contactor: Star contactor depends upon main contactor, normally close contacts of timer (T1), and normally close contacts of output delta contactor (Q3). So star contactor will only be energized if main contactor is ON, time output is not activated and delta contactor is not energized.

Timer T1: Timer T1 measures the time after which the winding connection of star delta starter is to be changed. It will start counting time after main contactor is energized.

Rung 3 Delta contactor: Delta contactor will be energized when main contactor (Q1) is energized, timer T1 is activated and star contactor (Q3) is de-energized.

Following video will show the plc tutorial of star delta motor starter in LOGO SOFT PLC software.

Temperature 3 Wire RTD meansurement circuit / transmitter

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Admin 09:09 0

Issues in 3 wire RTD temperature transmitter.

RTD varies its resistance with temperature which is not directly a measurable quantity. Change in RTD resistance with one unit of change in temperature is very less. It is very difficult to measure this little change.
Change in resistance with temperature is not linear therefore one single equation cannot predict correct temperature by sensing the change in resistance.
The resistance due to wire leads and connections can lead to false readings.

Properties of 3 wire RTD resistance temperature transmitter circuit.

Such RTD measurement circuit is needed which can convert this resistance into measurable quantity like voltage or current.
Therefore such RTD transmitter circuit is required which can not only correctly sense little variation in resistance but also amplify it to such level so that it can be utilized easily.
RTD transmitter should be able to neutralize the nonlinear relation between temperature resistances.
RTD measurement circuit should employ 3 wire connection method of RTD thermocouple so that possibility of false reading is eliminated.
RTD transmitter circuit should have the facility to adjust the amplification of signal for ease in sensing the signal.
Temperature 3 wire RTD meansurement circuit should have the facility to be calibrated at any point.

Working of 3 wire RTD meansurement circuit

Components which are required for RTD transmitter circuit are as follows

Input source (5V)
Fixed resistors of various values
Variable resistors ( values depend upon requirement. 100k and 20k used in this circuit
Operational amplifiers 741 or any suitable for this making instrumentation amplifier. IC having built in instrumentation amplifier can also be used.

For the sake of understanding, we divide the circuit into two main parts.
1) Bridge circuit

2) Instrumentation amplifier

1) Bridge circuit.

It consists of four resistors connected in Wheatstone bridge configuration. Three resistors R1, R2 and R3 are fixed resistors while fourth resistor is RTD Pt 100 thermocouple. How bridge circuit works and the whole theory behind it is not the scope of this article. To understand general concept of bridge circuit please visit the following link Analysis of bridge circuit
Bridge circuit allows both 2 wire and 3 wire connection RTD Pt 100 as shown in circuit diagram.

bridge circuit for 3 wire rtd transmitter circuit

The values of other 3 resistors can be used to fix the reference value (temperature). By reference value we mean that voltage difference between point A and B i-e V_AB will be zero at reference temperature. For instance if 0 ᵒC is to be set as reference temperature , the corresponding resistance of PT-100 at 0 ᵒC is 100Ω. By selecting other 3 resistors of 100Ω, voltage V_AB= 0 at 0 ᵒC. It is because same current flows in both branches of bridge and same voltage drop occurs in R3 and RTD PT-100.

Now if the 100 ᵒC is to be selected as reference value than such values of resistors is to be selected so that V_AB= 0 at 100 ᵒC. According to data chart of PT-100, its resistance value is 140Ω at 100 ᵒC. So if all the resistors values are selected as 140 Ω than V_AB will be 0 at 100 ᵒC. In this way any temperature can be selected as reference value by selecting appropriate value of resistors.

One other advantage of bridge configuration is that a slight change in resistance values can produces considerable difference in voltage V_AB which can easily be sensed.

2) Instrumentation amplifier

The second part of 3 wire RTD meansurement circuit is instrumentation amplifier. It consists of voltage followers and gain amplifier. To understand the concept and theory of instrumentation amplifier please visit the following explanation of instrumentation amplifier

instrumentation amplifier for 3 wire temperature transmitter

The gain of instrumentation amplifier is given by the following equation

$Gain=\left(1+\ \frac{2RV5}{RV6}\ \right)\ \frac{R9}{R8}$

To adjust the gain RV5 and RV6 can be varied. Due to this reason RV5 and RV6 are kept as variable. This gives user the facility to adjust gain according to exact requirement.

