REGULATOR_CIRCUIT

Published:2009/7/17 3:29:00 Author:Jessie | From:SeekIC

The special characteristic of this regulator is that the output voltage can be adjusted down to0 V. The regulation is provided by'an integrated regulator Type LM317. As is normal in supplies that can be adjusted to 0 V, this IC is used in conjunction with a zener diode. This diode provides a reference voltage that is equal, but of opposite sign, to the reference voltage (Ur) of the regulator, as shown in Fig. 74-1(a). Potential divider R1/R2 enables the output voltage to be adjusted. In this circuit, the negative reference voltage is derived in a different manner: from the regulator with the aid of an op amp (Fig. 74-1(b)). The op amp is connected as a differential amplifier that measure, the voltage across R1 and inverts this voltage to Ur. An additional advantage of this method is that at low-output voltages, a change in the reference voltage has less effect on the output voltage than the circuit in Fig. 74-1(a). The prototype, constructed as shown in Fig. 74-1(c), gave very satisfactory results. The op amp need not meet any special requirements: a μA741 works fine, although an LF356 gives a slightly better performance. The negative supply for the op amp can be obtained with the aid of a centertapped mains transformer.



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