Simple symmetrical power supply

Integrated voltage regulators LM317 and LM337 enable a simple, regulated, symmetrical power supply as shown in the diagram to be made. 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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