What is LM317 Voltage Regulator : Circuit & Its Working

The power supply received at the load end or consumer end has fluctuations in the voltage levels due to irregular loads or based condition of the local power grid. These voltage fluctuations may lead to the reduction of the lifespan of the electrical and electronic appliances of the consumer or damage to the loads.  So, it is required to protect loads from over and under voltages or need to provide a constant voltage to the loads and to maintain stability in system voltage using the regulation technique. Voltage regulation can be defined as maintaining constant voltage or maintaining the voltage level of a system within acceptable limits over a wide range of load conditions and thus, voltage regulators are used for voltage regulation. For linear voltage regulation, and occasionally adjustable LM317 voltage regulator is used wherein the non-standard voltage is intended.

What is the Voltage Regulator?

The voltage regulation in a power supply system can be achieved using an electrical or electronic device called as voltage regulators. There are different types of voltage regulators such as Fixed voltage regulators and Variable voltage regulators. These are again subdivided into many types as Electronic voltage regulators, Electro-mechanical regulators, Automatic voltage regulators, linear voltage regulators, Switching regulators, LM317 voltage regulators, Hybrid regulators, SCR regulators, and so on.

Voltage Regulator
Voltage Regulator

LM317 Voltage Regulator

LM317 Voltage Regulator
LM317 Voltage Regulator

It is a type of positive-linear-voltage regulators used for voltage regulation,  which is invented by Robert C. Dobkin and Robert J. Widlar while they worked at the National Semiconductor in 1970. It is a three-terminal-adjustable-voltage regulator and is easy to use because to set the output voltage it requires only two external resistors in the LM317 voltage regulator circuit. It is majorly used for local and on-card regulation. If we connect a fixed resistor between the output and adjustment of the LM317 regulator, then the LM317 circuit can be used as a precision current regulator.

LM317 Voltage Regulator Circuit

The three terminals are input pin, output pin, and adjustment pin. The LM317 circuit is shown in the below figure is a typical configuration of the LM317 voltage regulator circuit diagram including the decoupling capacitors. This LM317 circuit is capable to provide variable DC power supply with an output of 1A and can be adjusted up to 30V. The circuit consists of a low-side resistor and high-side resistor connected in series forming a resistive voltage divider which is a passive linear circuit used to produce an output voltage which is a fraction of its input voltage.

Decoupling capacitors are used for decoupling or to prevent undesired coupling of one part of an electrical circuit from another part. To avoid the effect of noise caused by some circuit elements over the remaining elements of the circuit, the decoupling capacitors in the circuit are used for addressing the input noise and output transients. A heat sink is used with the circuit to avoid the components getting overheated due to more power dissipation.

LM317 Voltage Regulator Circuit
LM317 Voltage Regulator Circuit


There are some special features of the LM317 regulator and a few are as follows:

  • It is capable of providing an excess current of 1.5A, hence it is conceptually considered as an operational amplifier with an output voltage ranging from 1.2V to 37V.
  • The LM317 voltage regulator circuit internally consists of thermal overload protection and short circuit current limiting constant with temperature.
  • It is available in two packages as 3-Lead Transistor Package and surface mount D2PAK-3.
  • Stocking many fixed voltages can be eliminated.

Working of Voltage Regulator LM317 Circuit

The LM317 regulator can provide excess output current and hence with this capacity, it is conceptually considered as an operational amplifier. The adjustment pin is the inverting input of the amplifier and to produce a stable reference voltage of 1.25V, an internal bandgap reference voltage is used to set the non-inverting input.

The output pin voltage can be continuously adjusted to a fixed amount using a resistive voltage divider between the output and ground, which will configure the operational amplifier as a non-inverting amplifier.

A bandgap reference voltage is used to produce constant output voltage irrespective of the changes in supply power. It is also called a temperature-independent reference voltage frequently used in integrated circuits.

The output voltage (ideally) of the LM317 voltage regulator circuit

Vout = Vref * (1+ (RL/RH))

An error term is added because some quiescent current flows from the adjustment pin of the device.

Vout = Vref * (1+(RL/RH)) + IQR

For achieving more stable output, the LM317 voltage regulator circuit diagram is designed such that to make the quiescent current less than or equal to 100 micro Ampere. Thus, in all practical cases, the error can be ignored.

If we replace the low-side resistor of the divider from the LM317 voltage regulator circuit diagram with the load, then the resulting configuration of the LM317 regulator will regulate the current to a load. Hence, this LM317 circuit can be treated as LM317 Current Regulator Circuit.

LM317 Current Regulator
LM317 Current Regulator

The output current is the voltage drop of reference voltage across the resistance RH and is given as

Output current in the ideal case is

Iout = Vref/RH

Considering the quiescent current, the output current is given as

Iout = (Vref/RH) + IQ

These linear voltage regulators LM317 and LM337 are frequently used in DC-DC converter applications. Linear regulators naturally draw much current as they supply. The power produced due to the multiplication of this current with the voltage difference between the input and output will be dissipated and wasted as heat.

Due to this, heat is required to be considered for significant design and leads to inefficiency. If the voltage difference increases, then the power wasted will increase, and sometimes this dissipated waste power will be more than the supplied power.

Even though this is insignificant, but as the linear voltage regulators with a few additional components is a simple way to obtain stable voltage, so, we must accept this trade-off. The switching voltage regulators are alternative for these linear regulators as these switching regulators are generally more efficient, but they require more number of components to design and thus need more space.

Hope this article gives a brief about the LM317 voltage regulator circuit with working. Furthermore, for any clarifications regarding the voltage regulators and their applications, you can feel free to contact us by posting your comments or queries in the comment section below.

Comments are closed.