Because a large power supply with a multikilogram transformer is bulky and unwieldy and cannot be easily carried in a pocket. A lower-accuracy, lower-oomph, single-channel-only stabilized power supply powered from a 12V wall-wart or a netbook power supply is a good pocket-sized alternative for many uses.
For maintaining high efficiency, the power circuit is realized as a buck converter, using a LM2576 chip, a 3-ampere step-down regulator, the adjustable version.
The voltage feedback is provided from the output via a 1-to-4 selector switch. A dual sliding switch is used with appropriate connection of the pins, to select the output of one of four resistor dividers; three are realized as trimpots for fixed voltages, the fourth one is the adjustable output potentiometer.
The pin 4, Feedback, of the regulator serves to keep its output voltage stabilized on the desired value; it has to be kept on value equal to the chip's internal reference (1.25 volts). When kept floating or grounded, the output voltage is as high as the chip can provide. When connected to the output voltage, the output is kept on 1.25 volts.
To protect the output against overvoltage during switching, a 100kOhm resistor is connected between the feedback pin and the output. When the switch momentarily breaks during selecting (or if a wire breaks off due to e.g. vibrations, or a pot slider momentarily loses contact), the resistor pulls the input up and the output voltage drops; if left hanging, the voltage would surge up instead, with possible adverse effects to the attached circuitry.
The chip is not normally able to provide less than 1.25 volts. To address this drawback, a pair of ordinary silicon diodes rated for sufficient amperage is used in series; each one has about 0.6-0.65 volts across itself when loaded; this is enough to bring the voltage down almost to zero.
As the voltage stabilization suffers when the diodes are present (the voltage dropped on them is somewhat dependent on the output current), a bypass switch is added to allow selection between a lower but less accurate voltage and a higher, more accurate one.
A pair of cheap VU-meters was chosen for orientational readout of voltage and current, for the reasons of availability and cost.
The voltmeter is connected on the output and has one fixed range.
The ammeter is connected in parallel to a shunt; the feedback for the regulator is taken from the shunt's output, in order to neutralize the voltage drop on the shunt (which is higher than if a "genuine" ammeter was used). To achieve easier setting/checking of lower currents, the ammeter has two ranges; 0..2A and 0..200mA. The 200mA-range shunt, 1.5 ohm, is fixed; the 2A-range one, 0.15 ohm, can be connected in parallel via the range selector switch.
A provision for connection of an external (presumably digital) ammeter is available on the pins of the header-connector; the ExtAm+ and ExtAm-, the positive and negative leads, are available there. The pins are bypassed with a switch when the external-ammeter feature is not used.
The input power is connected via a 2.1mm barrel jack, a standard for most wall-warts. A 3mm LED is located next to the connector, indicating when voltage is present on the input.
A green LED is built into the voltmeter, just above the main power switch, to indicate that the power supply is switched on and operational.
A green LED is connected to the output, partly to act as some minimal load for the chip (otherwise it tends to slowly charge the capacitors up and up), partly to indicate the output voltage with its brightness.
A pair of binding posts was used for the main output.
A 7x2 header connector was used for auxiliary outputs. The three topmost pairs are connected in parallel and assigned to the output voltage; a higher-accuracy digital voltmeter can be connected here. The two subsequent pairs are connected together and acting as a tiny solderless breadboard (e.g. to form an ad-hoc resistor divider or a LED tester etc...). The next pair is the positive and negative lead for the external ammeter. The last pair is the unstabilized input voltage, from both sides of the power switch; the left pin is energized only when the main switch is on, the right one is powered always when there is power on the input power connector.
Outside front top
Outside left, power connector
Outside front, diode/ammeter selector
Outside right, outputs
Outside top, heatsink