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Single Channel Data Acquisition System

  • Single Channel Data Acquisition System:

    The block diagram of a single channel DAS is shown in Figure. It consists of a signal conditioner, analog to digital converter. The output of the signal conditioner is given to to the A/D converter. This circuit performs repetitive conversions at a free running rate.

    The rate of conversion is internally determined. The digital outputs from the buffer are fed to a storage system or a printer or to a computer for further analysis.
    Block Diagram of Single Channel DAS
    A known example of the single channel DAS is the digital panel meter [DPM]. There are two disadvantages in using a DPM as a DAS. They are:

    It is slow and if the output is to be professed by digital equipment the BCD has to be changed into binary coding.

    While it is free running the data from the A/D converter is transferred to the interface register. This is done at a rate determined by the DPM itself, than the commands beginning from the external interface.

    Preamplification and Filtering:

    Mostly low resolution [8/10 bit] A/D converters are manufactured with a single ended input. They have a normalized analog input range of the order of 5 -10 V, bipolar or unipolar. Amplifiers are used for signal levels which are low compared to the input requirement.

    Amplifiers can be used to improve the level of the input signal to match the converter input. This provides optimum accuracy and resolution. An arrangement for preamplification is shown in Figure.
    Differential amplifiers are necessary when the input signal levels are below one tenth of a mV, or when resolution of 14 bits or 16 bits. Instrumentation amplifiers are used when differential output is to be handled from a bridge network.

    The circuit shown in Figure, consists of three amplifier instrumentation amplifier. The output of the amplifier is given to the conditioning circuit. The accuracy, linearity and gain stability specifications must be carefully considered to see that the system has no limitations.
    Preamplification for DAS
    If the input signals are to be isolated physically from the system the conductive paths are to be broken. This is done using transformer coupling or using optocoupled isolation amplifier. Such isolation is useful in handling signals from high voltage sources and transmission towers, Isolation is essential in biomedical applications.

    In order to eliminate noise and high frequency components the output of the preamplifiers can be fed to filters. Filters effectively compensate for transmission sensitivity loss at high frequency. Thus the dynamic range of measurement will be enhanced. To preserve the phase dependent data special filters like tracking filters are used.


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    Signal Conditioning Methods

  • Signal Conditioning with Methods:

    The data that will be acquired in a system will not be from identical sources. Therefore signal conditioning is necessary. Signal conditioning may involve some of the following operations.

    Attenuation to scale down the input signals may be necessary to match them to the input levels of the converter's full scale range.

    Linearization of data or linear approximation alters conversion.
    Analog differentiation precision rectification etc

    Signal conditioning can be done in two methods for data acquisition systems. They are:

    1. Ratiometric conversion
    2. Logarithmic compression

    The two methods mentioned above are explained here under:

    (1) Ratiometric Conversion:

    Ratiometric conversion is explained with reference to a transducer having four strain gauges in a Wheatstone bridge network. In such a bridge the output voltage will be a function of the change of resistance of each a ram and the excitation voltage of the bridge. 




    Let the strain gauges be under maximum constant unbalance. Now if the excitation voltage changes by ± X %, the output of the bridge also changes by ± x %.

    If we can condition the output voltage of the bridge such that the output of the signal amplifier is proportional to the strain only and independent of excitation voltage the system accuracy improves. This is due to the fact that the fluctuations in the excitation voltage do not affect the sensitivity of the system.

    In an analog method of obtaining this result an analog divider will be incorporated. To this analog divider the amplifier's output and the excitation voltage are supplied. Now the output of the divider is a ratio of the amplifiers output to the excitation voltage.

    Ratiometric Conversion
    Another method is illustrated in Figure. In this method the bridge excitation voltage is supplied as an external reference voltage to the analog to digital converter.

    In the A/D converter the conversion factor is proportional to the reference voltage. When such an arrangement is made the system sensitivity is independent of the fluctuations in bridge excitation voltage.

    (2) Logarithmic Conversion:

    Logarithmic conversion circuit permits measurement of fractional changes in the input as a percentage of the input magnitude rather than a percentage of a range.

