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.




Is this transformer potted in the middle?
If it isn't you could add some turns and connect them in series with the
primary to reduce the secondary voltages. This would be a lot easier
than removing turns from the secondaries, especially if the secondaries
are NOT wound on top!

On Sep 20, 7:43 am, Kenneth Scharf wrote:
Andy GD1MIP wrote:
Hi I am building a (240volt in) 13.8 volt (out) PSU along the lines of
this plan..

OK, we are all being technical here - can I ask if the crowbar IC is
still available? Is the specified relay still available?

At an overvoltage condition,there will be over 200 watts being
dissipated in the 2r2 resistor in series with the SCR - hopefully it
wont instantaneously go open circuit....if the overvoltage protection
works, wont the thing go into cyclic oscillation as it attempts to
restart?....(not sure here, perhaps the gate never goes low enough for
this)
LM723 regulators tend to be a bit unstable - VHF ground plane type
construction is recommended.
The transformer ratings - the manufacturer rates them at a specified
current for a reason. For low duty cycle use, eg SSB, you can get away
with pulling more current. But a solid state Class C FM amplifier? -
also, as mentioned, there is very little headroom at 16v - 18v would
be better.....and thats assuming the mains input voltage is reference
quality - here in OZ, its a design variable, up to 20% fluctuations
are not uncommon, especially at the end of a long run.
Motorola 2N3055 transistors are fine - BUT- when the original Motorola
patents expired, and Motorola stopped making them, a lot of
substandard crappy ones (typically Eastern European) came onto the
market. Be warned, they can be unpredictable. TO3 is a pretty well
obsolete case style anyway. Good luck. And the Motorola symbol was
widely forged as well.

The above, of course, are my Neanderthal rantings on a newsgroup and
will no doubt start an impassioned debate on points of theory , or
maths, or how it was done in 1935. Remember, free advice is worth what
you pay for it.

But I would advise putting a 5w 15V zener across the output terminals,
and before that put a 30amp fuse. Minimal cost for added insurance.

Go for it - build it, experiment. Blowing something up is always a
fantastic spur to learn the correct theory for the application..if it
works first up, then its just a mechanical exercise.

Andrew VK3BFA.