Another advantage of using this arrangement is that input impedance of voltage follower is infinite (very high) so by taking input, it does not disturb the bridge and its resistances.

Using RV5 and RV6 the gain of the amplifier is set at 10.

It means the voltage VAB will appear as 10 times of original at output of 3 wire RTD temperature transmitter circuit.

This value can be given to any controller or circuit to convert it into temperature and display on appropriate screen.

Summary

Following points summarizes the advantages of given 3 Wire RTD transmitter circuit.

Bridge configuration allows 3 wire connection modes which eliminate the error due to lead resistance.
Connecting RTD PT-100 in bridge configuration allows any small change to be sensed.
Any value of temperature can be selected as reference value by choosing appropriate values of resistors.
Voltage follower (instrumentation amplifier) has infinite input impedance which does not disturb actual reading of bridge.
By adjusting gain of instrumentation amplifier, the amplification can be set at required value. Variable resistors allows user to set according to his requirement.

RTD PT100 temperature sensor using Microcontroller

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Admin 09:08 0

PT-100 is a thermocouple which means it changes its resistance with temperature. However microcontroller cannot measure resistance directly. Its analogue pins can only measure voltage. For this reason PT-100 cannot directly be interfaced with microcontroller. It will require some external circuitry to convert resistance into voltage to interface RTD PT100 with microcontroller.

RTD transmitter circuit (external circuitry) must be able to sense variation in PT100 resistance due to change in 1 ᵒC and must have the facility to amplify it so that it can be read by microcontroller.
The detailed requirement, design and explanation can be found in article 3 wire RTD temperature transmitter circuit

RTD PT100 temperature sensor connection diagram

Now questions arise, why bridge circuit is used? Why 100Ω resistors are selected? Why instrumentation amplifier with above stated resistors values is selected? What are the advantages of above arrangement?

All answers to above questions is explained in my article 3 wire RTD temperature transmitter circuit.

Connection diagram for RTD PT100 temperature sensor using microcontroller

Following figure shows the connection diagram for temperature sensor PT100 using microcontroller. PT100 is connected in bridge circuit whose output is amplified by instrumentation amplifier. Using the resistors RV5 and RV6 of instrumentation amplifier, the gain is set at almost 10. Consider the following equation for.

$Gain=\left(1+\ \frac{2RV5}{RV6}\ \right)\ \frac{R9}{R8}$

Since the resistance of PT100 is 100Ω at 0 ᵒC, so by selecting the other resistors of bridge circuit of 100Ω value, 0ᵒC is set as reference point. It is because the output VAB at this temperature will be zero. The detailed explanation of reference value selection is also explained in 3 wire RTD temperature transmitter circuit article.

RTD PT100 temperature sensor using microcontroller connection diagram

The amplified Voltage VAB is given to the analogue pin of microcontroller. The microcontroller after necessary calculation displays temperature on LCD. The necessary calculations are explained in next portion.
LCD is interfaced with port C of microcontroller. To understand LCD interfcing click interfacing LCD with microcontroller.

How to find temperature from the voltage measured by Microcontroller

NOTE: For this article range of temperature to be measured is 0 ᵒ to 100 ᵒC.

Classical method

In this method, resistance is first determined from the measured voltage and temperature is then calculated.

Let us say the voltage V₀ is measured by the microcontroller. Say V₀ = 2V.

Now this is the voltage after multiplying gain “G” of amplifier. In our case gain G =10.

Now to get the actual voltage V_AB between point A and B

$V_{AB}=\ \frac{V_o}{G}=\ \frac{2}{10}=0.2$ …………………………………..1

This is the voltage between point A and B. From this value we can find the voltage drop across PT-100.
One leg of bridge rectifier has both resistors of 100Ω (R1 and R3). So voltage drop across each is 2.5V because input voltage is 5V and it is equally divided between two. It means voltage at point B is always 2.5V.

So voltage across R3 is 2.5V and voltage drop across PT100 should be 2.5+V_AB= 2.5+ 0.2 = 2.7V.

So voltage drop across PT100 V_PT100 = 2.7V

So now using voltage divider circuit, resistance of PT100 can be found

$V_{PT100}=\ \ V_{in}\left(\ \ \frac{R_{PT100}}{R_{PT100}+R2}\ \right)$ …………………………….2

By solving above equation

$R_{PT100}=\left\{\frac{\left(\frac{V_{PT100}}{V_{in}}\right)R_2}{1-\ \left(\frac{V_{PT100}}{V_{in}}\right)\ }\right\}$ ………………………..3

In our case V_in = 5V, R2= 100Ω, V_PT100 = 2.7V ( method shown)

By finding the resistance, we can find the temperature by using temperature coefficient of resistance method.

