An affordable bench power supply unit is presented here. It is capable to provide up to 1.5A, from 0 to 25V. The circuit is quite simple and it uses very common electronic parts.
The Bench Power Supply Circuit
The circuit can be divided in three sections. The first section, at the left part of the circuit, uses an ordinary transformer and a rectifier bridge for converting AC to about 33V DC. Diodes D1, D2 and C1 are used to generate a secondary, negative DC voltage.
At the right part of the circuit, we use an ordinary LM317 linear voltage regulator (U2). Normally, the minimum output voltage from U2 can be as low as 1.25V. In order to generate output voltage lower than 1.25V and as low as 0V, we need an additional minus (-) 1.25V voltage reference, which is provided from the voltage drop across D5 and D6. D5 and D6 are biased from the secondary negative DC voltage through Q2, which is used as a stable current source. Q2 stabilizes the voltage drop across D5 and D6 to about minus (-) 1.25V.Thus, using P2, the output voltage can be adjusted from almost 0 to 25V.
At the middle part of the circuit, we use a second LM317 regulator (U1), which acts as an adjustable current limiter. R6 is used as a current sensor. As the current increases, the voltage drop across R6 also increases and as soon as it reaches about 1.25V, it triggers the internal current limiter of U1. Using P1, we are able to adjust current limiter’s threshold at any desired level.
The circuit can be easily assembled using the printed circuit board provided below. Transformer connection to the PCB is achieved threw ordinary cables. It is recommended to use an appropriate metal enclosure and mount P2, P1 and also an appropriate voltage meter at the front panel. As in any linear power supply unit, heat loss increases as the output voltage decreases. To avoid triggering LM317’s auto shut-off due to excessive heat, use an appropriate heat sink for both regulators.
Click to download PCB Artwork and Composite drawing for the Bench Power Supply