[GreenKeys] Snubbing of selector magnet

[Jeffrey D Angus]

On 7/22/2013 12:04 PM, John Nagle wrote:
> Right.  The voltage for the first few nanoseconds when an
> inductor dumps can be huge.  Power semiconductors can often
> survive some of those, but there's cumulative damage.  The
> caps used in snubbers should be ceramics with high voltage
> ratings. I used 500V caps, after observing 400V spikes on a
> scope.
Back to my original model, 1600 ohms, 2 henries, 60 mA and 120 vol
loop. This is what happens without any snubber.

810 v spike on turn off. And an approximately 1 KHz ringing that
continues all the way through the entire 20 mS duration. Although it's
down to about 121.5 volts by then.

You'll notice the inductor has the current go negative as it dumps out
of the coil. So the selector magnet is seeing ringing that goes from + 60
to - 60 mA  as well and the huge transient voltage spike across the
switch.

In a day or so, I should have my interface project finished and I WILL
post a picture of the scope trace with the snubber in place. I'm using
0.47 uF @ 630 vdc  Mylar cap and 470 ohms @ 2 watt carbon
composition for the snubber.

Jeff-1.0
wa6fwi

The next time I have the Lorenz apart, I'll measure the actual inductance
of the selector magnet.
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