Voltage regulator: Difference between revisions
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[[Image:Voltage-Regulator-IEC-Symbol.svg|right|thumb|130px|[[Electronic symbol]] for Voltage regulator]] |
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A '''voltage regulator''' is an [[electricity|electrical]] [[regulator (automatic control)|regulator]] designed to automatically maintain a constant [[voltage]] level. |
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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 [[Alternating current|AC]] or [[Direct current|DC]] voltages. |
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With the exception of shunt regulators, all modern electronic 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]] [[Control theory|servo control loop]]. If the output voltage is too low, the regulation element is commanded to produce a higher voltage. For some regulators if the output voltage is too high, the regulation element is commanded to produce a lower voltage; however, many just stop sourcing current and depend on the current draw of whatever it is driving to pull the voltage back down. 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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== Electromechanical regulators == |
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[[Image:Simple electromechanical voltage regulator.PNG|thumb|300px|Circuit design for a simple electromechanical regulator.]] |
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[[Image:Stabilizer.JPG|thumb|300px|Inside the old electromechanical Voltage regulator.]] |
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[[Image:Simple electromechanical regulation.PNG|thumb|300px|Graph of voltage output on a time scale.]] |
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In older electromechanical regulators, voltage regulation is easily accomplished by coiling the sensing wire to make an electromagnet. The magnetic field produced by the voltage attracts a moving ferrous core held back under spring tension or gravitational pull. |
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As the voltage increases, the magnetic field strength also increases, pulling the core towards the field and opening a mechanical power switch. |
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As the voltage decreases, the spring tension or weight of the core causes the core to retract, closing the switch allowing the power to flow once more. |
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If the mechanical regulator design is sensitive to small voltage fluctuations, the motion of the solenoid core can be used to move a selector switch across a range of resistances or transformer windings to gradually step the output voltage up or down, or to rotate the position of a moving-coil AC regulator. |
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Early [[automobile]] [[electrical generator|generators]] and [[alternator]]s had a mechanical voltage regulator using one, two, or three [[relay]]s and various [[resistor]]s to stabilize the generator's output at slightly more than 6 or 12 V, independent of the [[internal combustion engine|engine]]'s [[rpm]] or the varying load on the vehicle's electrical system. Essentially, the relay(s) employed [[pulse width modulation]] to regulate the output of the generator, controlling the field current reaching the generator (or alternator) and in this way controlling the output voltage produced. |
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The regulators used for generators (but not alternators) also disconnect the generator when it was not producing electricity, thereby preventing the battery from discharging back through the stopped generator. The [[rectifier]] [[diode]]s in an alternator automatically perform this function so that a specific relay is not required; this appreciably simplified the regulator design. |
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More modern designs now use ''solid state'' technology ([[transistor]]s) to perform the same function that the relays perform in electromechanical regulators. |
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== Mains regulators == |
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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. |
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== Coil-rotation AC voltage regulator == |
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[[Image:Moving Coil Voltage Regulator.png|thumb|250px|Basic design principle and circuit diagram for the rotating-coil AC voltage regulator.]] |
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This is an older type of regulator used in the 1920's that uses the principle of a fixed-position field coil and a second field coil that can be rotated on an axis in parallel with the fixed coil. |
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When the movable coil is positioned perpendicular to the fixed coil, the magnetic forces acting on the movable coil balance each other out and voltage output is unchanged. Rotating the coil in one direction or the other away from the center position will increase or decrease voltage in the secondary movable coil. |
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This type of regulator can be automated via a servo control mechanism to advance the movable coil position in order to provide voltage increase or decrease. A braking mechanism or high ratio gearing is used to hold the rotating coil in place against the powerful electromagnetic forces acting on the moving coil. |
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The overall construction is extremely similar to the design of standard AC [[dynamo]] windings, with the primary difference being that the rotor does not spin in this device, and instead is held against spinning so the fields of the rotor and stator can act on each other to increase or decrease the line voltage. |
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== AC voltage stabilizers == |
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A voltage stabilizer is a type of household mains regulator which uses a continuously variable [[Transformer#Autotransformer|autotransformer]] to maintain an AC output that is as close to the standard or normal mains voltage as possible, under conditions of fluctuation. It uses a [[servomechanism]] (or [[negative feedback]]) to control the position of the tap (or wiper) of the autotransformer, usually with a [[electric motor|motor]]. An increase in the mains voltage causes the output to increase, which in turn causes the tap (or wiper) to move in the direction that reduces the output towards the nominal voltage. |
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An alternative method is the use of a type of saturating transformer called a '''ferroresonant transformer''' or '''constant-voltage transformer'''. These transformers use a [[tank circuit]] composed of a high-voltage resonant winding and a [[capacitor]] to produce a nearly constant average output with a varying input. The ferroresonant approach is attractive due to its lack of active components, relying on the square loop saturation characteristics of the tank circuit to absorb variations in average input voltage. |
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Older designs of ferroresonant transformers had an output with high [[harmonic]] content, leading to a distorted output waveform. Modern devices are used to construct a perfect [[sinewave]]. The ferroresonant action is a [[magnetic flux|flux]] limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage even as the input voltage varies widely. |
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The ferro resonant transformers, which are also know as Constant Voltage Transformers (CVTs) or ferros are also a good surge suppressors, and it provides high isolation and an inherent shortcircuit protections. |
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It can operate with an input voltage range as wide as ±40% or more of the nominal voltage. |
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Output power factor remains in the range of 0.96 or higher from half to full load. |