    As an example if we take an input range of 100 µV to 100 mV, the output voltage may correspond to 0 for 100 µV and 3 V for 100 mV, if the logarithm conversion gain is 1% per decade. Consider a change of 1% that is if the input changes from 100 mV to 101 mV. The output of the logarithm amplifier will change by

    Δ V = [log 101 mV/100 mV] x 1 V = 4.3 mV

    As the output change is related to the ratio of the input, it is clear that the change in output is the same i.e. 4.3 mV. That is whether the input changes from 10.0 mV to 10.1 mV or from 100 µV to 101 mV the change in the output will be only 4.3 mV.

    The output of the logarithm amplifier can be converted into digital form. This can be done using a 12 bit BCD counter. Then the resolution of the counter will be 3 V/1000 = 3 mV, for a 3 V full scale. This can be achieved by properly scaling the logarithm amplifier. 

    It is possible to monitor and record changes as low as 1 mV, for an input of 100 µV or 10 µV for 1 mV with this resolution of the converter. It is to be noted that in the absence of the logarithm amplifier the resolution would be 100 µV. Therefore we conclude saying that a 100 to 1 improvement is possible using a logarithm amplifier.

    The logarithm amplifier can enhance the resolutions at low inputs. However at the high inputs it offers a poor resolution. The logarithm conversion distributes the resolution on a "percentage of reading" basis against a "percentage of full scale" as with A / D conversion. This type of conditioning will be advantageous in systems having an output relationship involving the logarithm of the measured. Further it will be advantageous where a moderate accuracy measurement around 1% is required over a large range of 1:105.

    The log function is inherently unipolar. Therefore other types of compression will be used when handling bipolar inputs.


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    Classification, Elements, Configuration of Data Acquisition System

  • (a) Classification of Data Acquisition System: 

    Based on the data the DAS can be classified as:

    • Analog data acquisition system and
    • Digital data acquisition system

    Based on the environment DAS can be classified as:

    • System suitable for favorable environments i.e. with minimum RF interference and electromagnetic induction
    • System suitable for hostile environment

    (b) Elements of Data Acquisition System:

    Analog data acquisition system will consist of some or all of the following elements:

    1. Transducer
    2. Signal conditioner
    3. Visual display device
    4. Graphic recording equipment
    5. Magnetic recording equipment

    A digital data acquisition system will consist of some or all of the following elements of

    1. Transducer
    2. Signal conditioner
    3. Scanner or multiplexer
    4. Signal converter
    5. Analog to digital converter
    6. Auxiliary equipment
    7. Digital recorder.

    System suitable for favorable environments:

    These are laboratory instrument applications, test systems for collecting long term drift information and routine measuring equipment etc. These systems are designed to perform tasks oriented more towards making sensitive measurements. They do not concentrate on protecting the integrity of analog data.




    System suitable for hostile environment:

    These are designed to measure and protect the analog data under hostile conditions. Such conditions are encountered in aircraft control system, turbovisous in electrical power systems and in industrial process control systems. Hostile measurements require devices that are capable of withstanding wide range of temperature. They also require excellent shielding.

    (c)  Aim of Data Acquisition System: 

    The aims of data acquisition system are listed here under:
    1. To acquire the necessary data at precise speed and at the correct time.
    2. To inform system operator the status of plant using all the data collected.
    3. To monitor the total plant operation and maintain on-line optimum safe operation.
    4. To provide highly effective human communication and identify and notify problematic areas.
    5. To collect and store data.
    6. To compute unit performance indices using on-line real-time data.

    (d) Factors that Determine the Configuration and Subsystems of DAS: 

    The following are the important factors that determine the configuration and the subsystems of a data acquisition system

    1. Resolution and accuracy expected
    2. Number of channels to be monitored
    3. Sampling rate per channel
    4. Signal conditioning requirement of each channel
    5. Cost

    (e) Configuration of Data Acquisition System: 

    The factors to be considered to determine the configuration for a DAS are listed below

    1. Accuracy and resolution
    2. Number of channels to be monitored
    3. Whether the signal is analog or digital
    4. Whether it is a single channel or multichannel system
    5. Required sampling rate per channel
    6. Signal conditioning requirements
    7. The cost

    The different configurations that are possible in a DAS are listed here under:

    Single channel configurations: 

    1. Direct conversion
    2. Preamplification and direct conversion
    3. Sample and hold and conversion
    4. Preamplification, sample and hold and conversion
    5. Preamplification, signal conditioning and any of the above

