I'm still a bit uncertain whether Bob W4OAS is speaking about a constant
current power supply, or a constant voltage power supply which has current
limiting. They're very different creatures, but as a practical matter, it
may not make much difference, because either can be used to power a filament
without any current surge at startup. Just to make sure we're all on the
same page, here's how I normally describe the two types of power supplies:
1. Constant Current Supply: One sets the desired current Io, and the voltage
adjusts itself to whatever is required in order to fix that current, even if
the load resistance varies all over the place, including R=0 (a short
circuit). A theoretically ideal CC supply has an infinite output impedance,
which means that if you graph the current output as a function of the
voltage output on an I-V curve (with I being the vertical axis), you get a
horizontal line at Io, as the output voltage varies from zero to infinity. A
horizontal line means deltaV/delta I = infinity, which is the dynamic output
impedance of the supply. (Remember, we're talking "ideal" here!). In a real
CC supply, the line is horizontal (or nearly so) only up to some maximum
volage Vo, which is called the compliance voltage. Vo is the maximum voltage
a CC Supply can source.
2. Constant Voltage Supply: One sets the desired voltage Vo, and the current
adjusts itself to whatever is required in order to fix that voltage, even if
the load resistance varies all over the place, including R= infinity (an
open circuit). A theoretically ideal CV supply has zero output impedance,
which means that on an I-V graph the curve is a vertical line at Vo,
parallel to the I-axis, all the way from I=0 to I= infinity (again,
theoretically speaking). In a real-world CV supply, the line is vertical
only up to some maximum limiting current Io. In other words, if you power
your transceiver with a regulated 12 Volt/30 Amp power supply, that's a
real-world constant voltage supply with roughly zero output impedance at any
current below 30 Amps.
3. Constant Voltage - Constant Current Supply: Most bench power supplies are
CV-CC supplies, which means one has two adjustments, one for the output
voltage, and the other for the output current limit. For this kind of
supply, there is a smooth crossover between constant voltage operation (for
load resistance R>Ro), and constant current operation (for load resistance
R<Ro). On an I-V graph, such a supply has a horizontal line at Io up to a
voltage Vo. and then a vertical line that goes from Io down to zero.
Personally, for a regulated filament supply, I prefer a constant voltage
supply, with the current limit set just above the maximun current that the
fully warm filament will draw. If the voltage is set at, e.g., 12.6V, then
there it will stay as the tube ages, or is replaced with another tube. The
voltage can never exceed 12.6V up to the current limit.When the filament is
cold, the voltage will drop below 12.6V, but the current will never exceed
the current limit. On the other hand, with a constant current supply, the
voltage will drift around as the filament changes, but will never exceed the
compliance voltage setting. The high (nearly infinite) output impedance of a
constant current supply will be more prone to RF pickup, hum fields, etc.
The near-zero output impedance of a constant voltage supply will likely be
much more immune to such things. The power supply itself acts like a huge
bypass capacitor for AC fields (up to a frequency that depends on the
particulars of the supply. On lab grade supplies, such as Agilent, Kepco,
Lambda, and Keithley, the frequency response is carefully specified; you
probably won't find any mention of it on an MFJ or Astron supply.
73,
Jim W8ZR
>
> One great advantage to current limited filament supplies is there will
> be no large surge of current when powered up, unlike voltage source
> supplies.
>
> 73, Bill W6WRT
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