LM340 Series Voltage Regulators

 A voltage regulator is one of the most commonly used electronic circuitry in any device. A synchronized voltage (without fluctuations & noise levels) is very significant for the smooth functioning of many digital electronic devices. As a regular case with microcontrollers, a smooth regulated input voltage must be supplied to the microcontroller to function smoothly. A voltage regulator is found in electronic devices; it is consumed to maintain the voltage of the power source to ensure that the voltage stays within suitable limits.  This article discusses the types of Voltage regulators and Lm 340 series voltage regulator.

Voltage Regulators
Voltage Regulators

What is a Voltage Regulator?

A voltage regulator is an electrical or electronic machine that maintains the voltage of a power source within suitable limits. The voltage regulator is desired to keep voltages within the prescribed range that can be tolerated by an electrical apparatus using that voltage. Such a device is commonly used in motor vehicles of all types to ensure equal output voltage of the generator to the electrical load and to ensure the charging requirements of the battery. Voltage regulators are also used in electronic apparatus in which excessive variations in voltage could be detrimental.

IC Voltage Regulator
IC Voltage Regulator

LM340 Series Voltage Regulator

The voltage regulator using the LM340 IC is the mostly used voltage regulator IC. A build-in reference voltage is shown in the block diagram of the LM340 IC below.

3 Terminal Voltage Regulator
3 Terminal Voltage Regulator

Vref drives from the non-inverting input of the operational amplifier. There are various stages of the voltage gain of the op-amp used here. This high gain helps the op-amp to build an error voltage between inverting and non-inverting terminals to almost zero. Thus, the inverting input terminal value will be similar as the non-inverting terminal, Vref. Thus, the current flowing through the potential divider can be written as

I = Vref/R2

The resistor R2, as shown in the diagram, is not an exterior component connected to the IC, but an internal resistor, which is built inside on the IC by the manufacturer. Due to the above conditions, the same current flows through the R1. Thus the output voltage can be written as


Vout = Vref/R2 (R1 + R2)

This shows that the output of the regulator can also be controlled by putting the desired values for R1 and R2. The IC has a series pass transistor, which is capable of handling more than 1.5 A of load current provided that sufficient heat sinking is provided along with it.

LM 340
LM 340

Like other IC’s, this IC also has thermal shutdown and current warning options. Thermal shutdown is a feature that turns off the IC as soon as the inside temperature of the IC rises above its preset value. This rise in temperature may mostly be due to the excessive exterior voltage, ambient temperature, or even due to heat sinking. The preset cut off temperature value for LM340 IC is 175° C. Because of the thermal shutdown and current limiting, the devices in the LM 340 series are almost indestructible.

LM340-15 Circuit
LM340-15 Circuit

The above diagram shows the application of the LM340 IC as a voltage regulator. The Pins 1, 2, and 3 are the input, output and also ground.

If there is quite a bit of distance (in cms) from the IC to the filter capacitor of the unregulated power supply, then there may be a chance of unwanted oscillations happening within the IC due to lead inductances within the circuit. In order to remove this unnecessary oscillation, the capacitor C1 has to be placed as shown in the circuit. The capacitor C2 is sometimes used to develop the transient reaction of the circuit.

Any device in the LM 340 series needs a minimum input of the voltage, which should be at least 2 to 3 V greater than the regulated output voltage – otherwise , it stops regulating. Furthermore, there is a maximum input of voltage because of excessive power dissipation.

Types of Regulators

Basically, there are two types of Voltage Regulators: – Linear voltage regulator and Switching voltage regulator. In this article only linear voltage regulator is being discussed. The Linear voltage regulators are of two types: Series and Shunt.

Linear Regulator

Linear regulator acts as a voltage divider. In the Ohmic region, it uses a FET. The resistances of the voltage regulator are a variation with load resulting in constant output voltage.

Advantages of the Linear Voltage Regulator

  • Gives a low output ripple voltage
  • Fast response time load or line changes
  • Low electromagnetic interference and less noise

Disadvantages of the Linear Voltage Regulator

  • Efficiency is very low
  • Requires a large space heat sink
  • Voltage above the input cannot be increased

Series Voltage Regulator

A series voltage regulator is also named as a series pass voltage regulator. It uses a variable element located in series with the load. Due to the unreliability of the resistances in the series element, the voltage dropped across it can be varied to ensure that the voltage across the load remains constant.

Series Voltage Regulator
Series Voltage Regulator

The benefit of the series voltage regulator is that the quantity of current drawn can be efficiently used by the load, although some current would be consumed by any circuitry connected to the regulator. In contrast to the shunt regulator, the series regulator does not draw full current even when the load does not need any current. As a result, the series regulator is significantly more efficient.

Shunt Voltage Regulator

A shunt voltage regulator works by providing a path from the supply voltage to the ground through a variable resistance. The current through the shunt regulator is diverted away from the load and then flows uselessly to the ground, making this form generally less efficient than the series regulator. It is, however, simpler, sometimes consisting of a voltage-reference diode; it is used in a very low-powered circuit wherein the wasted current is too small to be of concern. This form is very general for voltage reference circuits. A shunt regulator can usually only sink (absorb) current.

Shunt Voltage Regulator
Shunt Voltage Regulator

Applications of Shunt Regulators

  • Low output voltage switching power supplies
  • Current source and sink circuits
  • Error amplifiers
  • The adaptable voltage or current linear and switching power supplies
  • Voltage monitoring
  • Analog and Digital Circuits that require precision references
  • Accuracy current limiters

This is all about Lm340 series voltage regulators and their applications. We believe that the information given in this article is helpful to you for a better understanding of this concept. The second-generation IC regulators are three-terminal devices that can hold the output voltage constant. The LM340 series is a typical case of second-generation IC regulators. The regulated voltages of the LM340 series are from 5 to 24 V. LM340 devices include current limiting and thermal shutdown. When an IC regulator is more than a few inches from the supply, it may be necessary to connect a bypass capacitor across the regulator input. The input voltage to an LM340 device should be at least 2 or 3 V greater than the regulated output.

Furthermore, for any queries regarding this article or for any help in implementing electrical and electronics projects, you can contact us or comment in the comment section given below.

Photo Credits: