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## Re: [Amps] Regulated filament current

 To: Re: [Amps] Regulated filament current "Jim Garland" <4cx250b@miamioh.edu> Thu, 3 Sep 2015 21:46:14 -0600 mailto:amps@contesting.com>
 ```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 > 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 _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps ```
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