120VAC to a2VDC 30A power supply circuit needed

[jgeidl]

120VAC to 12VDC 25A power supply circuit needed

[audioguru]

19VAC makes an unregulated 24.9VDC which is much too high for a 12V/25A regulator. The regulator will heat with (24.9V - 12V) x 25A= 323W!

If the transformer was 12VAC then its unregulated voltage would be 15VDC and the regulator would dissipate "only" 75W.

[ante]

If the transformers are toroide type its easy to fix. Normally the secondary winding is on top, just wind of until you get the desired voltage or make a tap and wind it all back again. If its not a toroide transformer you might still be able to rip of some turns to lower the secondary voltage even though it

[jgeidl]

Thanks Guys.

I don't pretend to understand how you  get from 19 VAC to 24VDC through rectification but will assume that to be true.  Modifying the transformer is really not a practical possibility.  These transformers are from UPSs so I am wondering how they do it; they provide the power to keep the internal battery charged.  I may have to go to plan B...Oh, wait, I don't have a plan B. :-)

How about a big honkin' Zener diode after the rectifiers and before the voltage regualtor?

Jim

[mvs sarma]

Case.1
You try TRIAC control on primary with ofcourse suitable line filteres to meet local regulatory needs.
then see that the output is stepped down to 12V. I feel this concept should serve your needs.

Case2:
you try to have halfwave rectification and try to regulate with nominal filter. then the output voltage should fall down.  this time you may neeed  higher power diode to supply larger average current in the absence of second halfwave.

Case3:
ypu go for non isolated dc-dc converter to convert the rectified 19VAc (may be around (1.4.14*19V)-2*diode drop

as the supply is already isolated you dont need another transformer. a high current Frerrite inductor will do.

Sarma

[Kevin Weddle]

If the transfomer is center-tapped, you could get +- 12. Then add parallel regulators for as much current as you need. 25A is a lot of DC currrent.

Otherwise, they make good variable regualtors with high input voltage and any ouput voltage and good current.

Or you could throw in some high power transistors to step down the voltage. It's a little more complicated because you have to design it around your usual load. If the load is light, you 'll need a shunt.