    Multichannel configurations: 

    1. Multiplexing the output of single channel converters
    2. Multiplexing the output of sample hold
    3. Multiplexing the inputs of sample hold
    4, Multiplexing the low level data
    5. More than one multiplexers

    Signal conditioning methods: 

    1. Ratiometric Conversion
    2. Wide dynamic range options:
    a. High resolution conversion
    b. Range biasing
    c. Automatic gain switching.
    d. Logarithm conversion

    3. Noise reduction options:
    a. Filtering
    b. Integrating converters.
    c. Digital processing

    System measurement time, error and cost are some of the useful estimates for the finalization of system configuration.

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    Data Acquisition System with Block Diagram

  • DATA ACQUISITION SYSTEMS:

    In industrial and instrumentation environment we come across signals of two types.

    1. Signals obtained by direct measurement of electrical quantities. They are generally DC or AC voltages, frequency, resistance etc.
    2. Signals obtained from the output of transducers.

    The instrumentation systems are of two types the analog systems and the digital systems. Therefore we have the data or information available in the above two forms. The data available is to be collected processed, stored, displayed and transmitted depending on the requirements.

    A data acquisition system consists of the following elements:

    (a) Individual sensors
    (b) Necessary signal conditioning system
    (c) Data conversion equipment
    (d) Data processing equipment
    (e) Multiplexing equipment
    (f) Data handling equipment
    (g) Storage system
    (h) Display system
    (i) Data transmission system

    Data Acquisition System Definition: 

    A data acquisition system is a complex system that consists of all the required subsystems for collection, conditioning, conversion, processing, display, storage and transmission of data.

    To obtain the best characteristics of the system in terms of performance handling capacity and economy, the subsystems of a data acquisition system can be clubbed together. Usually analog information [data] is converted into digital form for processing, display, storage and transmission.

    The processing of data may involve different operations. They can be simple or complex mathematical manipulations. The data collected from various points is to be transformed into a useful format. It will be necessary that the data is to be transmitted. The transmission may be from one collection point to the other or may be from the collection point to a computer.

    General Data Acquisition System: 

    A block diagrammatic representation of data acquisition system in general is shown in Figure.

    General Block Diagram of Data Acquisition System
    It can be observed from the block diagram that the outputs of the different transducers are supplied to the signal conditioners. In the process of signal conditioning the outputs of the transducers may be amplified, attenuated, linearized as per requirement.

    The outputs originated from the signal conditioners are given to the inputs of a multiplexer. From the output of the multiplexer the analog data can be supplied to recorders meters for indication or for a display device for display.

    Multiplexers output will be converted into digital format. This digital signal will be used with printer, digital display, magnetic recorder [tape recorder] for printing a hard copy, monitoring and recording respectively. The output of the A/D converter can be supplied to a computer and can also be transmitted.



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    Comparison between Open and Closed Loop Control System

  • No:
    Open Loop System

    Closed Loop System
    01
    It is characterized by definite relationship between desired system output and actual system output without monitoring the actual system.
    The closed loop control system is characterized by definite relation between desired system output and actual system output by monitoring the actual system. The monitored error signal will be utilized to control the system to obtain the desired output.
    02
    Open loop systems have moderate accuracy.
    Closed loop systems have high accuracy.
    03
    Open loop systems are sensitive to surrounding conditions like vibrations, voltage, aging etc.
    Closed loop systems are not sensitive to surrounding conditions like vibrations voltage aging etc.
    04
    Open loop systems have slow response to input command and changes. Therefore the system will be slow and sluggish.
    The response is fast.
    05
    The accuracy of the open loop system depends on:
    a. Designed output relationship.
    b. The performance of the calibration over long intervals of time.
    c. The varying effects of environmental conditions.
    The accuracy of the closed loop system depends on:
    a. Accuracy of the closed loop system.
    b. Accuracy of the comparing device.
    c. Accuracy of the control element and its sensitivity.
    06
    The advantage of open loop system is that they are simple and economical.
    Advantages of closed loop system are:
    a. Human errors are avoided.
    b. High degree of accuracy efficiency is obtainable.
    c. Economical.




