Lecture 2
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- Words: 1,394
- Pages: 27
Diploma project ME 111
Lecture 2 Instructor: Akram Ahmed Habib
Content
What is AC and DC Building a Power Supply The Parts of a Power Supply The Transformer The Rectifier Smoothing The Regulator A simple 5V DC regulated power supply system Power supply with 5V & 12V Negative volt supply +5V & -5V Variable voltage power supply 0V TO 24 V Ammeters Voltmeters Fuse
What is AC and DC
A representation of an Alternating Current (AC) supply is shown in figure. The voltage (and current) alternates between positive and negative over time and the resulting waveform shape is a sine wave. the frequency of this sine wave is 50 or 60 Hz,50 or 60 cycles per second.
What is AC and DC
A Direct Current (DC) supply, shown in figure, stays at a fixed, regular, voltage all of the time, like the voltage from a battery. A DC supply is needed by most circuits as a constant reference voltage. Also, some components would be damaged by the negative half-cycles of an AC supply. mhtml:fle://C:\Documents and Settings\akram\My Documents\My Pictur es\\مقرر دبلومBuilding a Power Supply - Electronics in Meccano.mht!http://www.eleinmec.com/fgures/016_02.gif
Building a Power Supply
Most of the circuits need a smooth DC power supply in order to function correctly. Some other circuits, particularly those using digital ICs, also need their power supply to be regulated. In this article and the articles that follow in this series you will learn the meaning of terms such as 'smoothing' and 'regulation' and find out how to build a simple power supply for your circuits.
The P arts o f a P ower Supply mhtml:fle://C:\Documents and Settings\akram\My Documents\My Pictur es\\مقرر دبلومBuilding a Power Supply - Electronics in Meccano.mht!http://www.eleinmec.com/fgures/016_03.gif
The Transformer The Rectifier Smoothing The Regulator
A simple 5V DC regulated power supply system mhtml:fle://C:\Documents and Settings\akram\My Documents\My Pictur es\\مقرر دبلومBuilding a Power Supply - Electronics in Meccano.mht!http://www.eleinmec.com/fgures/016_04.gif
The Tra ns fo rme r
How Transformers Work A transformer consists of two coils (often called 'windings') linked by an iron core, as shown in figure. There is no electrical connection between the coils, instead they are linked by a magnetic field created in the core. Transformers are used to convert electricity from one voltage to another with minimal loss of power. They only work with AC (alternating current) because they require a changing magnetic field to be created in their core. Transformers can increase voltage (step-up) as well as reduce voltage (step-down).
Transformers have two great advantages over other methods of changing voltage They
provide total electrical isolation between the input and output, so they can be safely used to reduce the high voltage of the mains supply. Almost no power is wasted in a transformer. They have a high efficiency (power out / power in) of 95% or more.
The Rectifier
The purpose of a rectifier is to convert an AC waveform into a DC waveform. There are two different rectification circuits, known as 'half-wave' and 'full-wave' rectifiers. Both use components called diodes to convert AC into DC.
The Ha lf-wav e R ec tifie r Figure shows the AC input waveform to this circuit and the resulting output. As you can see, when the AC input is positive, the diode is forward-biased and lets the current through. When the AC input is negative, the diode is reverse-biased and the diode does not let any current through, meaning the output is 0V. Because there is a 0.7V voltage loss across the diode, the peak output voltage will be 0.7V less than Vs.
The F ull-w ave Re ctifie r
the circuit in figure addresses the second of these problems since at no time is the output voltage 0V. This time four diodes are arranged so that both the positive and negative parts of the AC waveform are converted to DC.
Smoo th in g
Most circuits will require 'smoothing' of the DC output of a rectifier, and this is a simple matter since it involves only one capacitor, as shown in figure
Smoothing
waveform
The output in figure shows how smoothing works. During the first half of the voltage peaks from the rectifier, when the voltage increases, the capacitor charges up. Then, while the voltage decreases to zero in the second half of the peaks, the capacitor releases its stored energy to keep the output voltage as constant as possible. Such a capacitor is called a 'smoothing' or 'reservoir' capacitor
Rip ple
If the voltage peaks from the rectifier were not continually charging up the capacitor, it would eventually discharge and the output voltage would decrease all the way down to 0V. The discharging that does occur between peaks gives rise to a small 'ripple' voltage. The amount of ripple is affected by a combination of three factors:
Three factors effect on the ripple
The value of the capacitor. The larger the capacitor value, the more charge it can store, and the slower it will discharge. Therefore, smoothing capacitors are normally electrolytic capacitors with values over 470μF. The amount of current used by the circuit. If the circuit connected to the power supply takes a lot of current, the capacitor will discharge more quickly and there will be a higher ripple voltage. The frequency of the peaks. The more frequent the voltage peaks from the rectifier, the more often the capacitor will be charged, and the lower the ripple voltage will be. If you want to calculate the ripple voltage, you can use this formula...
