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?

 

[JohnRoberts]

I would consider a slightly different approach. Imagine if you just provide a full voltage high power winding. Then instead of using a diode to charge the reservoir cap to full unregulated voltage, use a solid state switch that turns on at some lower voltage after the peak.  This works kind of like a light dimmer to deliver a lower and possibly variable unregulated rail.

I did this circuit trick decades ago to pull a high current 5V supply from a 20V transformer without a ton of power dissipation.  You could vary this adjustable unregulated voltage to track above the regulated output to manage dissipation in the regulator pass element.

JR
[edit]  it might be simpler to just use a switching regulator. [/edit}

 

[moamps]

Any ideas? Anyone build something similar before?

I have somewhere schematic for Leader LPS-164 which is 0-30V/0-5A. It uses three taps and two simple discrete comparators for switching taps. If you are interested please PM me.

 

[CJ]

you can get a small variac for about 69 bucks,

 

[Samuel Groner]

I think I'd sense the voltage drop across the regulator, and then use count up/down logic to switch the windings. I'd look into solid-state switches (SRCs?) instead of mechanical relays--much better reliability.

I remember an article in the Hewlett-Packard Journal that described their approach for a lab supply; I check if I can find it.

Samuel

 

[Samuel Groner]

Here we go: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1977-06.pdf

Samuel

 

[moamps]

...I remember an article in the Hewlett-Packard Journal that described their approach for a lab supply..

Here is service manual for this HP power supply. It looks a little bit complicated for DIY .
http://www.mcs-testequipment.com/resources/Datasheets_Downloads/Agilent/Agilent%206002A%20DC%20Power%20Supply%20operating%20and%20service%20manual.pdf