$R_{P??100}=\ R_{ref}\ (1-a(T-T_{ref})$ …………………4

Since our reference point is T_Ref is 0ᵒC at which resistance of PT100 is R_refis 100 Ω.

Rewriting above equation

$T=\left\{\ \frac{\left(1-\ \frac{R_{PT100}}{R_{ref}}\right)}{a}\ \right\}$ ………………………..5

Where R_PT100is found from equation 3, R_ref= 100 Ω, α= temperature coefficient of resistance and its value can be determined from its datasheet which is 0.00385.

NOTE: The above method is long and contain too many calculations. Also the temperature cofficient of resistance α is not constant and varies with temperature. Therefore in next section we will discuss alternate method.

Direct method

In this method, a direct relation can be determined between the output voltage and temperature. The line equation is used and modified according to requirement. General line equation is

$y\ =mx+C$

Where m = slope, C= y intercept and y = independent quantity x= dependent quantity.

To find the relation between voltage and temperature, output voltage for various values of temperature is found from the transmitter circuit and given in the table.

Temperature T	Bridge output V_AB (mv)	V output of instrumentation amplifier ( V _ABx 10) Volts	Increase in voltage per degree rise in temperature ( VAB / T) (volts/ ᵒC) Slope (m)
0	0	0	N/A
5	24	24	0.0048
10	47.4	48	0.0047
15	70.4	72	0.0046
20	92.5	95	0.0046
25	115	117	0.0046
30	137	139	0.0045
35	158	161	0.0045
40	179	182	0.0044
45	199	203	0.0044
50	219	223	0.0043
55	239	243	0.0043
60	258	263	0.0043
65	277	282	0.0042
70	295	301	0.0042
75	314	320	0.0041
80	331	338	0.0041
85	350	357	0.0041
90	367	374	0.0040
95	384	391	0.0040
100	400	409	0.0040

Since at 0ᵒC, V_AB = 0. So put Y intercept C=0 in line equation

So equations becomes

$T=\ \frac{V_{AB\ }(in\ volts)}{m}$

It can be seen from the table that value of m (slope) varies with temperature so a single value cannot be used. For each 10 ᵒC range, separate value of m is used. This is simplest form of linearization. Other techniques of linearization can also be used.

This article uses the direct relationship between temperature and voltage to construct microcontroller based temperature sensor using PT100.

Simulation and result for microcontroller temperature sensor using RTD PT100

Above circuit is simulated in ISIS Proteus and results are shown below. RTD temperature sensor using microcontroller is checked for different values of temperature and results are accurate with maximum error up to 0.5 ᵒC.

RTD PT100 temperature sensor results and simulation

Program for RTD PT100 temperature sensor using microcontroller

The code is written and complied in MIKRO C.

long double inputread, voltage1, voltageAB, resistance, resistance2, temperature1, temperature2;
	// Lcd module connections start
sbit LCD_RS at LATC3_bit;
sbit LCD_EN at LATC2_bit;
sbit LCD_D4 at LATC4_bit;
sbit LCD_D5 at LATC5_bit;
sbit LCD_D6 at LATC6_bit;
sbit LCD_D7 at LATC7_bit;

sbit LCD_RS_Direction at TRISC3_bit;
sbit LCD_EN_Direction at TRISC2_bit;
sbit LCD_D4_Direction at TRISC4_bit;
sbit LCD_D5_Direction at TRISC5_bit;
sbit LCD_D6_Direction at TRISC6_bit;
sbit LCD_D7_Direction at TRISC7_bit;
	// Lcd module connections ends
	// copy above statments into your code for every project involving LCD
char txt[4];	// declare a char array
void main(){
TRISA=0XFF;	// PORTA is input
TRISC=0X00;	// PORTC is Output
TRISD=0X00;	// PORTD is Output
ADC_Init();	// Initialize ADC
Lcd_Init();	// Initialize Lcd

