hi barry,
Regarding the student who wanted to convert his +24v dc bench power supply
to a general purpose +/-12 volt supply.
Download the diagram, *.jpg format, it uses a -12v reg [7912] to give +/-12v.
The technique is sometimes referred to as a phantom ground. If the current drawn from the
-12v is less than 100mA, the 7912 will not require any heat sinking.
The cost of a 7912 reg, is around $2.
The IMPORTANT detail is that the 24v bench psu outputs must be floating.
Most bench psu's are, so that they can be used, as a negative voltage with the
positive terminal connected to your equipment ground.
The same method can be used on battry supplies, provided that the battery
is not already connected to ground.
The +12v side will always be +12v, provided that the +24v supply output is greater
than +15v. The nominal -12v will however, be the difference between 24v psu's actual
output and 12 volt.
Example;
1. If the 24v is 24v then you will get +12v and -12v.
2. if the 24v is actually 25v the +12v is +12v and the nom -12v becomes -13v
3. if the 24v is actually 23v the +12v is +12v and the nom -12v becomes -11v.
This negative voltage change when using op amps has little effect on the
op amp, as they design to be tolerant regarding voltage supplys [within limits].
A point to remember is that if you are using a +/- voltage rail divider for your
op amp experiments, use a zener reference diode in the divider supply, especially
the -v rail.
The psu could be configured using a +v reg [7812], in the 24v positive line but
the +12v would become nominal. I prefer to have the +12v supply stable.
You could connect a +5v reg between the +12v rail and ground to provide a +5v
voltage for any TTL i/c's devices you may use in your experiments.
Don't exceed the maximum current rating of the 24v supply.
Keep us posted.
Regards
EricG