[GreenKeys] Re: Rheostat

[WA5CAB at cs.com]

Well, you're correct that using an active device can wash out loop current 
changes due to the quite large line voltage excursion you listed.  No way to do 
that with the resistor other than by manual adjustment.  But I don't recall 
reading why it was you needed to turn off the loop current.  Doing that will let 
the machine run open.

In a message dated 4/10/2007 12:53:15 PM Central Standard Time, 
nerd at verizon.net writes: 
> Wow I must be a space cadet - or getting old - as I completely missed 
> the part of the sentence after "EMP."
> 
> My logic in using the transistor is that it serves both as a constant 
> current device as well as the loop switch.  When I had just the keyboard 
> in series with the magnet all I used as, in fact, just a simple resistor 
> to set the 60 ma.  But now that I need a switch... why not "neaten up" 
> the loop regulation against line voltage variations (105 to 130 here) at 
> the same time.  Once I get the whole thing together I will post the 
> diagram.  I think it does everything necessary easily, cheaply, and 
> hopefully reliably.
> 

Robert Downs - Houston
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