26-11-2012, 11:43 AM
15 - STEP DIGITAL POWER SUPPLY
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Here is a simple circuit to obtain variable DC voltage from 1.25V to 15.19V in reasonably small steps as shown in the table. The input voltage may lie anywhere between 20V and 35V. The first section of the circuit comprises a digital up-down counter built around IC1— a quad 2-input NAND schmitt trigger (4093), followed by IC2— a binary up-down counter (4029). Two gates of IC 4093 are used to generate
up-down logic using push buttons S1 and S2, respectively, while the other two gates form an oscillator to provide clock pulses to IC2 (4029). The frequency of oscillations can be varied by changing the value of capacitor C1 or preset VR1. IC2 receives clock pulses from the oscillator and produces a sequential binary output. As long as its pin 5 is low, the counter continues to count at the rising edge of each clock pulse, but stops counting as soon as its pin 5 is brought to logic 1. Logic 1 at pin 10 makes the counter to count upwards, while logic 0 makes it count downwards. Therefore the counter counts up by closing switch S1 and counts down by closing switch S2. The output of counter IC2 is used to realise a digitally variable resistor. This section consists of four N/O reed relays that need just about 5mA current for theiroperation. (EFY lab note. The original circuit containing quad bilateral switch IC 4066 has been replaced by reed relays
operated by transistorised switches because of unreliable operation of the former.) The switching action is performed using BC548 transistors. External resistors are connected in parallel with the reed relay contacts. If particular relay contacts are opened by the control input at the base of a transistor, the correspond-ing resistor across the relay contacts gets connected to the circuit.