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    Automatic Room Temperature Control

  • Automatic Room Temperature Control:

    A closed loop automatic control system for the temperature control in a room or enclosure is shown in Figure. The system is the required temperature. The input of the system is the required temperature. The output of the system is constancy of temperature or zero change in temperature.

    Automatic Room Temperature Control System

    The temperature in the room can be sensed by a sensor placed in the room. The sensor produces a signal that is proportional to the temperature change inside the room.


    The signal is compared with the desired temperature and the error is produced. The error is detected by a controller that changes the heating time of the heaters. Thus the controller regulates the temperature. Therefore the temperature inside the room will be steady. Always the temperature of the room is compared with desired temperature. Therefore changes in temperature are sent to the comparator through the feed back path. Hence the input is influenced by the output and disturbances through the feedback path. 

    Advantages of Automatic Room Temperature Control:

    The following are the advantages of automatic control system:

    1. Human errors are avoided.
    2. Precession, accuracy and high efficiency are obtainable.
    3. Automatic control system is economical.


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    Manually Controlled Closed Loop System

  • Example of a Manually Controlled Closed Loop System: 

    As an example of a manually operated closed loop system we take the case of driving of an automobile. The block diagram of the closed loop system is shown in Figure.
    Manually Controlled Closed Loop System
    The route, speed and acceleration are determined and are controlled by the driver. The driver observes traffic, running details on road. Accordingly he operates the accelerator, breaks, clutch, gears, steering wheel, lights etc. If the driver wants to maintain a speed of say 50 km/h, which is the desired output, he will apply that force on the accelerator pedal required to maintain the speed constant.

    He also holds the accelerator pedal constantly with the same pressure. The speed of the vehicle will be constant as long as there is no gradient or other problem of traffic. Normally no vehicle on road could go at a constant speed. The road conditions, traffic on road will have their effects on the speed of a moving vehicle.

    The speed of the vehicle is indicated by the speedometer. The driver views the speed in it and regulates it. He mentally calculates the speed shown by the speedometer with the desired speed. So he will regulate the speed by changing the pressure on the accelerator pedal. This is done using the foot [muscular power]. The sequence of all these operations can be represented in a diagram. This is called a closed loop system of the manually controlled type. The instantaneous speed of the vehicle is observed by the driver who reacts suitably and maintains the desired speed. Hence the driver performs the important role in this control loop.

    Disadvantages of Manually Controlled System:

    1. In a complex fast acting system the system response may be very rapid. Under such conditions the operator may fail to operate or control the system.
    2. The operator must possess extremely high skills.
    3. Failures are expected due to human errors in spite of good skills.
    4. In certain applications such as in defence the systems are self destructive. No operator should be employed in such systems.
    5, Elimination of human interference is economical, accurate, error free and efficient.

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    Examples of Open Loop and Closed Loop Control System

  • (a) Example of an Open Loop System: 

    In order to explain the open loop system we take the example of an automatic toaster. The block diagram of the toaster is shown in Figure.

    Block Diagram of a Toaster 

    The quality of the toast [bread slice] depends on the time for which the toaster is heated. Calibrated relays control the system. They control the heating time of the toast in the toaster. Relays are operated from external power source. They are adjusted by the calibrated dial to a particular time for the desired quality of the toast. The toaster heaters are energized after putting the toaster inside the heating chambers. Depending on the relay setting either the toasts are thrown out of the toaster or the toast is put off after the set interval of time.



    The quality of the toast is to be judged by user. He will know it only after the toast is released out of the toaster. As the user is not a part of the system this system is called open loop system. The system input is also ineffective to disturbance produced in the toaster due to loss of addition of heat from surroundings.

    (b) Practical Closed Loop System: 

    An automatic toaster with feedback is taken as an example of the practical closed loop control system. The block diagram is shown in Figure. The input of the system is desired quality of the toast. The output of the system is the actual quality of the toast. The actual toast quality in the toaster is measured by a quality measuring device like a colour detector placed in the toaster.
    Automatic Toaster with Closed Loop Control
    It produces a signal proportional to toast quality. This quality is then compared with the desired toast quality to generate an error signal. This error signal is detected by the controller that changes the heating time. The heating time is changed through relays such that the error is reduced. This makes the actual toast quality same as desired one.

    The disturbance in the toaster due to temperature changes from surrounding conditions change the toast quality. This is because the toast heating intensity is changed. It is to be noted that the quality is important here. It is very important compared with the desired toast quality.

