Automatic transformer tap selector for bench power supply

[diydidi]

HI Guys.
I'm in the process of building a variable bench power supply. 0-50V @5A.
I have wound the transformer secondary for 0-6-12-22-44 taps. By doing this I can geep the dissipation through the output transistors to a minimum.
I would like to build a circuit that can automatically switch the taps in and out as I advance the voltage pot etc.
Obviously only one relay should be on at any given time, otherwise two of the taps may be shorted. BOOM!!
I will have to power whatever circuit and reference voltage from one of these taps as I don't have space for another auxilary power supply to power this circuit too. I was thinking 4 comparators driving npn transistors, switching 4 relays in and out.
Any ideas? Anyone build something similar before?

[J M Fahey]

1) just use a multitap switch and a fine tuning pot for the intermediate steps.
If nothing else, it will give you finer tuning.

2) you should have wound it in a linear fashion, so each step is, say, 5 or 10 V away; as is 0/6 and 6/12 would make pass transistors dissipate max 30W but 12/22 is 50W and 22/44 is staggering 110W

In reality it's much worse, because I considered raw DC voltage steps instead of AC steps, just to explain the problem,

3) for most jobs you'll need *dual* supplies , +/-V , single supplies being *very* limited in what they are good for.;
Above 9V for pedals or 12V high current supplies, useful for testing car or battery type equipment, higher voltage high current single supplies are those boat anchors you get for pennies at Ham Fests or School/Lab surplus auctions ... because today nobody needs them.

If anything, you'd need two of them, in series.

[Enzo]

I think you are struggling to make it overly complex. There are good variable voltage regulator circuits all over online. No relays needed, just a good heat sink.