# Basic Symmetrical Power Supply

4.4 5 5 Product

A basic stabilized symmetrical-power supply unit is presented here. A stabilized symmetrical power supply unit is an embedded circuit, or stand alone unit, the function of which is to supply two complementary stable voltages to a circuit or device that must be operated within certain power supply limits. A “stabilized” power supply unit ensures that the output remains within certain limits under various load conditions, or it may also include compensation for variations in its own supply source.

The presented circuit is quite simple and it is based on the well known 78xx and 79xx, linear voltage regulators. These regulators are easy to use and require only some few external capacitors. They allow over 1.0A of load current if adequate heat sinking is provided.

The 78xx and 79xx series of three terminal regulators are available with several fixed output voltages making them useful in a wide range of applications. The xx is replaced with two digits, indicating the output voltage (for example, the 7805 has a 5 volt output, while the 7812 produces 12 volts). The 78xx line are positive voltage regulators: they produce a voltage that is positive relative to a common ground. There is a related line of 79xx devices which are complementary negative voltage regulators.

The stabilized symmetrical-power supply unit uses 78xx and 79xx ICs in combination, to provide positive and negative supply voltages simultaneously. Below, we present the complete electronic schematic of the symmetrical- power supply unit.

The main supply source is a double – winding transformer. D1, D2, D3 and D4 provide full wave rectification. C1 and C2 are charged at about 1.4 * Vin (where Vin, is the voltage provided at each winding). For 78xx and 79xx regulators, the input voltage must always be higher than the output voltage by some minimum amount (typically 2volts). This means that the condition 1.4*Vin>XX+2 must always hold true, to ensure normal operation. Choosing Vin to be exact equal to XX, we ensure that for XX>5V, the above condition holds true. We could choose Vin to be much higher but it is better to keep 1.4*Vin as low as possible to minimize heat losses. At this point it is good to remember that in any linear regulator, the power loss due to heating is the current times the voltage dropped across the regulator (Heat loss in Watts = (V input – V output)* current.