    The disturbances are also transmitted to the comparator, through the feedback path. They also influence the error. Thus in this system the input is influenced by the output and disturbances through the feed back path.


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    Open Loop and Closed Loop Control System

  • Block Diagram Illustrating a System:


    Control System:

    Control systems can be classified into two types depending on whether the output (controlled variable) effects the actuating signal or not. They are:

    1. Open loop control system
    2. Closed loop control system

    (a) The Open Loop Control System:

    A block diagram of system is shown an open loop system is shown in Figure. An open loop control system is that in which the control action is independent of the system input. Therefore an open loop system in utilizes a controller or controlling device to control the system process. The controller controls in such a way as to obtain the desired output without considering the actual system input.
    Block Diagram of  Open Loop System
    The controller uses an external power source to provide proper control action to the controlled system process. The open loop control system is characterized by definite relationship between the desired system output and actual system output without monitoring the actual system.



    (b) The Closed Loop Control System:

    In a closed loop control system the control action is dependent on the system output. In a closed loop control system the actual system output will be measured. This output is compared with the input. The resulting error signal is used for controlling the system output to obtain the desired value. The sequence of operations in a closed loop system is illustrated in Figure.
    Sequence of operations in a Closed Loop System
    (c) Features of the Open Loop Control System:

    The outstanding features of the open loop system are
    1. Moderate accuracy
    2. Sensitive to surrounding conditions like vibrations voltage aging etc.
    3. Slow response to input command and changes there by making the system slow and sluggish.

    (d) Factors for the Accuracy of Open Loop Control System:

    1. Accuracy of the designed input output relationship
    2. The performance of the calibration over long intervals of time
    3. The varying effects of environmental conditions

    (e) Advantages of Open Loop System:

    The following are the advantages of open loop system

    1. Simplicity
    2. Low cost.       

    (f) Performance and Characteristics of the Closed Loop System:

    A block diagram of the closed loop control system is shown in Figure. The operations in the closed loop system are performed by output measuring devices, comparators, amplifiers, controllers and system plant. The measuring devices produce signal proportional to the actual system output.
    Block Diagram of a Closed Loop System

    This signal is compared with the reference input. An error signal is generated. This error signal is amplified to produce an actuating signal. The actuating system operates the controller applying the control signal to the system plant. This changes the controlled system output to the desired value.
    High accuracy fast responses to input signals and relatively independent to operating conditions are the characteristics of the closed loop control system.

    (g) Accuracy of Closed Loop System

    The accuracy of the closed loop control system is governed by the following factors:

    1. Accuracy of the closed loop system.
    2. Accuracy of the comparing device.
    3. Accuracy of control element and its sensitivity.                                               

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    Regulated Power Supply with Short Circuit Protection

  • The simple way of providing over current protection is to use another transistor that controls the series transistor. The circuit is shown in Figure
    Regulated Power Supply with Short Circuit Protection
    In this circuit a current sensing resistor is connected in series with the series transistor. The voltage drop across the current sensing resistor, R4 will be the base emitter voltage of the transistor T3.
    The value of this resistance R4 is chosen to be small. For example we connect a 1 W resistor and if the load current is less than 500 mA; the voltage across the resistor will be less than 0.6 V. Therefore T3 will not function. The circuit works normally. If the load current increase beyond 600 mA the voltage drop across the resistance R4 , will be sufficient to turn on the transistor T3.
    The collector current of T3 flows through R3. It decreases the base voltage to the series transistors T2. This in turn decreases the output voltage preventing further increase in load current. If we represent circuited load current to be ISC, the voltage drop across R4 is ISC R4. This is equal to the VBE of the transistor T3. Therefore;

    VBE = ISCR4 ----------------- (1)

    ISC = VBE/R4 ----------------- (2)

    By properly choosing the value of resistance R4, the level of current limiting can be adjusted.
    The disadvantage of this circuit is that large power dissipation takes place at the series transistor, when there is a short circuit at the output terminals.


