How to make a Dual Power Supply

Key topics : Dual Power Supply uses, Dual power supply using two batteries, Dual power supply using two Linear regulators, Dual power supply using resistor dividers

Uses of Dual Power Supply

Most of the Op-Amp applications require dual power supply due to oscillation around ground. Also they are designed to work with dual power rails. Therefore it is important to provide dual power supplies. Also audio applications like power amplifiers, pre-amplifiers, audio applications with vacuum tubes etc require dual power supplies to work.

Lets see some basic circuits we can used to obtain dual power supplies.

1) Dual power supply using two batteries

Dual Power Supply using two Batteries

This may be one of the easiest way to get a dual power supply with a high current requirement. Voltages of the system changes with the batteries used and the current it can cater depends on the batteries used. in high current applications internal resistance of the batteries will have an impact on the output. This is a very efficient but expensive way to get a dual power supply. This is very popular to get +9 V and -9 V dual power supplies due to availability of cheap 9 V batteries. 

2) Dual power supply using two Linear regulators

Dual power supply using two Linear regulators

This is one of the famous method to obtain a accurate dual power supply using a center tap transformer, full wave bridge rectifier and linear regulators. It is recommended to add two small capacitors on the outputs of the linear regulators depending on the regulator used. Disadvantages of this are, low current capability ( around 1A in 78XX series, which depends on manufacturer, heat sinks, temperature etc) and inefficiency. Read this article to find more about linear regulators. Also see this article to understand the inefficiency of linear regulators. Output voltage of this depends on the linear regulator used. (ex : 7805 & 7905 for 5V power supply, 7815 & 7915 for 15V power supply etc). Also it is important to use a transformer which provides at least 3V higher than what is required in order to cater the voltage drops at linear regulators and diode bridge. However if the voltage difference is too high it will cause too much inefficiency through the linear regulators also.

3) Dual power supply using resistor dividers

Dual power supply using resistor voltage dividers

This simple and cheap arrangement can be used to cheat😏 circuits with small current requirements. (ex: Op-Amp circuits) Since the potential of the GND depends on the voltage divider created using resistors and capacitors (equivalent series resistance) this cannot be used for circuits with higher current requirements. However if the load is balanced around the +V and -V with reference to the ground this can be used. It is really important to choose the resistors with higher wattage as shown in the diagram.