X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Wed, 26 Sep 2007 16:31:53 -0400 Message-ID: X-Original-Return-Path: Received: from lifecomhealth.com ([72.11.90.8] verified) by logan.com (CommuniGate Pro SMTP 5.1.12) with ESMTPS id 2353465 for lml@lancaironline.net; Wed, 26 Sep 2007 15:19:34 -0400 Received-SPF: none receiver=logan.com; client-ip=72.11.90.8; envelope-from=jim.moon@lifecomhealth.com Received: from JimMoon ([192.168.1.249]) by lifecomhealth.com (8.13.8/8.13.8) with ESMTP id l8QJIWCF030075 for ; Wed, 26 Sep 2007 12:18:40 -0700 Reply-To: From: "Jim Moon" X-Original-To: Subject: RE: Lancair Legacy Avionics Master Switch Wiring Question X-Original-Date: Wed, 26 Sep 2007 12:19:42 -0700 Organization: Lifecom, Inc. X-Original-Message-ID: <008501c80072$314a3e00$f901a8c0@JimMoon> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook, Build 10.0.6822 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3138 In-reply-to: Thread-Index: AcgAYVOA255sIRF/RHyETtP9nKE/uAACRo0g Importance: Normal 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=3DUS&V=3D41&M=3D143= 2790-1 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.=20 -----Original Message----- From: bob mackey [mailto:n103md@yahoo.com]=20 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,=20 > needless to say the radio stack runs allitle over the switch's=20 > capacity. I guess I'll have to use a 30 AMP relay for the switching.=20 > Any info. on finding the correct relay to use, wiring insights,=20 > 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=3D67-191= 4-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.