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Once an ac voltage has been rectified and filtered, the resulting dc output still contains a certain amount of ripple voltage. If the filter is a single capacitor and the load is typical for the power supply, the ripple voltage will be approximately 10% of the dc output voltage. An output of +12±1 volts is a reasonable example. And more complex filters can of course reduce this amount of ripple to a large extent.
While some electronic circuitry can operate perfectly well with that ripple present, other circuits can be sensitive to the ripple and will react to it. Certainly nobody wants to hear a 120-Hz hum from their radio receiver or CD player. For these circuits, it is necessary to remove even the relatively small ripple that the filter allows to pass.
A second factor that must be considered is that both the line voltage and the load circuit may change over time. You have probably seen this effect yourself, when the picture on a television receiver balloons out each time the air conditioner turns on. This happens because your local line voltage drops briefly when the air conditioner turns on, and in some cases may remain low as long as the air conditioner stays on. This reduced line voltage causes the high voltage on the picture tube to also be reduced, which in turn changes the working characteristics of the picture tube.
A change in load resistance can also change the output voltage of the power supply, which can cause similar kinds of effects. To prevent such things from happening, the power supply must include some circuitry that will minimize such reactions, and maintain a constant output voltage over a range of line voltage changes or load resistance changes. That circuit is known as a regulator, and it is the last circuit in the power supply that affects the output voltage which actually reaches the load circuit.
There are two general classes of regulators: linear and switching. A linear regulator continually controls the output voltage and attempts to remove all ripple and variations. A switching regulator operates by rapidly turning the filter output voltage on and off to maintain the desired average output voltage. The switching regulator is much more efficient and can easily handle large amounts of power, but the linear regulator produces a much smoother output voltage and is preferred for most low-power applications as well as for audio frequency applications.
A very simple regulator consists of a Zener diode and a current-limiting series resistor, as shown to the right. The Zener diode is a specialized semiconductor diode that is designed to have a predictable and stable reverse-bias breakdown voltage. That is, when it is reverse biased with an applied voltage that exceeds its design breakdown voltage, it maintains a nearly constant voltage across itself over a wide range of currents. As long as the current flowing through the diode is restricted to a range the diode can tolerate, it will continue to maintain the voltage across itself.
The limitation of a Zener diode is that it cannot handle large changes in current through itself. Typical Zener diodes are rated at either ½W (500mW) or 1W. This doesn't mean the load is restricted to a low current, but it does mean that the load can't change very much, because the Zener diode must accept whatever current flows through R, but does not pass through the load. And if the load is disconnected for any reason, the Zener diode must immediately handle all of the current no longer flowing through the load.
For example, the type 1N963B Zener diode is rated at 12 volts, with an absolute maximum power dissipation of ½ watt (500 mw), and a maximum current rating of 26 mA. Note that the product of voltage and current is substantially less than 500 mw. This is because the diode will heat up if operated near its limits, and the heat will continue to accumulate until the diode is eventually destroyed. To avoid this, always be sure to operate a Zener diode within the limits specified by the manufacturer. In the case of the 1N963B, the design current through the diode is 10.5 mA, which is close to half the specified maximum. Therefore, load current can increase or decrease by about 10 mA without causing problems, but any greater change will drive the Zener diode out of its working range. That doesn't leave much room for variations.
There are a number of ways we can enhance the effectiveness of the Zener regulator, which we will explore in detail once we have begun our study of transistors and active circuits. Meanwhile, however, there is a special kind of voltage regulator that we can use as a single component, which will nevertheless provide an accurate and stable output voltage. We'll examine its properties shortly.