The Regulator
While there are many circuits that will tolerate a smoothed power supply, some must have a completely regular supply with no ripple voltage. This article discusses regulator ICs which can provide this regular power supply
He ats ink s
In use, a regulator IC will get quite hot, so a heatsink will need to be attached to it to dissipate the heat. The type of heatsink you choose depends on the regulator's case style, the amount of heat it must dissipate, and the way in which you wish to mount it. The examples shown below all suit the 78xx, 78Mxx, and 78Sxx regulator series TO-220 case style. The first two are the simplest and are clip-on types. The third has a hole so it can be bolted to the regulator, and two legs that can be soldered to a piece of stripboard or PCB. The fourth again has a mounting hole for the regulator, and also has a mounting hole for fixing to a board. To be sure of good heat transfer from the regulator to the heatsink, you can sandwich heat transfer compound between them.
Th e 7 8xx S erie s of Re gula tors
There are many types of regulator IC and each type will have different pin-outs and will need to be connected up slightly differently. Therefore, this article will only look at one of the common ranges of regulator, the 78xx series.
A simple 5V DC regulated power supply system mhtml:fle://C:\Documents and Settings\akram\My Documents\My Pictur es\\مقرر دبلومBuilding a Power Supply - Electronics in Meccano.mht!http://www.eleinmec.com/fgures/016_04.gif
Power supply with 5V & 12V
Negative volt supply +5V & -5V
Variable voltage power supply
Ammeters
Ammeters measure current. Current is measured in amps (amperes), A. Ammeters are connected in series. To connect in series you must break the circuit and put the ammeter across the gap, as shown in the diagram. Ammeters have a very low resistance.
Voltmeters
Voltmeters measure voltage. Voltage is measured in volts, V. Voltmeters are connected in parallel across components. Voltmeters have a very high resistance
Fuse
Fuse is a very thin wire, which either melts or vaporizes when current flow through it exceeded the fuse rating. The thin wire of fuse may be made of aluminum, tin-coated copper or nickel. The resulting open in the circuit stops current to flow. In electronic equipment, most fuses are cylindrical glass or ceramic type with a metal cap at each end! The current rating and voltage also can be seen in one of the two metal end caps.
Wait us in L3 in practical work in
Buildin g a P ower S upp ly
Lecture 2 Instructor: Akram Ahmed Habib
Content
What is AC and DC Building a Power Supply The Parts of a Power Supply The Transformer The Rectifier Smoothing The Regulator A simple 5V DC regulated power supply system Power supply with 5V & 12V Negative volt supply +5V & -5V Variable voltage power supply 0V TO 24 V Ammeters Voltmeters Fuse
What is AC and DC
A representation of an Alternating Current (AC) supply is shown in figure. The voltage (and current) alternates between positive and negative over time and the resulting waveform shape is a sine wave. the frequency of this sine wave is 50 or 60 Hz,50 or 60 cycles per second.
What is AC and DC
A Direct Current (DC) supply, shown in figure, stays at a fixed, regular, voltage all of the time, like the voltage from a battery. A DC supply is needed by most circuits as a constant reference voltage. Also, some components would be damaged by the negative half-cycles of an AC supply. mhtml:fle://C:\Documents and Settings\akram\My Documents\My Pictur es\\مقرر دبلومBuilding a Power Supply - Electronics in Meccano.mht!http://www.eleinmec.com/fgures/016_02.gif
Building a Power Supply
Most of the circuits need a smooth DC power supply in order to function correctly. Some other circuits, particularly those using digital ICs, also need their power supply to be regulated. In this article and the articles that follow in this series you will learn the meaning of terms such as 'smoothing' and 'regulation' and find out how to build a simple power supply for your circuits.
The P arts o f a P ower Supply mhtml:fle://C:\Documents and Settings\akram\My Documents\My Pictur es\\مقرر دبلومBuilding a Power Supply - Electronics in Meccano.mht!http://www.eleinmec.com/fgures/016_03.gif
The Transformer The Rectifier Smoothing The Regulator
A simple 5V DC regulated power supply system mhtml:fle://C:\Documents and Settings\akram\My Documents\My Pictur es\\مقرر دبلومBuilding a Power Supply - Electronics in Meccano.mht!http://www.eleinmec.com/fgures/016_04.gif
The Tra ns fo rme r
How Transformers Work A transformer consists of two coils (often called 'windings') linked by an iron core, as shown in figure. There is no electrical connection between the coils, instead they are linked by a magnetic field created in the core. Transformers are used to convert electricity from one voltage to another with minimal loss of power. They only work with AC (alternating current) because they require a changing magnetic field to be created in their core. Transformers can increase voltage (step-up) as well as reduce voltage (step-down).
