Dual Polarity Power Supply

This dual polarity power supply is easy to build, requires few parts, and is adjustable from 0-15 volts. It is great for powering op amp circuits, as well as other circuits that require a dual supply voltage.

Schematic

Parts

Part	Total Qty.	Description	Substitutions
C1, C2	2	2200uF 35V Electrolytic Capacitor
C3, C4, C5, C7	4	1uF 35V Electrolytic Capacitor
C6, C8	2	100uF 35V Electrolytic Capacitor
R1, R4	2	5K Pot
R2, R3	2	240 Ohm 1/4 W Resistor
BR1	1	2A 30V Bridge Rectifier
U1	1	LM317 Adjustable Positive Regulator
U2	1	LM337 Adjustable Negative Regulator
T1	1	30V Center Tapped 2 Amp Transformer
S1	1	SPST 2 Amp Switch
MISC	1	Heatsinks For U1 And U2, Line Cord, Case, Knobs For Pots, Wire

Notes

1. Since this project operates from 120 (or 220, or 240, etc.) volts AC, it MUST be built inside a case.

2. U1 and U2 get quite hot and will require heatsinks. A fan is usually not needed.

3. You can, of course, add a volt and amp meter.

4. U1 and U2 can only go down to a minimum of +-1.2V. If you need to go lower, you can add two 1N4003 diodes in series with the output of the regulator. The diodes drop about 0.6V each, which will allow the supply to go to 0. Note that this will also decrease your maximum output voltage by 1.2V.

Dual-rail Variable DC Power Supply

Dual-rail Variable DC Power Supply

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Simple add-on for a single-rail supply ±2.5V to ±15V output

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Circuit diagram:

Parts:

R1,R2____________4K7 1/2W 1% or 2% Metal Oxide Resistors
C1,C4,C5_______100nF  63V Polyester Capacitors
C2,C3__________220µF  25V Electrolytic Capacitors
Q1____________BD437   45V 4A NPN Transistor
Q2____________BD438   45V 4A PNP Transistor
IC1___________LM358   Low Power Dual Op-amp

Input and output connecting terminals etc.

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Comments:

This design was conceived as an add-on for the Variable DC Power Supply, a very successful circuit posted to this website in the year 2000.
This simple unit provides a dual-rail variable output ranging from ±2.5V to ±15Vdc with precise tracking of the positive and negative output voltages, still retaining the current limiting and short-proof capabilities of the "master" circuit.
As the purpose of such a dual-rail design is to supply experimental or under-repair circuits, the maximum current output delivered was deliberately kept to about 500 - 600mA per rail, thus avoiding the use of expensive power transistors and complex circuitry.

Notes:

• The circuit can be placed into the existing Variable DC Power Supply metal cabinet.
• Q1 and Q2 must be mounted on heatsinks. Usually, bolting them to the metal case (through insulating washers etc.) proved effective.
• The full ±15V output can be achieved only if the secondary winding of the supply Transformer used in the Variable DC Power Supply is rated at 48V minimum (center tapped).
• When using this circuit, please set the Current-limit control (P1) of the Variable DC Power Supply to any value comprised in the 50mA - 1A range but not higher.
• The second Op-amp (IC1B) contained in the LM358 chip was not used, but its input pins were tied to the negative supply and the output was left open.