Basic Symmetrical Power Supply

Small Symmetrical - Power Supply
Small Symmetrical - Power SupplyBasic Symmetrical Power Supply
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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.

Basic Symmetrical Power Supply

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.

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2021-05-10 15:47
C2 and C3 in the 3 way tone control board are very close to the op amps. This is essential for best decoupling. In theory, a voltage source has an internal impedance equal to zero. In practice, due to cables length and other parasitic impedances, this is not always the case in AC (especially in high frequencies). Bad decoupling may lead to oscillations and noise. This is why we use decoupling capacitors as close as possible to the ICs - in order to provide a zero impedance path to ground for the AC – i.e to make equal to zero the internal impedance of the source in high frequencies. The capacitors in the power supply unit are for stabilizing the regulators (to avoid oscillations) and they are as close as possible to the regulators but not as close as possible to the 3 way tone control ICs. Many hobbyists, think that smoothing and decoupling are the same thing but this is not true. Smoothing is for ripple rejection while decoupling is to provide a low impedance path to the ground at AC.
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2021-05-10 10:06
I am new to circuit building, and my experience is more then 40 tears old. However, I am looking forward to getting started. My question is concerning the output capacitors. In the schematic for the 3 way tone control, they show the output of the a 15V Power Supply with C2 and C3 at 100 nF each. Are these necessary given the 2 100 nF capacitors that are already in the o/p circuit?
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2020-02-08 12:56
Is the ability of C7 and C8 to handle the high frequencies because they have much smaller capacitance values and thus much smaller time constants, allowing them to charge and discharge faster?
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2020-01-27 19:50
Never mind, confused them for series.
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2020-01-22 08:21
But then doesn't c1 become 0.47uf?

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