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.

In article ,
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.

By using it the way it is your only design worry is that you will be
dumping more power across the 2N3055 Regulator Transistors, and will
need more of them, for the same current output, due to Dissipation
Limits of the heatsinks and transistor cases, because of the higher
Voltage Drop across the regulators. Also recognise that you will have a
much higher DC Voltage going into the regulators, after some Capacitor
Filtering. Likely up near 25 Vdc, which means you will be dumping
roughly 50% of the provided transformer power in the Regulation System.

On Tue, 15 Sep 2009 17:12:41 +0100, 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.

If you _really_ want to do some work you could remove all the secondary
windings and replace them with a single 16V winding; that'll give you 25%
more current just from the drop to 16V from 20V, plus whatever extra
power you can take because you used the space formerly taken up by the
55V winding to put in bigger wire.

Lots -o- work, though.

--
www.wescottdesign.com

"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

On Sep 15, 12:12*pm, 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.

--
Andy GD1MIP

As others have indicated the increased voltage will cause the
transistors to have to dissipate more power but the result will be
better regulation. I recommend that you don't use the 3055
transistors, there are better choices. I blew many of them before
replacing them with ECG 181, there is a 2N number this is equivalent
to but I cant remember it right now.

Jimmie

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

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!

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

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 I want to use, its spec is as follows
(from maker)...
73 Andy

Now that the ideas are cooking.... consider this.
1100 watt defunct microwaves can be had for the asking.
They have a particular transformer core design that makes it easy to
saw off the 1 KV secondary, and with the generous core window, you could
wind 18 to 20 turns of wire good for 10 amps. The turns per volt is not
much over 1 tpv.... And you have a decent rectifier diode for the
spares box too....

Admitted - a bandsaw in the workshop makes a 15 minute job out of this
transformer hack.(if you don't nick the primary)

Brian W
KA5WOG

On Sep 16, 8:46*pm, brian whatcott wrote:
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 I want to use, its spec is as follows
(from maker)...
73 Andy

Now that the ideas are cooking.... consider this.
1100 watt defunct microwaves can be had for the asking.
* * They have a particular transformer core design that makes it easy to
saw off the 1 KV secondary, and with the generous core window, you could
wind 18 to 20 turns of wire good for 10 amps. The turns per volt is not
much over 1 tpv.... * And you have a decent rectifier diode for the
spares box too....

Admitted - a bandsaw in the workshop makes a 15 minute job out of this
transformer hack.(if you don't nick the primary)

Brian W
KA5WOG

Brian, I have done this with old uwave transformers, the ones I could
knock out the magnetic shunt with a hammer and punch, but on the newer
ovens the shunt is a weld. Will these still work?

Jimmie