Oct 262018
 

Dual polarity variable voltage source

This dual polarity variable voltage source allows for two independent voltage outputs. One positive and one negative with a maximum amplitude of +/- 15 volts, an a current up to 1.5 amps.

This power supply is ideal for powering dual polarity circuits such as some audio amplifiers and applications with operational amplifiers, among others.

How does the double polarity variable voltage source work?

This voltage source uses the variable voltage regulator LM317 for the positive voltage output (from 1.2 to 15 volts) and the variable voltage regulator LM337 for the negative voltage output (from -1.2 to -15 volts). To vary the output voltage, two potentiometers are used. See the diagram.

Dual Polarity Variable Voltage Source

It must be remembered that these regulators have the limitation of not being able to deliver voltage equal to or less than 1.2 volts in the case of positive voltage output and -1.2 volts in the case of negative voltage output.

To solve this problem, two rectifier diodes (for example 1N4003) can be placed in series with the output of each regulator, which will give a voltage drop of: 0.6V + 0.6V = 1.2V. This will cause the final output to be always diminished in this voltage, and thus also deliver “0 volts”, but for most applications it is not a problem. (diodes are not shown in the diagram)

The electrolytic capacitors: C1, C2, C3, C4 are the filters that flatten the output wave of the diode bridge before it enters each regulator. Capacitors C5, C6, C7 and C8 are used to improve the transient response.

LM317 and LM337 Voltage Regulators Pin Distribution

LM317 and LM337 Voltage Regulators Pin Distribution

Each voltage regulator must have a heatsink, the larger the better. If the use of the voltage source is constant, a fan can be placed to cool the heatsinks.

List of the dual polarity variable voltage source components

  • 2  240 ohms / ¼ watt resistors: (R3, R4)
  • 2  5K potentiometers (R1, R2)
  • 2  2200uF / 35 volts electrolytic capacitors (C1, C2)
  • 4  1uF / 35 volts electrolytic capacitors (C3, C4, C5, C6)
  • 2  100uF / 35 volts electrolytic capacitors (C7, C8)
  • 1  2 amps diode bridge (DB)
  • 1  LM317 positive voltage regulator (U1)
  • 1  LM337 negative voltage regulator (U2)
  • 2  heatsinks, for U1, U2
  • 1  switch to place on the primary of the transformer
  • 1  110/220 to 30VAC, 2 amps central tapped transformer, (T).
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