Zener diodes are manufactured to any of a wide range of voltages. Unfortunately, the design voltage also affects how a Zener diode reacts to changes in temperature. The amount of such a change is known as the temperature coefficient (TC) of the diode, and is measured in volts or millivolts per Centigrade degree (mV/°C). A Zener diode rated at 4.7 volts has a nearly zero TC. Below 4.7V, a Zener diode's TC will be negative, with a minimum TC of about -2 mV/°C occurring close to 3V. Above 4.7V, a Zener diode's TC will be positive, and will become greater at higher voltage ratings. Above a rating of about 7V, the curve becomes nearly linear, so we can calculate the TC of higher-voltage Zener diodes directly. Simply subtract 3.5 from the Zener voltage rating to get the TC in mV/°C. Thus, a 12V Zener diode will hace a TC of 8.5mV/°C.
We can still use Zener diodes as precision voltage references if we can compensate for the temperature coefficient. For instance, a silicon PN junction will have a TC of around -2.2mV/°C, and this can be adjusted somewhat during manufacture. This is used to advantage in commercial Zener diodes designated 1N821 through 1N829A. These devices consist of a 5.6V Zener diode (TC = +2mV/°C) in series with a silicon diode made to have a TC of -2mV/°C. As long as the current flowing through this combination remains steady at 7.5mA, the output voltage will remain essentially unaffected by changes in temperature. The output voltage is rated at 6.2V with a ±0.3V tolerance.
The requirement of 7.5mA flowing through the device means we cannot directly use it as a voltage regulator. If we were to try it, any change in supply voltage or load current would change the current flowing through the Zener diode, so it would no longer be insensitive to changes in temperature. However, we can easily use it as a reference voltage for a more sophisticated regulator circuit.
If we add just a few components to a Zener diode, as shown to the right, we can make the output voltage nearly independent of external variations. This is accomplished by several factors:
Of course, there are a few issues with this circuit as well, especially if we're dealing with low-voltage circuitry:
Under the above conditions, this circuit is perfectly fine for some applications, but is problematical if a very low voltage is wanted for a particular circuit.
Another solution is to turn to circuit integration. This allows us to construct a large number of resistors, transistors, diodes, and even small capacitors on a single small piece of silicon. The schematic diagram to the right depicts such an integrated circuit (IC) combined with two electrolytic capacitors to perform all of the functions of both the filter and the voltage regulator. The fact that the IC contains a rather complex circuit is utterly transparent to the user and the circuit designer.
In addition to voltage regulation and current limiting, most IC voltage regulators include thermal sensing and shutdown circuitry. In a hot environment, electronic components have a harder time dissipating lost energy as heat. In addition, that dissipated heat itself can warm up components and change their operating limits and parameters. With discrete components, the answer is to allow sufficient air flow for cooling, and perhaps include a fan for ventilation.
However, the use of a fan or heat sink, while helpful, does not prevent the IC from getting quite warm as it handles significant amounts of power. If it gets too hot, it can easily be destroyed. To prevent this possibility, the IC contains additional circuitry that reacts to temperature increases before any damage can occur. This additional circuitry will reduce the output current if necessary to prevent overheating the IC itself. This thermal shutdown circuit overrides both the voltage regulation and current limit circuits.
IC voltage regulators come in several categories. For the most common voltages, there are two lines of fixed-voltage regulators. The 78xx series of positive voltage regulators provides an output voltage within 5% of its nominal value, which is specified as the "xx" part of the IC designation. Thus, the 7805 produces a +5 volt output, while the 7812 and 7815 produce +12 and +15 volt outputs, respectively.
The 78xx series of voltage regulator ICs is rated at 1 ampere of output current. For lower current requirements, you can use the 78Mxx (500 mA) or the 78Lxx (100 mA) series, respectively. For negative output voltages, the 79xx series does the same job.
IC regulators of this type require a certain amount of "extra" input voltage (usually about 2 volts) across themselves so their internal circuitry can operate properly. This extra input voltage represents wasted power, which is dissipated as heat. These ICs are built to tolerate a certain amount of this, but they become very inefficient if they must drop larger input voltages, or if the output current requirement is too high. A prime example of such a case is the power supply for a modern personal computer. The +5 volt output of such a supply must be able to provide at least 20 Amperes to the system, and there are other output voltages as well. For this sort of application, a switching power supply or switching regulator is required. We'll deal with that issue on a separate page.~
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