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    Transistor Shunt Voltage Regulator Working

  • Transistor Shunt Voltage Regulator
    Transistor Shunt Voltage Regulator Working is explained below,

    In the transistor shunt voltage regulator shown in Figure, the regulating transistor is in shunt with the output terminals. If the output voltage Vo increases, the base-emitter junction of the transistor T1 becomes more forward biased so that its collector current increases. This collector current is the base current for the transistor T2. An increase of base current of T2 raises its collector current which is an amplified version of the collector current of T1. The collector current of T2 is the base current of the transistor T3. An enhancement of the base current of T3 increases its collector current which is a still further amplified form of its input current. An increase of the voltage V0 thus causes an increase of the collector current of T3, increasing thereby the voltage drop across the resistance R. The circuit is thus self-correcting, the increased drop across R reducing the voltage V0 which is thus stabilized. The transistor T3 has to handle a large collector current. So, it must be a power transistor.


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    Transistor Series Voltage Regulator Theory

  • Regulated Power Supply for continuous variation of Output:

    The circuit of a continuously variable regulated power supply employing negative feedback is shown in Figure. The unregulated dc supply is fed to the voltage regulator. The circuit maintains constant output voltage inspite of variations in load or input voltage. T1 and T2 are the two transistors connected in the circuit, as direct coupled feed back amplifier. In this circuit the output voltage variations are returned as feed back to oppose the input changes. The base of transistor T2 is connected to the variable tap of the potentiometer R3.

    Supposing the output voltage increases by any reason, it causes an increase in the voltage across the P & Q points, which is the part of the potential divider, consisting of R2, R3 and R4. This raises the base emitter voltage of the transistor T2. This increases the collector current of T2 and most of it flows through R1. This makes the voltage at the base of T1 to decrease. This low voltage on the base emitter junction of T1, tends to decrease the output voltage, resorting it to the original value. The reverse will happen if the input voltage decreases. The zener diode is used for providing a reference voltage and the difference is amplified by the transistor T2. The potentiometer R3 serves as the voltage controller. The output can be made to vary from zero to the maximum by varying the potentiometer R3. The Circuit Diagram of Transistor Series Voltage Regulator is shown in the circuit below. Also Working is explained.
    Transistor Series Voltage Regulator circuit diagram
    The Transistor Series Voltage Regulator Theory is explained as follows.

    From the circuit of Figure, we can observe that the feedback voltage VF, is the voltage developed across the part of the potential divider P & Q. If we represent that resistance by Rf, the feed back fraction will be, Rf/(Rf + Ra) where Ra is the part of the potential divider R & P as shown in figure.
    The closed loop voltage gain of the transistor T1 is proportional to the reciprocal of the feedback fraction.

    Therefore, the closed loop gain A' = (1/Rf)/ (Rf + Ra) = 1 + (Ra/Rf) ----------------- (1)

    Considering the closed loop of the Zener diode which is the reference, the base emitter junction and the part of the potential divider, the sum of the voltages must be zero.

    Hence -VZ - VBE + Vf= 0 or Vf = VZ + VBE ----------------- (2)

    The feedback voltage is given by feedback fraction multiplied by the output voltage. If we represent the feed back fraction by ‘b’, we have

    Vf = bVout = VZ + VBE ----------------- (3)

    Therefore, Vout = (Vz + VBE)/β

    As the closed loop gain is equal to 1/b we can write Vout = A' (Vz + VBE) ----------------- (4)

    Where, Vout is the regulated output voltage.
    A' is the closed loop voltage gain.
    VZ is the zener voltage.

    VBE is the base emitter voltage of T2

    Power Rating Disadvantage: 

    Normally the series regulator uses a power transistor as its series transistor. This transistor must be capable of withstanding the power dissipated during full load current. A proper heat sink is to be provided. The power dissipation is given by the product of the collector current (load current) and collector to emitter voltage.

    When heavy load currents are expected, power transistors may be connected in darlington pair to act as the series transistor for the regulator circuit. This enhances the power handling ability of the regulated power supply of this type.

    Though the series regulator type regulated power supply works satisfactorily, it has the disadvantage that it has no short circuit protection.

    In any regulated power supply or in ordinary power supply the series diode will carry the load current. Accidental short circuiting of the output terminals or over drawing of current from the power supply leads to the excessive dissipation of power in the series transistor or diode, resulting in the destruction of the transistor or and the diode of the power supply circuit.

