From ibmwrNoSpam@NoSpamworld.std.com Wed May 21 08:48:48 1997 From: "Tom Deaver" To: "Sam Lepore" Cc: "IBMWR" Subject: Re: BMW: Lighted rocker switches for KLT? Date: Wed, 21 May 1997 06:18:53 -0600 X-Priority: 3 Reply-To: "Tom Deaver" Thanks for the info., Tom D. High in the Rockies ---------- > From: Sam Lepore > To: Tom Deaver > Subject: Re: BMW: Lighted rocker switches for KLT? > Date: Tuesday, May 20, 1997 3:48 AM > > Tom Deaver wrote: > > > > I seem to vaguely remember a thread some > > months ago about putting little lights in > > our rocker switches so they light up when > > operated. Was I dreaming or was this > > actually a possibility? > > See below for a message from the LDriders list ... > _____________________________________________ _________ > Sam Lepore, San Francisco, 1988 R100RT and 1995 K75RTA > > From: bmwldrNoSpam@NoSpamrof.net (George R. Barnes) > Date: Sat, 26 Apr 1997 18:44:42 -0600 > Subject: LDRider: BMW acessory switches > > Howdy y'all, > > If anyone out there wants to add electrical acessories to your BMW, I > found some really neat switches. They are lighted, in green or red, and > fit into the dummy switch holes on the fairing of LTs and probably most > other BMWs. Some trimming of the plastic of the fairing is necessary, > but the look of the switch makes it wort it. They are well made in the > USA and are a bit pricy at $16.00 each, but I think they are the best > looking switches I've seen. They come from NAPA auto parts stores and > are part numbers RS1005 (red) & RS1006 (green). > > George Barnes/CO > > From ibmwrNoSpam@NoSpamworld.std.com Fri May 23 03:41:20 1997 Date: Thu, 22 May 1997 23:07:36 -0700 From: Jim Larsen Organization: Larsen Aviation Services To: "'Peter C Roussos'" Cc: bmwmcNoSpam@NoSpamworld.std.com, bmw-techNoSpam@NoSpamroadkill.com, airheadsNoSpam@NoSpammicapeak.com Subject: BMW: Re: ac to dc switch rating conversion. Reply-To: Jim Larsen Pete Roussos started this thread 22 May 97 when he wrote: > I have a toggle switch rated at 10 amps at 120vac. Can someone advise > what the switches amp capacity at 12vdc would be? Thanks very much. After I replied to Pete's request many on this list questioned me by email regarding the derating of an AC type switch when used in a DC type application. This has caused me to dig up an excellent old article in which Art Bianconi explains what I was trying to say. A SWITCH IS A SWITCH....OR IS IT? -------------------------------- Some years ago I was fortunate to be able to work alongside engineers from Underwriters Labs (UL) during destructive testing of electrical devices. This was a part of my apprenticeship as a designer for a major electrical manufacturer and it was during this period that I acquired an appreciation for the real-world differences between AC and DC current and the impact those differences have on switch design and applications. I share this with you because I am growing increasingly concerned at the widespread lack of awareness most homebuilders demonstrate when selecting switches for the cockpit environment. Each time a builder asks me to perform a pre-FAA inspection of an aircraft, I carefully examine the switches and, to date, over three-fourths of the projects inspected have turned up AC rated or non-rated switches in DC circuits. CURRENT IS CURRENT; WHAT DIFFERENCE DOES IT MAKE DC OR AC? --------------------------------------------------------- The difference between AC and DC load carrying capability are dramatically non-linear and are best appreciated by carefully inspecting a high-duality switch carrying both AC and DC ratings. Typical of this is the roller and bar micro switch made by MICRO Corp. Rated at 10 amps at 125 or 250 volts AC, the same switch can carry only 0.15 amps at 250 volts DC! In real terms, we have lost more than 98% of the original load carrying ability and all we did was go from AC to DC! The voltage stayed the Same! BUT I'M USING 120 VOLT AC SWITCHES WITH ONLY 14 VOLTS DC. -------------------------------------------------------- Those of you who can still remember the old Kettering coil ignition systems will recall that when the condenser in the distributor went bad, the points generally turned blue and melted down in just a few minutes. Cockpit switches don't have the benefit of condensers to absorb the electrical inertia present in a DC circuit and as a result, the gap temperatures get hot enough to weld contacts. That includes AC rated switches, even those made with an exotic high temperature alloy. AC current changes directions 120 times in a 60-cycle circuit. As a result, there are 120 times each second when there is no current flow at all. The current actually helps turn itself off the moment it sees a gap and switch designers use this phenomenon to help reduce the cost of manufacturing AC switches. In DC circuits, however, the 'push" is constant even when the points begin to open and the resulting flash is DC current's way of demonstrating its resistance to termination. BUT WON'T MY CIRCUIT BREAKERS PROTECT ME? ---------------------------------------- No they won't. Fuses and circuit breakers provide overload protection and a welded set of contacts will not, by themselves, cause an increase in circuit load. What often happed during the UL testing was that the points welded shut making it impossible to open the circuit. Cycling the switch to the open position was often misleading; yes the lever moved but inside the switch, the cam had separated from the welded points. While it had appeared to break the circuit, the circuit was, in fact, still hot. If the consumer involved was your fuel boost pump and you thought that it was turned off when in fact it was still running, what would the consequences be? If it were a flap or elevator trim device or a landing gear motor, how would a tripped circuit breaker save you if the activating switch was welded closed and in a mode other than that required for a safe landing? A DC rated switch will cost you about 3 times more than an AC rated switch of identical current capacity. If your panel sports 10 switches (which is not likely) the difference would be less than $50. You've gotten this far. Is it worth jeopardizing your investment or your safety by cutting corners with even one cheap or improperly rated switch? ------------------------------------------------------------------------------------- For those that would like to print out the above and keep it in their FAQ file you can go to the following URL: Those of you in the aircraft industry should be aware of the derating required of even DC rated switches depending on the "Application" of the switch. If you have access to FAA AC 43.13-1A, Change 3, 1988, you can look at Chapter 11, Section 430 (Switches). This is quite long but I can make it available to read on this thread if requested, also for your personal FAQ file. It goes into derating a nominally rated DC Switch by up to 8 times for high inrush circuits such as those used by incandescent lamps that can draw an initial current up to 15 times greater than the continuous current. Switch contact burning or welding may occur when the switch is closed. I brought up the specs from a commercial switch manufacturer which shows that a 10 amp switch is rated at 120 VAC but only at 24 VDC. See OTTO Controls: Selecting A Switch at It states that: Stated ratings of a switch are expressed in various values depending on load, voltage, contact size, material, life etc. For example, the OTTO P1 is rated 10 amperes when the load is resistive in nature and the voltage is 115 VAC or 28 VDC. In-as-much as many readers of this list have seen the first part of this switch rating thread and are now unsubscribed for the 49er Rally and other events nationwide I'm going to repost this again next week. I think the bandwidth used is worth the knowledge and inclusion into personal FAQ files. Jim Larsen K100RT & K100RS SF Bay Area