Right Transformer for 12v ?

[Mark Amundson]

Steven Cook wrote:
> Hi all.
> A simple question: I am making a +/-12v PSU using halfwave rectifier / 1000u /
> 7812 / 7912 regulators. Would it be better to use a 12v or 15v wall wart? Also
> would a 20v wall wart be usable? Hope someone can help! Steve Cook.

The 12VAC wall-wart in the standard voltage doubler configuration should
give you about plus and minus 16 volts DC with 1N4001 diodes and 1000uf
caps. The 7812/7915 regulators operate best with at least 3 volts
difference between input and output pins. Don't forget .01uf ceramic
across the wall-wart output for RFI, 1uf tantalums from input to ground
for regulator stability, and back-bleed 1N4001 diodes pointing reverse
bias across regulator input and output pins.

You will need a reasonably beefy wall-wart (Volt-Amp Rating) because of
the inefficiencies of AC to DC rectification and regulation. If you need
more than 100mA of current, you need additional heatsinking of the
regulators. The traditional factor of 1.8 times the DC supply current
for the AC supply current should be doubled again for the voltage
doubler topology.

If you use this supply in real world rock concert situations, consider
moving up to the 15VAC wart for more voltage overhead under common
concert brownout conditions. Sometimes 25 percent of AC line voltage is
lost in wiring when stage power is tied into public address system
amplifiers. More regulator heatsinking is needed too.

If you can, track down an old National Semiconductor Voltage Regulator
Handbook for all the design tips and design margin equations.

Mark Amundson,