Re: Aristo cold head

"Jean-David Beyer" <jeandavid8@xxxxxxxxxxx> wrote

Now a gas discharge tube is not resistive

A gas discharge tube has a negative resistance: If you increase
the current it will lower the tube voltage. Modeling the lamp
as a constant voltage, though, is good enough for most circuit
analysis - the result is like driving current backwards through
a battery, the voltage on the 'battery' is the arc voltage.
In this model the lamp has zero resistance.

Since the lamp is pretty much a short circuit - zero resistance
- there needs to be a 'ballast' in series with the lamp. The
ballast can be a resistor, capacitor or inductor. The ballast
impedance (the general term for something that opposes current)
controls the lamp current.

In almost all equipment the ballast is either inductive or
capacitive - called a reactive ballast. Resistor ballasts are
sometimes found in very old equipment.

A reactive ballast will draw current 90 degrees out of phase
with the power line. The power that it blocks is stored in
either a magnetic field (inductor) or an electric field (capacitor)
and given back to the power line in the next half cycle (a sloppy
explanation, but probably good enough). A resistive ballast
dissipates the power as heat and the power is gone forever.

The reactive ballast causes the system to draw extra current,
though not power. This upsets the power company, means larger
wiring and transformers are needed and raises the electric bill
if your meter charges you for VARs. Large lamp power supplies
include power-factor correction by adding compensating inductance
to a capacitive ballast power supply or compensating capacitance to
an inductive ballast power supply. This makes the lamp system
draw the least amount of current and look like a resistor to the
power line. http://en.wikipedia.org/wiki/Power_factor

Putting a capacitor of the right value at the input to an Aristo
lamp will make the system look like a regular old light bulb and
there will be no inductive kick when it is turned off. However,
because of the capacitor there will now be a current surge when the
lamp is turned on. "Pay now or pay later."

These problems can be eliminated by connecting the power when the
AC voltage is zero and disconnecting the power when the AC current
is zero. This is what Aristo's 'solid state contactor' achieves.

--
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.darkroomautomation.com/index2.htm
n o lindan at ix dot netcom dot com


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