    To prevent the destruction of the series transistor or the rectifying diode when accidental short circuit occurs or excessive current is drawn, we have to incorporate a current limiting device. When such a device is incorporated we call the circuit to be consisting of short circuit protection.

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    Load Regulation Formula

  • LOAD REGULATION

    Load effect or load regulation indicates the extent to which the load current changes effect the output voltage. It is expressed as a ratio of the difference between no load voltage to full load voltage and no load voltage. It can be expressed as a fraction or as percentage. 

    Load Regulation Formula

    % load regulation = (No load voltage — Full load voltage) / No load voltage

    The smaller the percentage the better is the performance of the power supply.

    (a) Line Regulation or Source Effect:

    It is the extent to which the output voltage of a regulated power supply will be held constant under the influence of the mains voltage variations. Typical value of line regulation is 0.01%.

    The point to be noted here is that there must be a reasonable limit to the ability of the regulator to maintain output constant for line voltage variations. Therefore a range will be specified for the input voltage.

    Example: 230 V, ± 10%.

    That is the output of the regulated power supply will be constant even if the supply voltage has its changes between 207 V and 253 V.

    (b) Periodic and Random Deviation:

    The periodic and random deviation and ripple factor express the quality of the DC output from a regulated power supply. Periodic and random deviation (PARD) includes all periodic and random variations in the output voltage of the power supply. It is expressed as millivolts RMS or millivolts pp. Typical value is 1 mV RMS.

    (c) Overload Protection:

    It describes the ability of fixed and adjustable current limiters employed in the regulator circuit, from protecting the supply, or the circuit under test from excessive current. An overload indicator is usually provided that indicates the overload condition.

    (d) Warm up Time and Drift:

    Warm up time is the time the supply takes to stabilize its circuits to present a constant output. It is expressed in minutes.

    Drift is the deviation from the rated output voltage with respect to time. It is expressed as a minimum percentage of change in the output voltage from the specified voltage over a time period.

    Example: less than 3% in 12 hours.

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    Regulated Power Supply Specifications

  • Regulated Power Supply: 

    The output voltage of a power supply stage changes due to the line supply voltage variation as well as the load current variation. When the line voltage decreases the output decreases and when it increases it increases. The output voltage of the power supply gets reduced with increasing load currents. The reason for the decrease in output voltage in a power supply stage with increase in load current is the drop across the rectifying diode, the drop across the transformer and the drop across the filter choke or the resistor.

    From the characteristics desired out of a power supply stage we know that the output must be maintained constant irrespective of the line voltage change or load current change. In order to keep the output voltage constant for changing line voltage or changing load current we include a regulator circuit in a power supply stage.

    A regulated power supply is one which generates constant supply voltage irrespective of the load variations or input variations.

    Voltage regulation and current regulation also is carried on in the regulator circuit. From simple zener voltage regulator to feedback controlled voltage regulator there can be many types of voltage regulator circuits. Switched mode regulator circuits are also used.

    Voltage regulation is to maintain the magnitude of the output voltage constant. Current regulation is to maintain a specific amount of load current and to limit the maximum magnitude of the load current. The regulator circuits will also be provided with short circuit protection.


    SPECIFICATIONS OF REGULATED POWER SUPPLY

    The following are the important specifications of a regulated power supply. The specifications differ from manufacturer to manufacturer. However, majority of them are common.

    1. Input voltage: 230 V 50 Hz mains voltage
    2. Output voltage: As per requirement of the circuit
    3. Load regulation: 0.01% or even better
    4. Ripple: Less than 1 mV RMS or even better
    5. Protection: Short circuit protection for indefinite period set by current limit control.

    Output Voltage:

    The output voltage may be a fixed value. This is the condition for power supplies used with equipment where the output voltage desired may be for example 48 V, 24, V 12 V, etc. Further there can be more than one independent output voltages derived from the same circuit.
    Output voltage in case of laboratory type power supplies has a facility for coarse and fine variation of voltage and in such cases the maximum voltage will be specified. Ex. 0-30 V, This voltage will be maintained constant, by virtue of the regulator circuit, irrespective of variation in line voltage or load current variation.

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    Power Supply Characteristics

  • Any power supply ideally must have the following characteristics

    1. The magnitude of the output must be presented accurately and precisely.
    2. The ripple content of the output must be zero.
    3. The output voltage must have high stability irrespective of load current and line voltage changes.
    4. The source impedance must be zero.
    5. It should be portable.

