"Andy GD1MIP" wrote in message
...

Hi I am building a (240volt in) 13.8 volt (out) PSU along the lines of
this plan..
http://www.warc.org.uk/proj_psu.php it is very similar to the old
Marchwood practical wireless unit from the early 1980's using 2n3055
transistors.

My question. The spec says the transformer must have a secondary of
16.5 volt. Now when I last built a PSU using a similar plan the
transformer output could be much higher, in the region of 15 to 20
volts.

I have a Talema branded toroid I want to use, its spec is as follows
(from maker)
+++++
Toroidal transformer type 5028-P3S03:
Prim: 0-110V-230V-240V orange-yellow-violet-blue
Sec1: 20V/6A green-red
Sec2: 20V/6A black-white
Sec3: 55V/2,5A braun-braun
If you use only the 2x20V you can get out 9A of each winding (no load
on the 55V)
+++++


as you can see the secondary output is 20 volt, now folks do you think
this transformer will suffice at 20 volts, or should I removed a little
winding to bring it down?

73 Andy

PS the variac is just there for scale.




--
Andy GD1MIP


The transformer will work in the project that you are looking at, however,
the pass transistors will run significantly hotter than if you used the
transformer voltage specified. Your transformer will produce about 28VDC
after the rectifier/filter. The pass transistors must drop the difference
between 28 VDC and the output voltage (13.8VDC), which is roughly 14 VDC.
Multiply that by the output current to get the power that the pass
transistors must dissipate. Watts = Voltage * Current. Watts create heat,
which must be dissipated by the heatsink.
Since you're wanting some respectable current from this project, you could
do as you suggested; remove a few windings from the 20VAC secondaries to
reduce the power that the pass transistors dissipate. But, depending on the
construction of the transformer, it might be difficult to do.
Another way to reduce the voltage from the transformer is to put a "bucking"
transformer in the primary of your transformer to reduce the primary voltage
to it.

--
Dave M
masondg44 at comcast dot net