[ARC5] Shunt vs. Parallel

[don davis]

Scott:  "...arc=over is easier at low pressures..."   That's 1/2 of the
pressure effect on arcing.  Take a look at the Paschen curve breakdown for
typical air (O2/O3 not included) below (or Google it yourself; I'm
apparently not authorized to know how to copy and paste this crap).  The
interesting thing about the breakdown point is that there is a sharp minimum
at some intermediate pressure / altitude.  From my aerospace experience most
modern parts are most susceptible at ~50,000-75,000ft.  the curve is
affected by partial gas pressures so was heavily influenced by materials
used (lubricants, conformal coating, paint, ink, flux, etc.)  Not an easy
thing to do in wwii era.  The tricky part is controlling the atmosphere the
part will see in worst-case ops.  In cold war we generally used part
selection, out-gas ports, post launch heaters, etc. to operate properly near
a perfect vacuum or low pressure.  The Russians used a better / easier /
cheaper method by pressurizing the critical electronics In sealed tubs
followed by a spritz of 1,1,1,Trichlor or similar.  Pressurization also
helped with thermal control.  USA had some RT units that were available
unpressurized or not.

 

Most MIL-SPEC capacitors are monitored over the entire Gp. A, B, C test
conditions at a voltage ~4.3 times the rating.  Most mags were impregnated
in epoxy-like material (or organics, tars and other).  If I recall correctly
the voltage rating of most magnetic part types were not over-qualified in
the same manner that capacitors were due to the infinite time required for
stabilization of magnetics out-gassing.  So, I would imagine that trade-offs
in designs where capacitors were fungible with magnetics would have been
made during design / production phases to maximize the arc-over points.

 



 

 

 

73 de don ad6pb

 

 

-----Original Message-----
From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net] On
Behalf Of Scott Robinson
Sent: Saturday, July 15, 2017 9:59 PM
To: WA5CAB at cs.com; arc5 at mailman.qth.net
Subject: Re: [ARC5] Shunt vs. Parallel

 

Hi Robert,

 

I'm with you; I bet the altitude is key. Most or all of WWII warbirds were
not pressurized, and arc-over is easier at low pressures.

 

/scott 

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