Adjustable Symmetrical Power Supply Using LM317 and LM337

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The circuit was designed
to provide an adjustment with a power supply that is symmetrically
designed while providing a voltage range of 1.25V to 30V at 1A current.
LM317 – an adjustable 3-terminal positive voltage regulator capable of
supplying in excess of 1.5A over an output voltage range of 1.2V to 37V
and requires only two external resistors to set the output voltage due
to its internal current limiting, thermal shutdown and safe area
compensation, making it essentially blow-out proof LM337 – an adjustable
3-terminal positive voltage regulator capable of supplying in excess of
5A used as battery chargers, constant current regulators, and
adjustable power supplies due to its features such as protected output
from short circuit, product enhancement tested, current limit constant
with temperature, guaranteed thermal regulation, adjustable output down
to 1.2V, guaranteed 5A, and guaranteed 7A peak output current.


Circuit diagram

The circuit will serve as a voltage converter with an input voltage
of 35 V to produce an output voltage of 1.25 V to 30 V. The positive
voltage is being handled by LM317 IC while the negative voltage is
handled by LM337. The circuit can provide an output current of 1 A.
During the production of 1 A current, the regulator is dissipating too
much heat and without the presence of a heatsink, the regulator may get

Using these types of regulators provide features such as low noise
and low price in the market. It can be made operational even with few
components used. The only disadvantage that it will impose is the poor
conversion efficiency. With the output of 35 V to 5 V, the efficient
ratio of the output power with the input power is less than 42%. This is
the reason why the switching regulator became cheap recently although
the number of external components to be connected is minimally
increased. These regulators will work with better efficiency when used
in case where current is more than 1A for more than 15 V and 0.4 A for
less than 15 V from the power supply. Each regulator is adjusted for
single positive and negative voltage output using the 10K ohms
potentiometers RV1 & RV2. For dual outputs, a dual connected
potentiometer RV3 is made to operate by switch S1. The visual indication
on the voltmeter V1 is shown using the switch S2.


  • R1-2=270ohms
  • R3-4=2.2Kohms
  • R5-6=10Kohms
  • C1-5=100uF/63V
  • C2-4=100nF/100V
  • C3-8=10uF/25V
  • C6-10=100uF/63V
  • C7-9=100nF/100V
  • /RV1-2=10Kohms Lin.
  • RV3=2X10Kohms Lin.
  • IC 1=LM 317T
  • IC 2=LM 337T
  • D1-2=1N4001
  • D3-4=1N4001
  • L1-2=LED 3mm
  • F1-2=1A slow Blow Fuse
  • S1-2=2X ON-ON SW
  • V1=0-30V DC Voltmeter

The adjustable symmetrical power supply is suitable to be used in
audio amplifiers, microphone amplifiers, op-amp applications, impedance
converters and other devices that require regulated positive and
negative DC supply, since the output current is 1 A.