thrifty voltage regulator

Posted on Feb 6, 2014

One of the drawbacks of a three-pin voltage regulator is that the input voltage needs to be 2. 5 3 V higher than the output voltage. This makes these integrated regulators unsuitable for battery power supplies. If, for instance, the output voltage is 5 V, a 9 V battery could be discharged to 7. 5 V or thereabouts only. On top of this, most of these regulators draw a current of about 2 mA.

thrifty voltage regulator
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Special low-drop versions sometimes offer a solution, but they are not ideal either. The regulator described here is rather thriftier: it draws a current of only 300 µA and the difference between its input and output is only 100 200 mV In the circuit diagram, T1 is arranged as a series regulator, which means that the difference between input voltage and output voltage is limited to the transistor`s saturation potential. Therefore, a 9 V battery can be discharged to about 5 V, which is quite an improvement on the situation with an integrated regulator. Diodes D1-D2-D3, or a suitable zener diode (D4), in conjunction with R5 and P1, form a variable reference voltage source, which is used as the (output-dependent) base potential of T3. If the output voltage drops below a desired level, the base potential of T3 also drops. The transistor then conducts less hard and its collector voltage rises. The base voltage of T2 also rises, so that T1 is driven harder. This results in the near-instantaneous restoration of the output voltage. The design of the reference voltage source is clearly of paramount importance. The current through the LEDs or the zener diode is of the order of only 100 µA. This means that thedrop across a 5. 1 V zener diode is only 4. 3 V and across each LED, only about 1. 43 V. For a wanted output voltage of 4. 8 V, the three LEDs proved very effective, whereas the zener did not. It may well be necessary, if a...

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