Fully Adjustable Power Supply

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Based on a National Semiconductor
application note, this circuit uses an LM317 3-terminal regulator
(REG1), chosen because of its built-in over-current and over-temperature
protection. Its output is boosted up to just over 5A by the MJ2955
transistor (Q1). The output voltage is varied by adjusting the voltage
on REG1’s ADJ terminal using VR1 (a 10kO
potentiometer), via the 270O resistor. Adjustable current limiting is
provided by op amp IC1, used as a comparator, which monitors the voltage
across the 0.1O current sensing resistors. Once this voltage exceeds a
level set by potentiometer VR2, then its output goes low, dragging down
the adjust pin of REG1 and thus the output voltage.

Fully Adjustable Power Supply

Fully Adjustable Power Supply

LED1 illuminates when current limiting is occurring. The 10kO voltage
adjust potentiometer (VR1) has one side connected to -5V instead of 0V
so that the output voltage can be varied down to 0V instead of 1.2V
(normal limit of an LM317). Trimpot VR3 is adjusted to set the minimum
output voltage to +100mV or so. Note that because the -5V rail is used
as a reference, it should be regulated using an LM7905 or similar. The
LM317 3-terminal regulator and Q1 should be mounted on the same heatsink
to take advantage of REG1’s thermal control.
Author: Philip Chugg – Copyright: Silicon Chip Electronics