What Richard talks about is AWSOME Circuit called "pre regulator" But made of SCR's
Actually that is another good approach, but I am going 100% linear - for better or for worse.
The question is this - say you have four DC voltages from the transformer/rectifier circuit. How do you automatically switch between the four different voltages without any relays?
Here is the circuit. It is one of those circuits that you look at and say "How can that work", so I did it as a LTSpice project so people can play with it.
![](https://www.eevblog.com/forum/projects/lab-psu-design-ideaquestions/?action=dlattach;attach=41792)
The devices are just ones I picked quickly from LTSpice. You would need MOSFETs rated for high current linear operation - most are not. The NPN darlington pair does all the regulation of the output. For reasons I have mentioned previously, I think transistors are very much superior power supply regulator devices then MOSFETs.
V5 is replaced by the driver transistor from the regulator circuit.
It is simple reducing or increasing the stages - just follow the pattern. This approach does need extra mosfets, diodes and electrolytic capacitors which is why most modern budget power supplies use other circuits, but it does switch supply voltages instantly and I just think it is a neat idea. No relays. No weird glitched as you wind up the voltage - just a smooth transition.
All three MOSFETs can be mounted near each other as when one winding is active, the others are either hard on or hard off.
The plots show that there is a seemless transition of current from the appropriate winding:
![](https://www.eevblog.com/forum/projects/lab-psu-design-ideaquestions/?action=dlattach;attach=41794)
If you have two center tapped windings on the transformer, here is how you get 2 DC voltages out from each winding:
![](https://www.eevblog.com/forum/projects/lab-psu-design-ideaquestions/?action=dlattach;attach=41790)
If anyone needs an explanation, let me know, but see if you can work out how the circuit works. The LTSpice file is below.
Richard.