Variable Voltage power Supply Build using LM317T

This variable voltage power supply build is for the quadraphonic decoder that requires 20 V power supply. At first, I bought some 20 V regulators from China, but sadly they turned out to be faulty/fakes, hence I decided to build a variable voltage power supply using the LM317T which I happened to have. First I used my calculator, by entering 20 V for the required voltage, and 1 kΩ for R1. The calculator computed the value of R2 to be 15 kΩ. Since I happened to have some 100 kΩ presets, I decided to use that instead for R2. Although the target voltage was required to be 20 V, I felt being able to vary the voltage a few volts either way would be useful as I wanted to see how low I could set the voltage and still have the quadraphonic circuits operate well.

I realised the 20 V regulator was a fake when I measured the voltages and found that when the input voltage was 25 V, the output voltage was 24 V. This is not how a good voltage regulator behaves and therefore there was no option but to remove it. As you can see, I used a pair of pliers and snipped off the offending voltage regulator. I had to do this because I usually varnish and seal my boards from the underside and consequently it would have been difficult to remove the solder.

I then fashioned some crimp plugs and decided to connect those to the cut terminals.

This was the first time I started to use these blue-coloured presets and was impressed with their precision. They seem to be well-made and I was able to adjust the voltage very easily.

Although the circuit is a small one, I had to use a slightly bigger board due to the mounting holes at all the corners. My top tip when working with the LM317T is to double-check its pinout because it is not the same as the 78-series voltage regulators. On the LM317T, the centre pin is output, however if you are not sure, then my build will be useful.

I experimented with the supply voltage to see how low I could set the voltage and still have the quadraphonic circuits operate reliably. At 15 V the audio started to get fuzzy and distorted, however at 18 V it was clear; hence I set mine to 18.5 V. I have been using it at this lower voltage and have not noticed any side effects. Since 18 V regulators are very common, and if you happen to have a nice genuine one, then it is probably better to use that, then order expensive 20 V ones that might turn out to be fakes.

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