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Because it regenerates an output voltage waveform, output distortion, which is typically less than 4%, is independent of any input voltage distortion, including notching. |
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Efficiency at full load is typically in the range of 89% to 93%. However, at low loads, efficiency can drop below 60% and no load losses can be as high as 20%. The current-limiting capability also becomes a handicap when a CVT is used in an application with moderate to high [[inrush current]] like motors, transformers or magnets. In this case, the CVT has to be sized to accommodate the peak current, thus forcing it to run at low loads and poor efficiency. |
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Minimum maintenance is required beyond annual replacement of failed capacitors. Redundant capacitors built into the units allow several capacitors to fail between inspections without any noticeable effect to the device's performance. |
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Output voltage varies about 1.2% for every 1% change in supply frequency. For example, a 2-Hz change in generator frequency, which is very large, results in an output voltage change of only 4%, which has little effect for most loads. |
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It accepts 100% single-phase switch-mode power supply loading without any requirement for derating, including all neutral components. |
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Input current distortion remains less than 8% THD even when supplying nonlinear loads with more than 100% current THD. |
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One of the draw back of CVT(constant voltage transformer) is its higher size and high audible humming sound. |
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== DC voltage stabilizers == |
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{{main|Voltage stabilizer}} |
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Many simple DC power supplies regulate the voltage using a ''shunt regulator'' such as a [[zener diode]], [[Avalanche 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 operating capability of the shunt regulating device (commonly, by using a series [[resistor]]). In shunt regulators, the voltage reference is also the regulating device. |
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If the stabilizer must provide more power, the shunt regulator output is only used to provide the standard voltage reference for the electronic device, known as the [[voltage stabilizer]]. The voltage stabilizer is the electronic device, able to deliver much larger currents on demand. |
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== Active regulators == |
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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: |
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* Linear regulators |
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* Switching regulators |
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* SCR regulators |
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=== Linear regulators === |
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{{main|Linear regulator}} |
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Linear regulators are based on devices that operate in their linear region (in contrast, a switching regulator is based on a device forced to act as an on/off switch). In the past, one or more [[vacuum tube]]s were commonly used as the variable resistance. Modern designs use one or more [[transistor]]s instead. Linear designs have the advantage of very "clean" output with little noise introduced into their DC output, but are less efficient and unable to step-up or invert the input voltage like switched supplies. |
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Entire linear regulators are available as [[integrated circuit]]s. These chips come in either fixed or adjustable voltage types. |
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<!-- moved details to [[Linear regulator]] --> |
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=== Switching regulators === |
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{{main|Switched-mode power supply}} |
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Switching regulators rapidly switch a series device on and off. The [[duty cycle]] of the switch sets how much [[electric charge|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. |
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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. (Large-valued inductors tend to be physically large relative to almost all other kinds of componentry, so they are rarely fabricated within integrated circuits and IC regulators — with some exceptions.<ref>http://www.national.com/pf/LM/LM2825.html</ref>) |
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=== Comparing linear vs. switching regulators === |
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The two types of regulators have different advantages: |
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* Linear regulators are best when low output noise is required |
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* Linear regulators are best when a fast response to input and output disturbances is required. |
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* At low levels of power, linear regulators are cheaper. |
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* Switching regulators are best when power efficiency is critical (such as in [[portable computer]]s). |
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* Switching regulators are required when the only power supply is a DC voltage, and a higher output voltage is required. |
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* At high levels of power (above a few watts), switching regulators are cheaper. |
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<!-- (Does the above summary already include all this information ?) |
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Switching regulators overcome the main disadvantage of linear regulators: low efficiency. |
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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. |
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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). |
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--> |
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=== SCR regulators === |
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Regulators powered from AC power circuits can use [[silicon controlled rectifier]]s (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. |
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=== Combination (hybrid) regulators === |
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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. An efficient way of creating a variable-voltage, accurate output power supply is to combine a multi-tapped transformer with an adjustable linear post-regulator. |
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== See also == |
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* [[DC to DC converter]] |
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* [[Voltage regulator module]] |
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* [[Third brush dynamo]] |
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== References == |
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<references/> |
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[[Category:Electrical power control]] |
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[[Category:Electronics terms]] |
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[[de:Spannungsregler]] |
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[[es:Regulador de tensión]] |
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[[fr:Régulateur de tension]] |
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[[pl:Regulator napięcia]] |
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