My recommendation is to use a relay to switch power to your avionics stack.
I used Tyco - Potter & Brumfield VF4 style relays in Darrell Gerrard's panel
(N314DG).  These relays, rated at 30 amps, were designed for the automotive
world and are extremely reliable.  Millions of them are in vehicles all over
the world.  Check out Digikey part numbers PB682-ND (relay, $2.59), PB714-ND
(socket, 1.73) and A27926CT-ND (female spade terminals designed to lock into
the socket, $11.65/100).  Look here:
http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=41&M=1432790-1
and click on the Digikey catalog page reference for a pdf of the catalog
page.  If you ever have a failure and need a spare, you can find one at any
NAPA auto parts store almost anywhere in the country.

When it comes to component ratings, be careful.  All of the parts above and
those mentioned below are typically rated with their ABSOLUTE MAXIMUM
RATINGS.  These ratings are the values that are just below where the smoke
comes out and the part fails.  Maximum ratings for switches, relays and
connectors are typically the values at which the contacts are operating at
100 deg. C temperature rise above ambient.  This temperature, produced by
the resistive losses in the contact, is not a good design target.  If
ambient is 40 deg. C on a hot day, the contacts are pretty hot.  Remember
I^2xR ?  Power is proportional to the square of the current.  So, a contact
rated at thirty amps that is conducting 15 amps will be dissipating about
1/4 of the power, and should see about a 25 deg. C rise -- clearly a lot
more acceptable.  My rule of thumb is to derate all contact ratings by at
least 33%.  Use a 30 amp contact, switch or relay at currents not above 20
amps and 15 amps to be really safe.

With regard to the "solid state switch" mentioned below
(http://www.irf.com/product-info/datasheets/data/irf1324s-7ppbf.pdf), be
careful.  This part was designed for use in switching power supplies, not as
a continuous duty DC relay.  Yes, the ABSOLUTE MAXIMUM RATING is 303 amps
(at a case temperature of 100 deg. C), but you have to remember what these
absolute maximum ratings are intended for-- this is where smoke is
guaranteed to be visible as the part melts or catches fire.  If you read the
footnotes, it says "Calculated continuous current based on maximum allowable
junction
temperature. Package limitation current is 160A."  This is reiterated in
Fig. 9.  The package limitation is the current carrying capacity of the
leads on the part -- they are effectively a fuse that blows at 160 amps
(probably the first time you close the starter contactor).  Next, you have
to look at Fig. 8 -- Maximum Safe Operating Area.  This figure shows the
capability of the part when operating at various pulse widths (note that it
is for a single pulse) as well as in a continuous DC mode.  For DC
operation, the maximum continuous operation is limited to less than 10 amps
at 12 volts drain-to-source.  Note that the high current curves in Fig. 1
and Fig. 2 are for 60 micro-second pulses !!  Finally, these parts need to
be mounted to a circuit board with proper board layout and attention to
thermal design (see the thermal design specs and footnotes 8 and 9).  Bottom
line, I wouldn't recommend using this as a replacement for a master relay
anytime soon.

-----Original Message-----
From: bob mackey [mailto:n103md@yahoo.com]
Sent: Wednesday, September 26, 2007 5:22 AM
To: lml@lancaironline.net
Subject: Lancair Legacy Avionics Master Switch Wiring Question


> My second question, was on wiring a Garmin stack to an avionics master
> switch purchased from Lancair. The rocker switch is rated at 15 amps,
> needless to say the radio stack runs allitle over the switch's
> capacity. I guess I'll have to use a 30 AMP relay for the switching.
> Any info. on finding the correct relay to use, wiring insights,
> schematics, ect. . . would be appreciated.

Keep it simple, use a 30 Amp switch...

$3.66 at Digikey for the illuminated (green LED), SPST, 30A, 12V
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=67-1914-ND
http://www.lumex.com/pdf/CLS-RO11A12250G.pdf
Many others in varying specifications. You probably want at least a 30V
rating. This one is only 12V.


If you must use a relay, consider a solid state switch instead.
http://www.irf.com/product-info/datasheets/data/irf1324s-7ppbf.pdf
Smaller, lighter, cheaper, more reliable (if mounted properly), no flyback
voltages, 300+ amp rating. With careful design, you could even use these to
replace the main contactor and starter relays -- again in a 12V system.
Other similar parts would be suitable at higher voltages.