"klem kedidelhopper" wrote in message
...
On Feb 26, 5:16 am, "Arfa Daily" wrote:
I looked over what I had previously written and perhaps I didn’t
explain
this properly. I have a 24VCT transformer. The transformer needs to
power
24VAC equipment as well as 12VDC equipment. For the DC circuit I first
tried using a bridge directly off the 24V winding. As soon as I
connected
up the filter cap the DC output went to 35V, which is probably too high
to
feed my 12V regulator.

Several people came up with some great ideas to address this, and I
then
realized that I had a center tap that was not being used. So I
connected
my bridge across the center tap and one side of the secondary. This
time
the 13VAC when FW rectified using the bridge went to about 19VDC, which
is
a safe input to the regulator. Although this worked, I wasn’t happy
about
unbalancing the transformer this way so I posted my results.

Unless I seriously misunderstood It was suggested here that I come off
the
full secondary output with two diodes, cathodes tied together, (typical
FW
rectifier), and use the center tap as my negative return. I didn't
think
that would make any difference in the output voltage from using a
bridge
without the center tap however I tried it anyway. As I suspected it
would,
the DC output again went to 35V when I connected the filter up.

I've been playing with power supplies all my life and maybe my age is
catching up with me. I guess I just don't understand the explanation of
how to get a lower voltage to my DC regulator by what I thought sounded
like just configuring the transformer properly. So at the risk of
sounding
like a complete idiot here can someone please explain this further?
Thanks, Lenny

OK. A bit clearer now. What you were originally proposing now seems OK
provided that the 24v AC circuit is completely floating with respect to
anything that you want to do at 12v - ie driving your regulator input -
and
the current that your 12v circuit is going to draw, is not a lot compared
to
the rating of the transformer, and the current that the AC circuit is
going
to draw is not near to the transformer's rating, then you should get away
with running in the proposed 'unbalanced' way. I have done such things
over
the years, and gotten away with it. Although it's not very good practice,
transformers are pretty forgiving components. I guess that the simple
answer
is try it, and see what happens ?

Arfa

I'm grateful for everyone's input, and although this power supply as
well as the equipment that it will power is to be used in a family
business, never the less I'm trying to build this as "technically
correct" as possible. I found some 6.8V 10W stud mount zeners in the
junk box last night. I'm thinking that they may be a bit light for
this but I might try stringing two or possibly three of them together
on a heat sink as Arfa suggested or perhaps try the series diode
trick, (from Phil), both as previously mentioned on the output of the
filter and see what happens. Even though given the load it may work
fine it would be nice as Arfa said to not have to "unbalance the
transformer. Lenny

That would make the zeners good for about an amp and a half tops. Bear in
mind that in order to string them in series on a heatsink, the studs will
need to be insulated from it. These stud mount diodes often come with
insulating mounting hardware. I think as an initial try-out, I'd just put
one in the line and see what you get. Should knock you down to something
just above 28 v from your 35 v starting point. Let us know how you get on

Arfa