[GreenKeys] Current limiting

[Chris Elmquist]

Ok. Thanks for the education guys.  I get it now.  Makes sense.

I appreciate the info.

73, Chris N0JCF

On Tuesday (01/20/2009 at 09:58AM -0600), nerd at verizon.net wrote:
>    There is discussion of this in the archives. Most of the older selector
>    magnets have significant inductance so the drive voltage strongly
>    affects the rise time of the magnet current and thus the pull-in time.
>    To get "snappy" operation you need a higher drive voltage. In practical
>    terms you can get a huge selector range setting with a high voltage but
>    as you reduce the voltage the range narrows until there is no setting
>    that will work satisfactorily. The little driver power supply I made
>    (can be found in archives) ran on 170 VDC and had a very fast current
>    source/sink which was driveable by a low level circuit. I experimented
>    using a variac on the input and while the magnet pulled in just fine at
>    24 volts 60 ma it was too slugglish on pull-in to print any characters.
>    At 170 volts I could just about set the selector dial anywhere and have
>    it print just fine.
>    Peter
>    Jan 20, 2009 01:33:39 PM, chrise at pobox.com wrote:
>    I'm a little green on current loop fundamentals-- having grown up in
>    the RS232 age... but is the high voltage really neccessary if you are
>    not running a long distance with the loop?
>    In other words, could a modern constant current source using say, LM317
>    be built that provides a nice stable 20 or 60mA but with only 12, 24 or
>    48v DC source?
>    Is there something in the older TTYs that requires a higher voltage on
>    the loop?
>    My ASR-33 runs with 20mA sourced from 12v but that is pretty "modern"
>    compared to the 5-level stuff you guys are working with I am sure.
>    Chris
>    --
>    Chris Elmquist
>    mailto:chrise at pobox.com
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-- 
Chris Elmquist
mailto:chrise at pobox.com