Andy GD1MIP wrote:
Hi I am building a (240volt in) 13.8 volt (out) PSU ...
the transformer must have a secondary of
16.5 volt. ...
I have a Talema branded toroid ...
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)
+++++
...do you think
this transformer will suffice at 20 volts?...

73 Andy

[brian]
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....

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

Make that 240 V AC less 50 V AC = 190 V AC input to the toroid.

That would provide 190/240 X 20 volts = 15.83V Close enough.

[Stan]
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?

Well, let's see now. If you choose that rather intriguing approach
of applying its own secondary to buck the input, what might the 20 volts
outputs really become?
240 volts input into a 240 + 55 volt winding pair means that the 240
volt input winding gets 240 / 295 volts, so the outputs would be
(50 V nominal) = 40.7 volts
and
(20 volt nominal) = 16.3 volts

But WAIT: that 50 volt winding does NOT develop 50 volts, but rather
40.7 volts so the 240 V input winding gets 240V - 40.7 volts = 199.3 V
So the 20 Volt output is 199.3/240 volt X 20 V = 16.6 Volts....( or a
smidgeon higher?) This sounds good!

[Stan]
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!


Actually, with careful wiring, this concern for increased PDs on the
secondaries is unneeded. If the secondary bucking windings are placed
on the neutral end of the supply, then the increase in the peak primary
PD is percentage wise quite small....


In summary, using secondary windings of the SAME transformer to buck the
mains input is a neat idea if you can find the ratios you need, and
doesn't seem to add much risk to the enterprise

Brian W