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| A '''voltage regulator''' is an ] designed to automatically maintain a constant ] level. |
A '''voltage regulator''' is an ] ] designed to automatically maintain a constant ] level. | ||
| It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, It may be used to regulate one or more ] or ] voltages. | |||
| With the exception of shunt regulators, all voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element. This forms a ] ]. If the output voltage is too low, the regulation element is commanded to produce a higher voltage. If the output voltage is too high, the regulation element is commanded to produce a lower voltage. In this way, the output voltage is held roughly constant. The control loop must be carefully designed to produce the desired tradeoff between stability and speed of response. | With the exception of shunt regulators, all voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element. This forms a ] ]. If the output voltage is too low, the regulation element is commanded to produce a higher voltage. If the output voltage is too high, the regulation element is commanded to produce a lower voltage. In this way, the output voltage is held roughly constant. The control loop must be carefully designed to produce the desired tradeoff between stability and speed of response. | ||
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| (main article: ]) | (main article: ]) | ||
| Linear regulators insert a variable resistance in series with the load current. In the past, one or more ]s were commonly used as the variable resistance. Modern designs use one or more ]s instead. Linear designs have the advantage of very "clean" output with little noise introduced into their dc output |
Linear regulators insert a variable resistance in series with the load current. In the past, one or more ]s were commonly used as the variable resistance. Modern designs use one or more ]s instead. Linear designs have the advantage of very "clean" output with little noise introduced into their dc output. | ||
| Entire linear regulators are available as ]s. These chips come in either fixed or variable voltage types. | |||
| Entire linear regulators are available as ]s. These chips come in either fixed or variable voltage types. Common solid-state series voltage regulators are the LM78xx (for positive voltages) and LM79xx (for negative voltages), and common fixed voltages are 5 V (for ] circuits) and 12 V (for communications circuits and peripheral devices such as ]s). In fixed voltage regulators the reference pin is tied to ], whereas in variable regulators the reference pin is connected to the centre point of a fixed or variable voltage divider fed by the regulator's output. A variable voltage divider (such as a ]) allows the user to adjust the regulated voltage. | |||
| <!-- moved details to ] --> | |||
| === Switching regulators === | === Switching regulators === | ||
| (main article: ]) | (main article: ]) | ||
| Instead of controlling a variable resistance, the output of a switching regulator is controlled by rapidly switching a series device on and off. The ] of the switch sets how much ] is transferred to the load. This is controlled by a similar feedback mechanism as in a linear regulator. Because the series element is either fully conducting, or switched off, it dissipates almost no power; this is what gives the switching design its efficiency. Switching regulators are also able to generate output voltages which are higher than the input, or of opposite polarity - something not possible with a linear design. | |||
| Like linear regulators, nearly-complete switching regulators are also available as integrated circuits. Unlike linear regulators, these usually require one external component: an ] that acts as the energy storage element. (Unfortunately, the inductor must be external because large-valued inductors tend to be physically large relative to almost all other kinds of componentry; because of this, they are impossible to fabricate within integrated circuits.) | Like linear regulators, nearly-complete switching regulators are also available as integrated circuits. Unlike linear regulators, these usually require one external component: an ] that acts as the energy storage element. (Unfortunately, the inductor must be external because large-valued inductors tend to be physically large relative to almost all other kinds of componentry; because of this, they are impossible to fabricate within integrated circuits.) | ||
| === Comparing linear vs. switching regulators === | |||
| Sometimes only one or the other will work: | |||
| * Linear regulators are best when low output noise is required | |||
| * Linear regulators are best when a fast response to input and output disturbances is required. | |||
| * Switching regulators are best when power efficiency is critical (such as in ]s). | |||
| * Switching regulators are required when the only power supply is a DC voltage, and a higher output voltage is required. | |||
| In many cases either one would work. So the choice comes down to which costs less. At high levels of power (above a few watts), switching regulators are cheaper. At low levels of power, linear regulators are cheaper. | |||
| <!-- (Does the above summary already include all this information ?) | |||
| Switching regulators overcome the main disadvantage of linear regulators: low efficiency. | |||
| The principal disadvantage of linear regulators (compared to switching regulators) is that because the regulating device is acting as a variable resistor, it dissipates much more power. | |||
| Switching designs have largely superseded linear circuits where high levels of power are involved (above a few watts). The main disadvantages of the switching regulator is greater complexity and hence cost, and high frequency noise at the output (which usually must be filtered out). | |||
| --> | |||
| === SCR regulators === | === SCR regulators === | ||
Revision as of 14:34, 22 June 2005
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level.
It may use an electromechanical mechanism, or passive or active electronic components. Depending on the design, It may be used to regulate one or more ac or dc voltages.
