[ARC5] Shunt vs. Parallel

[Scott Robinson]

Hi Don,

Thanks for this info!  I didn't know what the curve was called and hence 
where to find it.

I think late WII fighters and such got as high as 40,000 feet in some 
cases--P-38 comes to mind. Looking at the ISO standard atmosphere 
model, that means a pressure of about 0.2 x sea level (760 mm Hg). This 
reduces the breakdown voltage by about a factor of four: 60 KV at sea 
level vs 15 KV at 40,000 feet. The test rig is two copper plates an inch 
apart.

Another altitude effect is reduced air cooling. For an unheated plane, 
that certainly isn't a problem (-40 deg C out there!), nor is it as 
fussy an issue in hollow state circuitry as in solid state.

Aerospace engineering has always had difficult environmental challenges. 
My own ground-based designs mostly have had to contend only with weird 
mains voltages, failed A/C, and customers shorting the outputs.

Peace,

Scott

On 7/16/17 2:11 AM, don davis wrote:
> 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.
> 
> Image result for paschen curve air
> 
> 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
>