While I am still working on my function generator kit (well, after my week-long break), I had originally planned to put together multiple kits, all in the same "How Analog Works" style. The damn FG is taking too long; I'm going to simultaneously put together a much simpler one, so that I will actually have
something completed before I grow old
This is a power supply (0-30V 500mA) made entirely from discrete components, but designed to have the line and load regulation of a "proper", opamp-controlled PSU. The regulation loop is fully differential with optional, external sense terminals (obviously not too necessary at 500mA, but they are used to cancel out any internal resistance including through an ammeter). Mostly SMD again, but unlike the FG there aren't as many parts, so it's all 0805. (Unfortunately SOT-363 is a bit tricky to solder and I needed three of them. Inexpensive matched pairs are a bit hard to find.)
I have tested the circuit, but using hand-matched transistor pairs without good thermal coupling, as I do not have the specified parts. Line regulation was 1mV on 30V over an input voltage range from 35 to 45V (0.03%), load regulation was about the same from 0 to 500mA. Once I order PCBs, I will fully characterize the circuit for line and load regulation, temperature coefficient and transient response with the correct parts installed, and will make a probably futile attempt at measuring the output noise with the equipment I have on hand...
The total BOM cost for the control PCB is $10 sourcing all parts from Mouser in decent-quality versions; I will be able to cut probably 20% from this by sourcing general-purpose transistors and resistors elsewhere. This does not include an enclosure, a transformer, a heat sink, the contents of the front panel (pots, meter, meter switch, banana jacks, power switch, power indicator), and the contents of the rear panel (mains connector, fuse holder and fuse).
Modification for power greater than 500mA is fairly simple and will be addressed in the assembly instructions, but will require significant heat sinking (40W for 1A, 80W for 2A).
I will order PCBs tonight; more info to follow after testing them.
P.S. It looks like KiCad's PDF export is fixed now. Previously, I had to export all greyscale PostScript and convert to PDF. If anybody has problems opening the schematic, please let me know.
P.P.S. I'm going to work on the documentation tonight, so if anybody has any "how does this part work?" questions, please wait until I post that. If you have any "you're an idiot and this will never work" comments, feel free to post them now before I spend too much time documenting a brainfart
P.P.P.S. If anybody has any good info on temperature-compensating a Zener, I'm all ears. I used a 6.2V diode + 1N4148, which seemed like a good combination according to the datasheets. There appears to have been a series of compensated Zeners at one point, but I can't find any now. I'm not going to use a bandgap reference because that violates the "all-discrete" design pattern here. I'm not
too worried about temperature compensation, as that's more of a long-term drift property, at least once the unit warms up, but I
am making an effort to keep it from being dreadful. It's not a precision instrument, but I do like a power supply that starts up to the same voltage I set it to yesterday.
P.P.P.P.S. This is the most P.S.'s I've ever used.