With the exception of shunt regulators, all voltage regulators operate by comparing the actual output voltage to some internal fixed reference voltage. Any difference is amplified and used to control the regulation element. This forms a negative feedback servo control loop. If the output voltage is too low, the regulation element is commanded to produce a higher voltage. If the output voltage is too high, the regulation element is commanded to produce a lower voltage. In this way, the output voltage is held roughly constant. The control loop must be carefully designed to produce the desired tradeoff between stability and speed of response.
Electromechanical regulators
Early automobile generators and alternators had a mechanical voltage regulator using two or three relays and various resistors to stabilize the generator's output at slightly more than 6 or 12 V, independent of the engine's rpm or the varying load on the vehicle's electrical system. More modern designs use solid state technology (transistors) to do the same.
These regulators operate by controlling the field current reaching the generator (or alternator) and in this way controlling the output voltage produced by the generator.
Mains regulators
Electromechanical regulators have also been used to regulate the voltage on ac power distribution lines. These regulators generally operate by selecting the appropriate tap on a transformer with multiple taps. If the output voltage is too low, the tap changer switches connections to produce a higher voltage. If the output voltage is too high, the tap changer switches connections to produce a lower voltage. The controls provide a deadband wherein the controller will not act, preventing the controller from constantly hunting (constantly adjusting the voltage) to reach the desired target voltage.
Shunt regulators
Many simple dc power supplies regulate the voltage using a shunt regulator such as a zener diode, avalanche breakdown diode, or voltage regulator tube. Each of these devices begins conducting at a specified voltage and will conduct as much current as required to hold its terminal voltage to that specified voltage. The power supply is designed to only supply a maximum amount of current that is within the safe operarating capability of the shunt regulating device (commonly, by using a series resistor). In shunt regulators, the voltage reference is also the regulating device.
Active regulators
Because they (essentially) dump the excess current not needed by the load, shunt regulators are inefficient and only used for low-power loads. When more power must be supplied, more sophisticated circuits are used. In general, these can be divided into several classes:
- Linear regulators
- Switching regulators
- SCR regulators
Linear regulators
(main article: Linear regulator)
Linear regulators insert a variable resistance in series with the load current. In the past, one or more vacuum tubes were commonly used as the variable resistance. Modern designs use one or more transistors instead. Linear designs have the advantage of very "clean" output with little noise introduced into their dc output.
Entire linear regulators are available as integrated circuits. These chips come in either fixed or variable voltage types.
Switching regulators
(main article: Switched-mode power supply)
Instead of controlling a variable resistance, the output of a switching regulator is controlled by rapidly switching a series device on and off. The duty cycle of the switch sets how much charge is transferred to the load. This is controlled by a similar feedback mechanism as in a linear regulator. Because the series element is either fully conducting, or switched off, it dissipates almost no power; this is what gives the switching design its efficiency. Switching regulators are also able to generate output voltages which are higher than the input, or of opposite polarity - something not possible with a linear design.
Like linear regulators, nearly-complete switching regulators are also available as integrated circuits. Unlike linear regulators, these usually require one external component: an inductor that acts as the energy storage element. (Unfortunately, the inductor must be external because large-valued inductors tend to be physically large relative to almost all other kinds of componentry; because of this, they are impossible to fabricate within integrated circuits.)
Comparing linear vs. switching regulators
Sometimes only one or the other will work:
- Linear regulators are best when low output noise is required
- Linear regulators are best when a fast response to input and output disturbances is required.
- Switching regulators are best when power efficiency is critical (such as in portable computers).
- Switching regulators are required when the only power supply is a DC voltage, and a higher output voltage is required.
In many cases either one would work. So the choice comes down to which costs less. At high levels of power (above a few watts), switching regulators are cheaper. At low levels of power, linear regulators are cheaper.
SCR regulators
Regulators powered from ac power circuits can use SCRs as the series device. Whenever the output voltage is below the desired value, the SCR is triggered, allowing electricity to flow into the load until the ac mains voltage passes through zero (ending the half cycle). SCR regulators have the advantages of being both very efficient and very simple, but because they can not terminate an on-going half cycle of conduction, they are not capable of very accurate voltage regulation in response to rapidly-changing loads.
Combination (hybrid) regulators
Many power supplies use more than one regulation method in series. For example, the output from a switching regulator can be further regulated by a linear regulator. The switching regulator accepts a wide range of input voltages and efficiently generates a (somewhat noisy) voltage slightly above the ultimately desired output. That is followed by a linear regulator that generates exactly the desired voltage and eliminates nearly all the noise generated by the switching regulator. Other designs may use an SCR regulator as the "pre-regulator", followed by another type of regulator.