Automatic transformer tap switcher for bench power supply

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?
 

Elvee

Member
2006-09-08 2:04 pm
You could inspire yourself from automatic voltage stabilizers: they were popular ~30 years ago, but you can still find schematics:

http://4.bp.blogspot.com/-JaW-OdnXs...s1600/TRIAC+CONTROLLED+STABILIZER+CIRCUIT.png

The purpose and voltages are different, but the principle can readily be reused: you just need to adapt the components value.

An advantage of triacs is that your supply won't sound like a castanets band in operation....
 

nec3

Member
2005-06-21 5:08 pm
Ankara
4 Comparators may not be enough to prevent "boom" . Any fault in comparators, rezistors would easily cause more than one output go high.

So, I think you should add another logic circuit that would drive only one tranzistor "in every case" .

Maybe, a circuit consisted of ex-or gates which will give you a high output only when inputs all are not same and use its "fault" voltage output to let relay drivers work.

And, i dont think a triac would be easy to use since you have AC voltages as low as 6 volts because of its conduction voltages and relatively high hold on currents. So, mechanic relay seems best option in your case.
 
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?
My, now getting quite old, lab supply uses noisy relays to switch the transformer taps as I turn the output voltage adjust knob.
Works well and flawlessly.
There may be a sch for a lab supply somewhere on the net.