Hi Guys,

Ages ago I saw a very simple circuit - a CT transformer, couple of
caps, 4 rectifiers and a change over switch.

With the switch in one position, the circuit was a bridge rectifier,
in the other, a voltage doubler.

Pointers please, I have forgotten where I saw this...

Thanks,

Mark

On Nov 28, 9:23*pm, MarkAren wrote:
Hi Guys,

Ages ago I saw a very simple circuit - a CT transformer, couple of
caps, 4 rectifiers and a change over switch.

With the switch in one position, the circuit was a bridge rectifier,
in the other, a voltage doubler.

Pointers please, I have forgotten where I saw this...

Thanks,

Mark

Just draw a standard bridge rectifier circuit, bridge across the full
transformer secondary, filter capacitor at the bridge output. Now--
break the connection from the capacitor (-) to the bridge, and put in
a SPDT switch, wiper to the cap (and to ground, if you will) and the
two possible connections to either the bridge negative output, or to
the transformer secondary center tap. There's no problem connecting
to the transformer center tap with the bridge (-) floating. You can
also use this basic circuit to supply two voltages at once: connect
up a bridge circuit, and ground the bridge/cap (-), and also put a cap
from transformer CT to ground. The bridge output (peak) will be
nominally 1.4* the RMS transformer full secondary (minus a couple
diode drops), and the CT output will be half that. Or, you can ground
the CT and have + and - nominally equal voltages, assuming the loads
on both sides are similar.

Cheers,
Tom

Bingo !

http://www.idesignz.org/misc/Simple%...tage%20PSU.pdf

Thanks,

Mark

On Sun, 29 Nov 2009 00:33:13 -0800 (PST), MarkAren
wrote:

Bingo !

http://www.idesignz.org/misc/Simple%...tage%20PSU.pdf

Thanks,

Mark
This is a circuit that does not suffer from temporary abcense of power
during switching. And it is using a transformer with a single
secundary winding.
http://www.gigawatts.nl/co/projects/...bel/switch.jpg

Joop

Hi Joop,

Interesting circuit.

Where does the R1/D1/Q/.Q2 reference point connect to ?

I assume the parasitic diodes in Q1/Q2 form a current path ?

-Mark


On Nov 30, 2:54*am, Joop wrote:
On Sun, 29 Nov 2009 00:33:13 -0800 (PST), MarkAren

wrote:
Bingo !

http://www.idesignz.org/misc/Simple%...tage%20PSU.pdf

Thanks,

Mark

This is a circuit that does not suffer from temporary abcense of power
during switching. And it is using a transformer with a single
secundary winding.http://www.gigawatts.nl/co/projects/...bel/switch.jpg

Joop

On Nov 29, 9:42*am, MarkAren wrote:
Hi Joop,

Interesting circuit.

Where does the R1/D1/Q/.Q2 reference point connect to ?

I assume the parasitic diodes in Q1/Q2 form a current path ?

-Mark
....

The idea is that by using two MOSFETs in (anti)series like that, the
parasitic diodes can't conduct through from q1.drain to q2.drain.
However, they (the parasitic diodes) insure that q1.source=q2.source
is at most a diode drop higher than the lower of q1.drain and
q2.drain. Then if the photocoupler is "on", the gates are pulled up
toward close to the most positive voltage in the circuit, limited of
course by the zener diode to protect against gate-source breakdown.
In that state, both Q1 and Q2 are hard on. They are happy to conduct
as "on" FETs in either direction. If the photocoupler is "off," the
gate-source potential drops to essentially zero, and both FETs are
off, leaving only the back-to-back parasitic diodes.

Essentially the same circuit is used in certain optically coupled
relays, such as the Panasonic "PhotoMOS" relays, and in other places.
Although the "on" resistance is twice as high as a single MOSFET of
the same type, the circuit blocks both polarities when "off," and
conducts either direction when "on."

Cheers,
Tom

On Nov 29, 5:54*am, Joop wrote:
On Sun, 29 Nov 2009 00:33:13 -0800 (PST), MarkAren

wrote:
Bingo !

http://www.idesignz.org/misc/Simple%...tage%20PSU.pdf

Thanks,

Mark

This is a circuit that does not suffer from temporary abcense of power
during switching. And it is using a transformer with a single
secundary winding.http://www.gigawatts.nl/co/projects/...bel/switch.jpg

Joop

But should note that it puts out twice the voltage from the same
transformer as compared with the bridge/CT circuit. On the other
hand, both voltages use the whole secondary, so the safe available
output power is nominally the same for either voltage, whereas the
output power in the full-wave CT circuit is somewhat less than in the
bridge configuration (assuming negligible loss in the diodes).

Cheers,
Tom

The idea behind http://www.gigawatts.nl/co/projects/...bel/switch.jpg
is the same as the PC PSUs: you close a connection and it works on
115Vac, you open it and plug the PSU into 230V.

And if you go as in http://www.idesignz.org/misc/Simple%...tage%20PSU.pdf
you're right that the full-wave center-tap mode does not make full use
of the transformer copper (by the way, just adding an electrolytic on
the center-tap in place of the switch you can have the two output
voltages simultaneously). It's unavoidable, unless wou have two
independent secondaries, of course: put both in series or both in
parallel.

But if you are looking for having two different output voltages (not
same DC voltage with 115 or 230 mains), and want to make the best use
of your electrolytics (specially if they are big ones), you should
need a means to put them in parallel for the low setting, and series
for the high. I did it with a DPDT non-shortcircuiting switch. No
center-tap (becomes a full-wave doubler as in the gigawatts link). If
you are interested I will gladly send it to your address, but you can
help me and the Group if you can suggest a convenient way to post
files somewhere (not a volatile place such as rapidshare).