Transformers have two great advantages over other methods of changing voltage They
provide total electrical isolation between the input and output, so they can be safely used to reduce the high voltage of the mains supply. Almost no power is wasted in a transformer. They have a high efficiency (power out / power in) of 95% or more.
The Rectifier
The purpose of a rectifier is to convert an AC waveform into a DC waveform. There are two different rectification circuits, known as 'half-wave' and 'full-wave' rectifiers. Both use components called diodes to convert AC into DC.
The Ha lf-wav e R ec tifie r Figure shows the AC input waveform to this circuit and the resulting output. As you can see, when the AC input is positive, the diode is forward-biased and lets the current through. When the AC input is negative, the diode is reverse-biased and the diode does not let any current through, meaning the output is 0V. Because there is a 0.7V voltage loss across the diode, the peak output voltage will be 0.7V less than Vs.
The F ull-w ave Re ctifie r
the circuit in figure addresses the second of these problems since at no time is the output voltage 0V. This time four diodes are arranged so that both the positive and negative parts of the AC waveform are converted to DC.
Smoo th in g
Most circuits will require 'smoothing' of the DC output of a rectifier, and this is a simple matter since it involves only one capacitor, as shown in figure
Smoothing
waveform
The output in figure shows how smoothing works. During the first half of the voltage peaks from the rectifier, when the voltage increases, the capacitor charges up. Then, while the voltage decreases to zero in the second half of the peaks, the capacitor releases its stored energy to keep the output voltage as constant as possible. Such a capacitor is called a 'smoothing' or 'reservoir' capacitor
Rip ple
If the voltage peaks from the rectifier were not continually charging up the capacitor, it would eventually discharge and the output voltage would decrease all the way down to 0V. The discharging that does occur between peaks gives rise to a small 'ripple' voltage. The amount of ripple is affected by a combination of three factors:
Three factors effect on the ripple
The value of the capacitor. The larger the capacitor value, the more charge it can store, and the slower it will discharge. Therefore, smoothing capacitors are normally electrolytic capacitors with values over 470μF. The amount of current used by the circuit. If the circuit connected to the power supply takes a lot of current, the capacitor will discharge more quickly and there will be a higher ripple voltage. The frequency of the peaks. The more frequent the voltage peaks from the rectifier, the more often the capacitor will be charged, and the lower the ripple voltage will be. If you want to calculate the ripple voltage, you can use this formula...
The Regulator
While there are many circuits that will tolerate a smoothed power supply, some must have a completely regular supply with no ripple voltage. This article discusses regulator ICs which can provide this regular power supply
He ats ink s
In use, a regulator IC will get quite hot, so a heatsink will need to be attached to it to dissipate the heat. The type of heatsink you choose depends on the regulator's case style, the amount of heat it must dissipate, and the way in which you wish to mount it. The examples shown below all suit the 78xx, 78Mxx, and 78Sxx regulator series TO-220 case style. The first two are the simplest and are clip-on types. The third has a hole so it can be bolted to the regulator, and two legs that can be soldered to a piece of stripboard or PCB. The fourth again has a mounting hole for the regulator, and also has a mounting hole for fixing to a board. To be sure of good heat transfer from the regulator to the heatsink, you can sandwich heat transfer compound between them.
Th e 7 8xx S erie s of Re gula tors
There are many types of regulator IC and each type will have different pin-outs and will need to be connected up slightly differently. Therefore, this article will only look at one of the common ranges of regulator, the 78xx series.
A simple 5V DC regulated power supply system mhtml:fle://C:\Documents and Settings\akram\My Documents\My Pictur es\\مقرر دبلومBuilding a Power Supply - Electronics in Meccano.mht!http://www.eleinmec.com/fgures/016_04.gif
Power supply with 5V & 12V
Negative volt supply +5V & -5V
Variable voltage power supply
Ammeters
Ammeters measure current. Current is measured in amps (amperes), A. Ammeters are connected in series. To connect in series you must break the circuit and put the ammeter across the gap, as shown in the diagram. Ammeters have a very low resistance.
Voltmeters
Voltmeters measure voltage. Voltage is measured in volts, V. Voltmeters are connected in parallel across components. Voltmeters have a very high resistance
Fuse
Fuse is a very thin wire, which either melts or vaporizes when current flow through it exceeded the fuse rating. The thin wire of fuse may be made of aluminum, tin-coated copper or nickel. The resulting open in the circuit stops current to flow. In electronic equipment, most fuses are cylindrical glass or ceramic type with a metal cap at each end! The current rating and voltage also can be seen in one of the two metal end caps.
Wait us in L3 in practical work in
Buildin g a P ower S upp ly
