I did not have a current-limited power supply. Just a Astrom rs-35m. anyway I just went for it and everything worked out okay. all my ohm reading were all in the safe range and I did take my time to make sure I did eveything right.
K3 turned on okay. Chris ________________________________ From: David Cutter <d.cut...@ntlworld.com> To: d...@w3fpr.com; Tom W8JI <w...@w8ji.com> Cc: elecraft@mailman.qth.net; Chris Hembree <w7...@yahoo.com> Sent: Fri, August 13, 2010 12:33:33 AM Subject: Re: [Elecraft] K3 inline fuse Good advice from you both on battery fusing, particularly in vehicles, but you've drifted off the subject a little. Chris W7CTH said: "Hi, I am ready to apply 12v for the first time to my new K3. " I would use a current-limited power supply to connect up ANY equipment for the first time. No fuse is as fast as electronic limiting. Even the fastest semiconductor fuses require a huge overload to achieve a sluggish response. Busman 25A fuse: 340A^2t. I'm not an expert, but divide 340 by the time you want it break and then take the square root, I think that's how it's done. So, to blow in 100ms it takes 58A. Someone put me right. UK 13A plug fuses need 2.1 x 13A to guarantee to blow in 30seconds! David G3UNA > Tom, > > You are absolutely correct. Take a mobile situation - the radio's power > ground is connected to the transceiver chassis ground, as is the coax > shield, microphone shield and other stuff. All that "stuff" can (and > will) connect to the vehicle chassis at some point. I know the coax > will connect at the point where it attaches to the antenna. > > Now imagine a situation where the negative lead fuse is open. The > vehicle chassis is still connected to the battery negative - so the > radio gets it positive lead directly from the battery (since that fuse > is still good), but the negative battery terminal voltage flows through > the vehicle chassis to the point where the coax shield connects to the > vehicle chassis (presumably at the antenna), and then follows the coax > shield to the transceiver. The transceiver still works, so you (the > operator) are not aware that the negative fuse has opened, but the DC > current paths will give rise to "strange happenings". > > Bottom line, follow Tom's advice and fuse only the positive lead, and in > a mobile situation, take the negative voltage from the vehicle chassis > close to the radio. > > The fusing of the negative lead started when some were concerned about > the vehicle's battery to engine block (or vehicle chassis) connection is > broken, which could lead to massive starting motor currents being > conducted from the coax ground at the mobile antenna, through the radio, > and onto the battery negative terminal. Yes, that situation is quite > real, but can be circumvented by periodically checking the engine block > to chassis grounding in the vehicle, as well as the battery negative > connection to the vehicle chassis. > > If the radio has an isolated (floating) negative connection (I believe > only commercial mobile radios do that), then fusing of the negative lead > is OK, but for most ham radios, the fusing of the negative lead can > cause as much damage as the reasons given for inserting a fuse into the > negative lead. Take your pick. > > 73, > Don W3FPR > > Tom W8JI wrote: >> Positive lead fuses are a good idea. >> >> Negative lead fuses never were a good idea unless the radio has a totally >> floating negative buss. >> >> If the negative lead fuse to the radio opens for any reason all the negative >> lead current for the radio will flow through the negative lead of any >> accessories connected to the power supply. >> >> They are a terrible idea for any system with the negative lead common to >> ports in and out of the radio, or to the chassis. You certainly won't catch >> me using one!The negative lead needs to be connected solidly to the power >> supply negative for a multitude of reasons, the most prominent of which is >> if the negative lead fuse to the radio opens you can blow up accessories or >> open ground traces including traces in the radio. >> >> They are not even recommended in vehicles any longer in some countries >> because of the fire and damage hazard they create. >> >> 73 Tom >> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html