Lcd_Cmd(_LCD_CLEAR);	// Clear display
Lcd_Cmd(_LCD_CURSOR_OFF);	// Cursor off
Lcd_Out(1,1,”Welcome to”);	// dsiaplay the welcome logo on the LCD screen at position ROW=1 COLUMN=1
Lcd_Out(2,1,”help2educate”);
delay_ms(3000);	// keep displaying logo for 5s
Lcd_Cmd(_LCD_CLEAR);	// Clear display
Lcd_Out(1,1,”Please wait”);	// dsiaplay the welcome logo on the LCD screen at position ROW=1 COLUMN=1
Lcd_Out(2,1,”loading…..”);
delay_ms(6000);
while(1){
inputread = ADC_Read(0);	// Read analog value from channel 0
delay_ms(4000);
Lcd_Cmd(_LCD_CLEAR);	// Clear display
Lcd_Cmd(_LCD_CURSOR_OFF);	// Cursor off

voltage1=(inputread*4.88)/1000;	//
voltageAB=voltage1/10;	// diving voltage by gain 10 to get acutal VAB voltage
if(voltageAB<0.048)	//for temperature 0-10 deg C
{ temperature2=voltageAB/0.0048; }	// value of slope m at 5 deg C is used
if(voltageAB>=0.048 && voltageAB<0.095)	//for temperature 10-20 deg C
{ temperature2=voltageAB/0.00469; }	// value of slope m at 15 deg C is used
if(voltageAB>=0.095 && voltageAB< 0.139)	//for temperature 20-30 deg C
{ temperature2=voltageAB/0.0046; }	// value of slope m at 25 deg C is used
if(voltageAB>=0.139 && voltageAB< 0.182)	//for temperature 30-40 deg C
{ temperature2=voltageAB/0.00451; }	// value of slope m at 35 deg C is used
if(voltageAB>=0.182 && voltageAB< 0.223)	//for temperature 40-50 deg C
{ temperature2=voltageAB/0.00442; }	// value of slope m at 45 deg C is used
if(voltageAB>=0.223 && voltageAB< 0.263)	//for temperature 50-60 deg C
{ temperature2=voltageAB/0.00434; }	// value of slope m at 55 deg C is used
if(voltageAB>=0.263 && voltageAB< 0.301)	//for temperature 60-70 deg C
{ temperature2=voltageAB/0.00426; }	// value of slope m at 65 deg C is used
if(voltageAB>=0.301 && voltageAB< 0.338)	//for temperature 70-80 deg C
{ temperature2=voltageAB/0.00418; }	// value of slope m at 75 deg C is used
if(voltageAB>=0.338 && voltageAB< 0.374)	//for temperature 80-90 deg C
{ temperature2=voltageAB/0.00411; }	// value of slope m at 85 deg C is used
if(voltageAB>=0.374)	//for temperature 90-100 deg C
{ temperature2=voltageAB/0.00402; }	// value of slope m at 95 deg C is used
floatToStr(temperature2, txt);	//convert float variable value to characters to be displayed on the LCD
delay_ms(5);
lcd_out(1,1,txt);	// dsiaplay the input voltage value stored in character text on the LCD screen at position ROW=1 COLUMN=1
Lcd_Out(1,10,”deg C”);	// dsiaplay the character input volts on the LCD screen at position ROW=1 COLUMN=5
delay_ms(1000);	// wait 2s before taking next value
}}

If you have any questions you can ask in comments. You can also share your ideas and suggest any improvments in above article.

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Thursday, 16 March 2017

+15V & -15V Regulated Dual Polarity Power Supply Circuit

Introduction

Components and ratings of dual regulated power supply

Circuit diagram: Positive & Negative voltage power supply circuit

Simulation of Dual polarity power supply

Simulation Video

PLC Tutorial: Star delta starter for induction motors

Introduction to star delta motor starter

Working principle of star delta starter

Star Delta starter diagram

PLC Tutorial program for star delta motor starter

Temperature 3 Wire RTD meansurement circuit / transmitter

Issues in 3 wire RTD temperature transmitter.

Properties of 3 wire RTD resistance temperature transmitter circuit.

Working of 3 wire RTD meansurement circuit

1) Bridge circuit.

2) Instrumentation amplifier

Summary

RTD PT100 temperature sensor using Microcontroller

Connection diagram for RTD PT100 temperature sensor using microcontroller

How to find temperature from the voltage measured by Microcontroller

Classical method

Direct method

Simulation and result for microcontroller temperature sensor using RTD PT100

Program for RTD PT100 temperature sensor using microcontroller

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