Small Variable Power Supply
Miroslav Adzic - Serbia & Montenegro
Features: 1.3-12.2 V, 1 A, over-current protection. This is a simple but reliable device based one of the oldest integrated
voltage regulators of them all - the LM723.
R2 sets the output voltage. The maximum current is determined by the value
of R3: the over-current protection circuitry inside the LM723 senses the voltage across R3 and
starts shutting the output stage off as soon as this voltage approaches 0.65 V. This way the
current through R3 can never exceed 0.65/R3, even if the output is shorted.
C3 and C4, both ceramic, must be placed as close as possible to the integrated circuit, because the LM723 can
be prone to unwanted oscillations. It is not an overkill to solder them directly (and very carefully) to the
pins of the IC. All other connections should also be kept short.
The LM723 works with input DC voltages from 9.5 to 40 V and the IC itself
can source some 150 mA if the output voltage is not more than 6-7 V below the input. When an external pass
transistor is used (in the usual emitter-follower mode), the base-emitter junction of T1 represents a
significant resistance and the integrated circuit's output stage is relatively lightly loaded. All the current
drawn by the load passes through T1 and it dissipates an amount of power that is directly proportional to the
current and the difference between the input and the output DC voltage.
Output (approximate values):
Vmin = (R4 + R5) / (R5*1.3)
Vmax = (7.15 / R5) * (R4 + R5)
Imax = 0.65/R3
Max. Power on R3: 0.42/R3
Min. Input DC Voltage (pin 12 to pin 7): Vmax + 5
C1 2200uF (3300uF even better)
D1 Green LED
F1 0.2A F
F2 2A M
IC1 LM723 (in a DIL14 plastic package)
R2 Pot. 5k
T1 2N3055 on a heatsink 5°K/W