    Sources of DC Voltage: 

    The following is the list of DC voltage sources and shows the application of the type of source:

    Source
    Application
    Primary cells (Dry cell)
    For use in low power applications like radios, Calculators, Multimeters, Tape recorders, etc.
    Secondary cells (Wet Batteries)
    For use as backup batteries for PA amplifiers, Inverters, UPS etc
    AIkaline batteries
    Low power sources mostly with portable instruments.
    Nickel Cadmium cells
    For large load currents.
    Lithium Sulphur dioxide
    For large load current requirement
    Lithium iodine cells
    For large load current requirement
    DC Voltage standard
    For calibration of instruments
    The Laboratory Power supply
    To conduct laboratory experiments
    Line operated Power supplies
    For low medium and High power applications
    Uninterrupted Power supplies
    Computer centers, telephone exchanges, transmitters, Microwave stations, repeaters etc


    Power supply:

    The commonly used power supply, with electronic circuits is the line operated power supply. In this the mains voltage is utilized in a transformer or directly, to work a rectifier. The rectified output, will be filtered using a filter circuit, that eliminates the ripple content of the rectified output. The output of the power supply will be pulsating unidirectional current. The block diagram of the power supply is shown in Figure.
    Block Diagram of a line operated power supply

    A generalized block diagram of the power supply stage is shown in Figure. The transformer isolates the load from the mains power supply ground. It provides either a step up voltage or step down voltage in its secondary. Taps or center tap or both can be provided at the secondary of the transformer.

    A rectifier converts the alternating current to pulsating unidirectional current. Mostly Full wave rectifier, or bridge rectifier circuits are used as rectifier circuits.

    The alternating component of the rectified voltage is called the ripple voltage. The ripple should not be present and it should be minimized. A filter circuit is used to filter off the ripple contained in the output of the rectifier.

    The bleeder resistor is connected across the output terminals of the rectifier, to present a light load on the power supply. It helps in maintaining constant DC voltage in the output of the rectifier. Further it can be of a potential divider type to tap the different voltages required as per the circuit conditions.

    The magnitude of the output voltage and the amount of current that can be drawn from the power supply depend on the design of the power supply and the demand of the circuit.

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    Power Supplies Introduction

  • Power Supplies Introduction: 

    For any electronic equipment to function a power supply stage is necessary. The power supply stage provides the different voltages required by the various devices incorporated in the equipment. The electronic equipment requires D.C. power supplies, with varied specifications. The specifications depend on the type of equipment or the instrument. Currents ranging from few microamperes to several hundreds of amperes and voltage from as low as 1.5 V to several kilo volts are required.

    In addition steady voltage (DC voltage) is also required to set the level in digital circuits to establish the high level (1 state) differentiating it from the low state (0 state). This voltage level will be dependent on the digital system requirement.

    Further control voltages for functioning of several systems in automatic control systems are also required.

    Initially use of primary cells (dry cells) and secondary cells (wet batteries / lead acid cells) were used to supply the DC requirement in electronic circuits. The primary cells are of the disposable type; where as the secondary cells are rechargeable. Disposable alkaline cells were used in portable instruments. Nickel Cadmium cells are used when the current requirement is large. They are rechargeable Lithium based primary cells will find their use in miniature equipment. Lithium iodine and lithium sulphur dioxide cells are of the rechargeable type, are used where the current requirements are large.

    It is to be noted that the cost of the batteries plays an important role in their use. Mostly disposable types are not economical. Rechargeable types can be used for low power equipment. Solar cells are used in satellites to energize the electronic circuitry. They are also used in miniature equipment like calculators, digital clocks etc, that take minute current for their operation.

    The most common method of supplying power to electronic equipment is to use line operated power supply. There are power supplies for low, medium and high power equipment, use rectifier circuits that take the alternating voltage, either from the single phase, three phase or poly phase transformer and work a rectifier circuit designed as per the requirement.

    Improvement in the performance of a power supply can be achieved using regulated power supply circuits that work on feed back principle. The present trend is to use a switched mode power supply that has many advantages over the conventional type.

    Uninterrupted power supplies are now in use for continuous power supply to electronic equipment.

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