3 May
2005
3 May
'05
12:11 p.m.
On Mon, 2 May 2005, Allison wrote:
Allison is correct; I'd like to amplify on it.
CV transformer power supplies are very inefficient; they draw tons
of current and make a lot of heat.
A CV transformer is basically a regular transformer with changes
to the core and usually an added winding that has a capacitor on
it. The extra winding and cap circuit is resonant at 60 (50) Hz as
Allison points out.
The core has a gap that makes it intentionally magnetically
inefficient; the core is magnetically saturated; when the line
voltage increases, more current flows, but no more magnetic field
is generated, the extra power going up in heat. This is a
"saturable core" transformer.
They're run saturated, with excess input current, so that when the
line voltage drops, the magnetic field does NOT drop -- because
input power caused saturation, magnetic field was at maximum,
dropping the line voltage to 100 (from 120, US) only lowers waste
heat, the magnetic field stays the same.
When the input goes below some point then yes, the output voltage
will start to drop.
THere's lots of variations, I don't know what your power supply
uses. Some put out "square" waves (clipped sines due to
saturation), or regulated sine.
Hard to imagine now, but this setup was once a lot cheaper than a
switcher.
Disconnecting
this capacitor "cures" the excess current draw and
the chassis voltages come up fine (still running through variac at
reduced AC voltage with series light bulb as I expect this cap is
part of a "line voltage regulator".
The transformer is labled "C.V." (Constant Voltage?)
The cap needs to be there. An unloaded CVT runs hot and draws
more current due to the highly reactive load. A side effect
of storing power in a resonant circuit (floating coil and cap).
As the transformer is loaded the current remains the same but
more stored energy is transfered to the active load. I have
a Compupro Chassis, TEI and even a spare supply of that style.
I also have 120V/120V CVT for systems that do not have one
internally. They tend to run warm under normal cases. It
should with a modest load (auto headlamps are handy for this)
behave and also not blow primary side fuse(s).
By current switchmode tech they are scary but represent old
magamp thinking and are reliable devices.