[GreenKeys] Loop supplies

Wed May 18 07:45:27 EDT 2016

Hi

Actually a loop supply is sort of an interesting thing. When the loop is open, voltage does not matter.
When the loop is idling closed (fully settled) the only thing that matters is having barely enough voltage
to keep the required current running through the magnets. That number will vary all over the place 
depending on what magnets you have. 

When you go from current to no current. The loop voltage also does not matter. The voltage performance
of the switch is what you worry about. That leaves only one point that you *do* care about loop voltage:
when you go from open loop to closed loop. Once you fully overcome the dv/dt stuff on the magnets (again
a variable depending on your setup) the voltage required is pretty modest. 

Net result - you can make a fancy loop supply that delays the data just long enough for a switcher to ramp
up to some high voltage (say charging a cap). That switcher only needs to do its thing for a short while. The
rest of the time, low voltage (or no voltage) works fine. 

To the points made elsewhere, this is sort of an academic thing. Normal loop supplies aren’t that hard to come up with. 
It is an interesting way to do it off of a USB supply …

Bob

> On May 18, 2016, at 7:23 AM, Steve Garrison <steve.n4tty at gmail.com> wrote:
> 
> Doesn't 'loop' by definition imply connected in series?
> 
> Steve G./N4TTY
> Sent from my iPhone
> 
> On May 17, 2016, at 22:09, JC White <jcw1231 at pacbell.net> wrote:
> 
>> 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.
>> > 
>> >  
>> > 
>> > http://www.ebay.com/itm/191015464867?_trksid=p2060353.m1438.l2649
>> > <http://www.ebay.com/itm/191015464867?_trksid=p2060353.m1438.l2649&ssPageNam
>> > e=STRK%3AMEBIDX%3AIT> &ssPageName=STRK%3AMEBIDX%3AIT
>> > 
>> >  
>> > 
>> > http://www.ebay.com/itm/191015464867?_trksid=p2060353.m1438.l2649
>> > <http://www.ebay.com/itm/191015464867?_trksid=p2060353.m1438.l2649&ssPageNam
>> > e=STRK%3AMEBIDX%3AIT> &ssPageName=STRK%3AMEBIDX%3AIT
>> > 
>> >  
>> > 
>> >  
>> > 
>> > I would like to have a much higher voltage, but should be ok to run around
>> > 20 to 24 volts on a local loop.  You might even be able to get some of the
>> > modules to boost the 5 to 12 volt supplies to 30 volts or so.  Sort of a
>> > design waste, but  not very expensive either if you have some low voltage
>> > wall warts.  Then all  you need is a plastic box to put them in and maybe
>> > some phone jacks.  
>> > 
>> >  
>> > 
>> > I used a large empty pill bottle to put the computer to machine converter
>> > board in.
>> > 
>> >  
>> > 
>> 
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