Step Digital Power Supply

CIRCUIT

IDEAS

15-STEP DIGITAL POWER SUPPLY
ere 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 maylie anywhere between 20V

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 justabout 5mA current for their

ing resistor across the relay contacts gets
connected to the circuit.
The table shows the theoretical output for various digital input combinations.
The measured outputis nearly equal to
the theoretically calculated output across
regulator IC3 (LM317). The output voltage is governed by the following relationship as long as the input-to-output differential isgreater than or equal to 2.5V:
Vout = 1.25(1+R2'/R1')
Where, R1' = R15 = 270 ohms (fixed)

and 35V.
The first section of the circuit comprises a digital up-down counter built
around IC1— a quad2-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 theother 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 pulsesfrom 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 pin5 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 countsoperation. (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IC 4066 has been replaced by reed relays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...