On Sep 16, 9:41*am, brian whatcott wrote:
Andy GD1MIP wrote:
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.

Find a 240/50 volt transformer , and wire the 50 volt output in
antiphase with the mains input to the target transformer, and don't fool
with windings at all....

Brian W- Hide quoted text -

- Show quoted text -

240 volt input with 55 volts (antiphase) i.e. 'bucking' the 240 input
is 240 minus 55 = 185 volts. The input voltage being educed from 240
to 185 volts. And 185/240 = 0.77

With that the two remaining secondaries would be 20 x 0.77 = 15.4
volts. Maybe that's a little bit too low for what is needed? But worth
a try?

But also if the 55 and 240 windings are connected in series (aiding or
in-phase) and 240 is applied to that; we could have a 240 + 55 = 295
winding.

Then with 240 connected to what is now, in effect, a 295 volt winding?
So 240/295 = 0.81
And 20 x 0.81 = 16.27 Very close to 16.5?

Also' and if NOT with two 6 amp rated secondaries I would certainly
'try' getting 9 amps from each of them.

BTW the insulation of any secondary connected in series with the mains
voltage primary must have adequate insulation to withstand peak mains
voltages (about 340 v AC). I haven't done this at 140 volts but have
done it at 120 volts; by connecting 12 volt secondaries in series with
the input to simulate voltage increase/decrease of plus/minus 10% etc.
It worked fine when I didn't have a variac!