Power Supplies

# Simple symmetrical power supply

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.

An embedded circuit, or stand-alone unit, having the purpose of supplying two complimentary stable voltages to a circuit or device that must operate within particular power supply constraints is known as a stabilized symmetrical power supply unit.

Integrated voltage regulators LM317 and LM337 enable a simple, regulated, symmetrical power supply as shown in the diagram to be made. The LM337 serves a wide variety of applications including local, on card regulation. This device can also be used to make a programmable output regulator, or by connecting a fixed resistor between the adjustment and output, the LM337 can be used as a precision current regulator.

This supply is variable from 0 V to ± 15 V, can provide a current of up to 1.5 A, is short-circuit proof, and has thermal protection. Moreover, the regulators have internal dissipation limiting which ensures, irrespective of their cooling, that’ the dissipation does not exceed 20 W.

To enable the output voltage to be varied from 0 V, auxiliary voltages of ± 1.4 V are created with the aid of two Zener diodes, D1 and D2. Although technically inelegant, this arrangement is the most cost-effective in a simple power supply.

The regulators must be fitted with a heat sink with a thermal resistance of 1.75 K W-1.

The maximum current of 1.5. A is available only if the maximum dissipation of 20 W has not been reached and (with an input voltage of ±18 V as shown), the output voltage is above 5 V. At lower output voltages, the maximum output current drops to 1 . 1 A (at U0 = 0 V).

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