Yeah, we all know that every electronics hobbyist needs a variable power supply. An off the shelf (normally 0-30v 3A variety) is readily available and some quite cheap Chinese models can be found on eBay from about £40+. I wanted however, to build my own.
I started by by buying this ‘Kit’ circuit from Banggood at just over £6 (about $7.50 at current exchange rates). This is specified to deliver upto 30V @ 3A but I doubt that level of performance is often if ever achieved.
The first potential hurdle I met was the transformer required to power this circuit. A 24V AC input is specified and in the UK, with a actual mains voltage around 240VAC (it should be 230VAC but it is normally a lot higher), that means that a transformer rated at 230VAC input – 24VAC output will actually deliver at least 25VAC on the secondary output. When rectified,the resulting DC voltage being delivered to the banggood circuit will be at least 35VDC. The particular transformer I had available actually resulted in an (unloaded), rectified output of 40VDC! This would have fried the TLO81 opamps in the kit so something had to be done. With some trepidation, I decided to take some windings off the toroidal transformer (I have never done this before) and successfully reduced the eventual rectified voltage to 31VDC
I will be describing the build in more detail in the next edit.
The completed power supply delivers 25VDC at 2.5A and I am perfectly satisfied with that power rating.