Trying to make a bench power supply. What would be a cheap easy way to reduce voltage (to protect the series pass transistor)? 3 Amp transformer has these terminals 12-6-0-6-12. No load voltage is 33 VAC (47 Vpeak DC). Primary taps are 0 and 230V. I came up with these options, and can't decide between them.
Couple of 5A rated household AC staircase switches between the 6V and 12V terminals. Cheap and easy, but manual control. Series transistor needs thermal protection against "accidents" (including output short circuits).
Couple of relay tap changer between 6V and 12V terminals. Maybe cheap, not easy. Needs zero crossover, Schmitt trigger control circuit. Relays will probably need eventual replacement.
Triac tap changer. Possibly more complex than above, and may need more parts list. Triac has voltage drop.
Triac (or SCR) to chop AC waveform on secondary side. I have no clue where to begin. Have seen the old HP manuals. But if bad comes to worse, I am ready to pop in an Atmega88.
60V buck converter. Where to get a reliable one? Is switching noise an issue?
Series pass regulator to reduce voltage to below 40V (max dissipation 2W @ 750mA), then a 40V buck converter, and then a series pass linear regulator? This is becoming confusing, and wasting a lot of volts. Will also have switching noise.
Roll my own buck converter???
I'm leaning towards #1 right now (couple of $0.15 switches). Power supply will be used for analog work mostly (test op amps and audio amplifiers etc and other hobby electronics stuff). Is the inability to smoothly change voltage from zero to 30V or 40V, a likely hindrance? I don't think I've ever had to change voltage by more than ±3V for anything (usually characterizing LED, or optimizing current draw for small projects). Why would someone want to smoothly change voltage all the way from 0 to 30V? Only convenience, or something more?
I'm hoping this works out, as transformers seem to cost less than a third of a good 36V SMPS (same wattage). Only problem with transformers is the >40V at no load, and then it goes downhill.
So, any suggestions?