On 2/18/2013 9:49 AM, klem kedidelhopper wrote:
I'm in the process of building a dual purpose power supply. The supply
will have two outputs. One will be 24VAC rated at 4.0 A. The load on
this supply will probably never exceed 2.0 A. The second supply will
consist of a bridge rectifier off the 24V tap connected to a suitable
filter cap of say 1000UF. This filtered DC will then be connected to a
small surplus 12V regulator board and heat sink assembly which by the
looks of it can handle 5A or better. I plan on mounting this regulator
in the cabinet and providing a terminal strip for the 12V output. The
DC load will probably never exceed .500 - .750 A.

Now here is the dilemma. I haven't tried to put all this together yet,
but I know that once I rectify and filter the 24VAC I'll probably end
up with something like 30VDC out. The regulator uses a ua723, a TO3
and a smaller TO transistor as well a whole bunch of discrete
components. I have no specs on this regulator, but it is well built,
appears to be commercial grade, and although it might handle it fine
I'd feel a lot better hitting it with something like 18VDC instead of
30. I would hate to blow it up trying to see if it would work on the
higher input.

I could add some series resistance either before or after the bridge,
but the voltage drop across this resistance would vary depending on
the total load and I'm not sure how well such a scheme would work. So
I was thinking about employing a voltage divider at the output of the
filter. The resistance ratio would be easy to figure out, however I'm
just not sure how to determine the optimum resistor values. Does this
seem like a viable plan, or perhaps someone my have other thoughts as
to how to address this? If someone could please help me with this I
would be very grateful. Thanks, Lenny

Not clear exactly what you're doing, but the devil is in the details.
Depending on the line voltage and the design of the transformer, but
the unloaded voltage of the transformer may result significantly higher
than 30V.

The word "24V tap" begs the question of how the transformer
is configured to get the 30VDC.

You really need to characterize the 12V board. It doesn't take a
"whole bunch" of discretes to make a 723 supply. There may
be other things going on. You don't know
what input min/max it can tolerate. You're gonna have a wide
range in input voltage depending on line voltage and load current.
And a couple of volts of ripple. Once you figure out how much
headroom you have, insert a zener in the power input.
You can make one from a small zener and a power transistor.
Doesn't have to be all that accurate.

And there are a lot of potential gotchas using an unknown
board. For example, I build supplies that sense output
current in the negative lead. They run from floating transformer
windings. If I were to try to use the AC for some other purpose,
stuffing current into my floating winding relative to ground
would make a real mess of things.

Making a dual-purpose power supply where both purposes are known
is much easier than building an dual-output supply where
none of the "purposes" are known in advance.