Re: IIc+ Power Supply mod.

["Luke Forat" <iceman@nospamdigidude.homeip.net>]

  Jorge,

  I don't think that's a good idea for several reasons - I'll try to
elucidate: Zeners need to be driven for reverse avalanche, you can't stick a
zener just "in series" with the bridge and by this I assume you meant on
it's Output (DC side) - it needs to be in parallel and "driven" by a
resistor in series. The output of a 240V full wave rectifier bridge across a
cap is not really 240V it is 240V * sqrt of 2 (340V) which is why the caps
are rated 350-400V on the output (on open load they will store up to the
peak). You would have to drop approx 200V across the resistor which "drives"
the zener which with a primary current of say .3 Amps being generously low
on the VA rating of the small PSU would mean about 60Watts of dissipated
heat (this output would then go to the main switching transistor circuit so
you would need to break the track(s) and rewire). You would need a large
enough Zener (say 5-10Watts as the current to keep a zener driven is in the
mA range) to keep the switched primary voltage "fluctuations" in tolerance
and avoid zener breakdown due to high voltage reverse peaks from the
switching primary (the primary transistor is protected for this, your zener
isn't and there is really no safe way you can with this wiring). In the
event of zener short the drive resistor would burn out. If your primary
current exceeds calculations your primary voltage will drop below 110V and
your switching circuit will "die" and/or eventually your resistor will burn
out as it is dissipating too much heat (W=I2R). You will need to naturally
change the ripple filter cap and place a 350V rated one.

   So, if you manage with all the above and you turn on your PSU, you will
have a little heater in there that is just dissipating heat and when you
reach the max load on it, it will dissipate even more heat equivalent to a
100Watt light bulb which you don't see (drop resistor dissipation + primary
transistor dissipation + switching transformer dissipation).

    Will it work? If wired and rated correctly it can. Is it a sensible way
to go? Not really because you risk a fire if not worse as the case is not
designed to dissipate that much heat - it will melt and/or the components
inside will die of overheating. So you would need to modify the case and
insert forced ventilation... but that is absurd on a "portable" switching
powersupply that lies on the floor and can be "chocked" so easily. It is one
of the main reasons why modern "dongle" switching laptop PSUs do not have
any external openings and must solely rely on internal dissipation - either
that or they can't be certified for that use. In other words it can be done
but it is not safe.

    Trust me, get a 220 to 110V  autotransformer, it is so much safer (and
it makes Tesla happy).

    Luke

-- 
"Jorge Chamorro Bieling" <bieling@terra.es> wrote in message
1haau6w.1v5iyy1stdq4gN%bieling@terra.es">news:1haau6w.1v5iyy1stdq4gN%bieling@terra.es...
> Luke,
>
> I think I'll try this BRUTE FORCE idea I've had:
>
> What if I insert a 50w 110v zener in series with the bridge rectifier ?
> The power supply is rated 36w, but that's maximum power.
>
> This switcher may have 80-90% efficiency ?
> Then the zener will have to dissipate just a bit more than the IIc+...
> How much may be the average power consumption of the IIc+ ?
> The how big is the radiator that I need ?
> What am I missing here ?
>
> :-)
>
> Thanks.
> -- 
> Jorge Chamorro Bieling