On Mon, 20 Oct 2003 20:28:06 GMT, Jim Higgins
wrote:

The basic design demonstrates a generally acceptable approach to
designing a power suppy, but some of the components chosen just
don't seem up to meeting the design criteria of 24 volts in, 12
volts out at 30 amps.

Such designs are simply brain dead.

If you need +12 V at 30 A and you are using NPN series pass
transistors, you should use about 13 V at the end of the capacitor
discharge cycle. This should be doable with far less than 24 V
capacitor nominal voltage.

Assuming the capacitor voltage drops from say 17 V to 13 V during each
half cycle, the power dissipation in the series pass transistors will
be ((17+13)/2 V)-12 V) x 30 A or 90 W, so this is doable with 3-6
transistors.

However, the required base voltage varies between 14 and 18 V, so a
separate power supply is required, either with a separate secondary
winding or by using some kind of voltage doubling, but this needs to
supply only 2-4 A.

The other alternative is to use PNP series pass transistors on the +12
V side, in which case a high, separate voltage is not needed.

Paul OH3LWR