Wednesday, January 16, 2019

Linear Voltage Regulator


Voltage Regulator

Voltage Regulator takes an input voltage and creates a regulated output voltage irrespective of the input voltage at either a fixed voltage level or adjustable voltage level. In simple words, a voltage regulator is used to regulate voltage level.

Types of Voltage Regulators

There are mostly two types of voltage regulators: first one is Linear voltage regulators and the second are switching voltage regulators. Here let’s discuss Linear Voltage Regulator;

Linear Voltage Regulator

A linear voltage regulator delivers a steady output voltage from a more or less steady input voltage source. In normal operation, even if the input voltage varies fast, the output voltage remains steady. This means they can also very efficiently filter out input ripple, not only at the fundamental frequency, but also for fifth or tenth harmonic. The constraint is only the response speed of the internal error amplifier feedback circuit.
7805
7805 Pinout Diagram
Most linear regulators have a closed loop control for voltage regulation, below figure shows it;
Linear Regulator
Linear Regulator Block Diagram

Linear Voltage Regulator working

  • The pass transistor is the regulatory element, efficiently a variable resistor that controls the current flowing from input to output.
  • The resistor divider Ra/Rb is chosen so that at the required output voltage, the divided down voltage at the error amp inverting input is the same as the Vref voltage at the non-inverting input.
  • The error amplifier controls its output in a way that the voltage variance between its inputs is always zero.
  • If the voltage at the output rises due to a drop in the load or an increased input voltage, the voltage at the inverting input of the error amplifier increases higher than Vref voltage and the output of the error amplifier goes negative, so decreasing the drive to the pass transistor and reducing the output voltage. 
  • If the load rises or the input voltage falls, the voltage at the inverting input drops below the Vref voltage and the drive to the transistor is increased to raise the output voltage to compensate.
  • Therefore the same feedback loop controls for both input voltage differences (line regulation) and changes in load (load regulation).
  • If the output is shorted to ground, the transistor would be turned ON hardly and a very high current would flow from input to output, so a second internal circuit is needed to limit the current.
Below figure shows the Linear Regulator with current limiting;
Linear regulator with current limiting
Linear regulator with current limiting

  • The sense resistor, Rs is used for the current limiting which uses the voltage drop across to monitor the output current. When the current is high enough so that the voltage exceeds 0.7V, transistor Q2 starts to conduct to take current away from transistor Q1, thus decreasing the drive and limiting the output current, thus Ilimit = 0.7V/Rs.
  • The current limit needs to be set well above the maximum current that would flows during usual process. Normally the limit is 150% - 200% greater than the rated current. As the regulator is not deactivated during a short circuit, it is in constant overload.
  • The dissimilarity between input and output voltage is dropped by the pass transistor. If the input voltage is 12V and the regulated output voltage is 5V, then 7V has to be dropped by the transistor.
  • This means more power is dissipated in the regulator ac compare to actually deliver to the load. This is the reason why many linear regulators require a heat sink.
  • If in the case the input voltage drops below the output voltage, the linear regulator cannot compensate and the output voltage will follow the down input voltage.
  • If the input voltage drops too low, the internal power supply to the error amplifier and Vref will be compromised and output may become unbalanced or start to fluctuate.
  • Linear regulators also execute poorly in stand-by mode. In the case of no load, a usual 78xx series regulator needs around 5mA to power the error amp and its reference voltage circuits.
  • The advantages of linear regulators are low price, good control features, it has low noise and it has low emissions and outstanding transient response.
  • The disadvantages are high quiescent consumption and very low efficiency for large input/output voltage changes.

Efficiency of a Linear Voltage Regulator

The efficiency of linear regulator is defined by the ratio of the supplied output power Pout to the power consumption Pin.

η = Pout/Pin

Where;
Pout = Vout x Iout
Pin = Vin x Iin
Iin = Iout + Iq

Iq is the quiescent current of the linear regulator which is measured at under no-load conditions. The equation can be rewritten as;

η = (Vout x Iout) / Vin(Iout + Iq)

The voltage regulator has to be equipped with a large sufficient heat sink to permit safe operation under the worst-case circumstances of maximum input voltage and maximum output current.

Problems with Linear Voltage Regulator

Linear regulators have a number of advantages on the one hand, but also have some disadvantages that require special precaution in their use.

Drop out problem with Linear Regulator
Drop out problem with Linear Regulator
As stated before, if the voltage difference between input and output is below the required range (typically 2V), then the regulation loop can no longer work correctly. A common application problem arises when a rectified AC input has a high voltage ripple because the smoothing capacitor is too small as shown in below diagram. If the input voltage falls below the fall-out voltage on each half cycle, then the regulated output will show periodic dips at twice the mains frequency. These quick dips will not show up on a multimeter which just measures the normal output voltage, but can cause circuit problems. This effect can be removed by either using bigger smoothing capacitors or increasing the turn’s ratio of the transformer.

Linear Voltage Regulator circuit diagram

Below circuit diagram shows a type of Voltage Regulator i.e. Linear Voltage Regulator;
Linear Voltage Regulator
Linear Voltage Regulator Circuit Diagram

Application of Linear Voltage Regulator

  • Voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other circuit elements. For example, it takes in 90V-240V and provides 12V, 5V, 3.3V, -12V for the system to run and for the laptop the output voltage of the power adapter is about 19-20V.
  • In automobile alternators and central power station generator plants, a voltage regulator regulates the output of the plant.
  • In an electric power distribution system, voltage regulators may be mounted at a substation or along supply lines so that all customers get stable voltage independent of how much power is drawn from the line.
  • Mobile phone charger, it takes in 100V-240V (AC/DC) and will give you a stable 5V DC output. If the input voltage stays within the range the output will continually be the same.
  • We are using micro-controllers everywhere and mostly all micro-controller runs on 5V and a very low current so in most cases a 7805 is placed with them to make sure that the controller runs correctly.
  • Any sensitive electronic device that needs steady input voltage is most of the time supplied by a regulator.
  • For AC voltage regulator we can use the voltage stabilizer that takes input a somewhat fluctuating voltage and provides a stable AC voltage. These types of regulators are usually used with freezers and televisions.

Conclusion

Voltage regulators like Linear Regulator are a great select for powering ON very low powered devices in which the difference between the input and output is small. They are easy to use, simple and cheap, a linear regulator is usually incompetent.

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