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A voltage regulator is an electrical device 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 (tranistors) to do the same.

These regulators operate by controlling the field current reaching the generator (or alternator) and in this way controlled 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. The principal disadvantage of linear regulators is that because the regulating device is acting as a variable resistor, the power dissipated by the regulating device is equal to the power supply output current times the voltage drop in the regulating device.

Entire linear regulators are available as integrated circuits. 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 transistor-transistor logic circuits) and 12 V (for communications circuits and peripheral devices such as disk drives). In fixed voltage regulators the reference pin is tied to ground, 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 potentiometer) allows the user to adjust the regulated voltage.

Switching regulators

(main article: Switched-mode power supply)

Switching regulators overcome the main disadvantage of linear regulators: low efficiency. 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. 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).

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.

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 regualtor.

See also

• Switched-mode power supply
• DC to DC converter

• Power supplies
• Electronics terms