Wednesday, 22 July 2020

Simple Variable Voltage Bench PSU

Back in the days of the desktop PC, a common DIY project was to re-use its ATX power supply as the basis of a Bench PSU. Sadly those days are slipping into the past as more and more of us move to laptops. All is not lost however, as most laptop transformers also deliver a reasonable output voltage which can easily be down-regulated and supplied to nascent bench projects.

The circuit schematic could hardly be simpler --- I am fairly embarrassed that the best I could do to obtain it for insertion here was "Print Screen" in KiCad. All of the action is provided by an LM317 linear regulator which costs peanuts these days. The output voltage is given by:

Vout = Vref * (1 + RV1 / R1), Vref = 1.25v

Note that this is independent of the input voltage, although obviously it can't be greater. Also we want to choose R1 so that the output is at sensible values for differing positions of the (linear) potentiometer RV1. Given a transformer rated at 19v, we choose R1 = 680R, giving values of Vout:

RV1    Vout 
0      1.25
2k5    5.8
5k     10.4
7k5    15
10k    19.6

Obviously the last value is a little too high, however this value of R1 gives the greatest precision for the transformer at hand.

The design is completed with a cheap LED voltmeter as shown below.


Simple extensions to this design would be to use a combined Voltmeter/Ammeter display (also costing peanuts); adding a current-limiting circuit, or a switched constant-current source, also supported by the LM317. Next time!

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