[GreenKeys] Loop power supplies

[Bob kb8tq]

Hi

The inductance of the “load” resistors in a normal resistor based loop supply is so low
that it is even less significant than the meter. The inductance of the magnets is the only
inductance that really matters. 

The big deal with a constant current source is that it keeps “full voltage” on the magnets 
longer than the equivalent resistor based loop. A less significant benefit is that you *might* 
put a bunch of magnets in the loop and it would still stay at 60 ma. It would be slow, but it 
would have a 60 ma current at idle. 

Bob

> On Mar 4, 2018, at 11:41 PM, drlegendre . <drlegendre at gmail.com> wrote:
> 
> @Jeff
> 
> Ok.. so by using a tube as the pass element, they're eliminating the inductive rise time of the high-value rheostat(s) which would otherwise be in series with the output current. Is this the whole reason for using this supply?
> 
> What then about the inductance of the meter winding? Of no consequence?
> 
> On Sun, Mar 4, 2018 at 9:28 PM, Jeffrey Angus <jdangus at att.net <mailto:jdangus at att.net>> wrote:
> On 3/4/18 9:01 PM, drlegendre . wrote:
> IOW it's not a constant-current supply, but rather a high
> voltage (~300V) current-adjustable supply.
> That is correct.
> Ultimately, its behavior isn't much different than that of a
> simple passive rheostat - nor am I exactly sure what advantage
> it provides over the former.
> Actually there is.
> This difference is small, but using a vacuum tube as a variable
> resistor allows the rise time of the current through the selector
> magnet rise a little bit faster.
> 
> 
> 
> 
> 
> -- 
> "I am a river to my people."
> Jeff-1.0
> WA6FWI
> www.foxsmercantile.com <http://www.foxsmercantile.com/>
> 
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