[GreenKeys] snubbery

[jhhaynes at earthlink.net]

By the way, I've just discovered a wonderful device for switching
loop current.  Fairchild HSR312 electronic switch, a couple of
bucks at Mouser.  It's optically coupled so you get ground isolation,
and in one connection it doesn't matter which direction current flows
in the loop.  So you get a four-terminal box with two terminals to
turn it on, the photodiode, and two terminals that can go in series
with a loop anywhere.  It will need snubbing, and after looking at
MOV devices I decided to use a Zener diode or two instead.  The
Zeners are pretty precise about the breakdown voltage and the wattage
they will handle - the MOV devices seem to be a lot more fuzzy, but
would probably work OK.

Don't worry too much about the snubber affecting the waveform.
The current rises at a rate limited by L/R, so there is some delay
between closure of the switch and the selector magnet pulling in.
It you didn't snub on opening the switch the current would fall to
zero instantly (and the voltage would rise to infinity) so you would
get a bit of spacing bias.  Ideally you would snub at twice the voltage
in the loop supply, so that the current would decay at the same rate
it rises and there would be no bias.  Of course this is complicated
by other things, like the difference between pullin and release
current of the selector magnet, so maybe you don't want the current
to decay at the same rate it rises.  But the point is just not to snub
too severely.

A diode in series with a resistor across the magnet would be another
way to snub.  You shouldn't get ringing with a capacitor because
the series resistor damps the tendency to ring.

Note that even when they used relays for loop switching they used
contact protecting networks, R and C in series, across the contacts
and those are effectively snubbers.  80 ohms and 0.25uf were typical
values, but you need to watch with an oscilloscope to see what
sort of snubbing you are getting.


jhhaynes at earthlink dot net