[GreenKeys] Fw: Loop supplies

[WA5CAB at cs.com]

I proved empirically by trying it in 1966 that you cannot run even a 
holding magnet selector equipped machine (M14 or M15) off of a 24 VDC supply.  The 
error rate with one machine in series with a 14 TD (essentially zero 
resistance) varied between 5 and 30% depending upon what the characters were.

You can't run a loop with the selectors of the second and subsequent 
machines in parallel with those of the first unless you do not want any machine to 
be able to talk to any other one (in which case you don't have "A" loop, 
you have several loops).  To set up a loop of two or more machines that can 
each talk to all the rest, all must be in series.  You can also not rewire the 
magnets of a pulling magnet selector assembly for 20 MA because the magnets 
are already in parallel.

In a message dated 05/17/2016 21:14:07 PM Central Daylight Time, 
jcw1231 at pacbell.net writes: 
> I have some concerns about using such a low voltage loop supply.  We are 
> talking considerable inductance in the loop (the selector magnets) and the 
> circuit time constant becomes such that it can take many milliseconds for 
> the current to rise to the required 60 ma, which is why the "usual" 
> recommended loop supply voltage is around (or above) 120 vdc (the higher the 
> voltage, the faster the current can rise).  Check out pp 108 - 110 of the Tab 
> Books RTTY Handbook for a discussion about this situation.  A fair constant 
> current loop regulator is also shown, one in which the current can reach 90% 
> of the 60 ma target within 2 milliseconds.  The cc circuit shown CAN 
> function below 120 volts but the lower limit for it appears to be around 40 vdc 
> (with the time constant due to magnet inductance being the major 
> constraint).  I have seldom seem perfect copy from test tapes at this loop supply 
> voltage.
> 
> 
> 
> BTW, if one needs to run a loop driving more than a single magnet, it is 
> important to have the added magnet(s) in parallel with, rather than in 
> series with, the first magnet, since placing the magnets in series dramatically 
> increase the circuit time constant and degrades machine copy.
> 
> 
> John
> WB6BLV
> 
> ----- Forwarded Message -----
> From: "epvgk at limpoc.com" <epvgk at limpoc.com>
> To: Ralph Mowery <rmowery28146 at earthlink.net> 
> Cc: greenkeys at mailman.qth.net
> Sent: Tuesday, May 17, 2016 4:48 PM
> Subject: Re: [GreenKeys] Loop supplies
> 
> 
> On Tue, May 17, 2016 at 07:18:01PM -0400, Ralph Mowery wrote:
> >While I have not tried them, ebay has some modules from China that might
> >just work for a local loop.  Buy one of the modules which is just a 
> circuit
> >board and look around for a 24 volt wall wart DC supply.  The modules are
> >only about $ 2 to $ 6.  Some of them can be adjusted for a constant 
> current
> >and should be able to put out about 20 volts for the 60 ma loop.
> >
> >  
> 
> You should be able to use one of those with my CL adapter board, but 
> you'll need
> a small current limiting resistor as the board already puts 187 ohms total 
> in 
> series with the loop. (plus the dc resistance of the selector magnet 
> coils, etc)
> 
> eric
> 
> >
> >I have played with a couple of just the voltage regulated supplies.  If 
> you
> >need to measure the current, then a low cost ma meter or even one of the
> >'free' ones from harbor freight could be used.
> >
> >  
> >
> >Here are some of the ebay  links.
> >

> >  
> 
> 
> 
> 
> 

Robert & Susan Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20160518/63e67003/attachment.html>