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 > > >> In both Neg and Pos leads, supply a fuseholder and fuse. I use the >> "stubby" fuses from Motorola @ 30A, you should use something like >> that. These are T-rated, meaning they're designed to open with a DC >> load. Don't use a fuse out of your home's fusebox... >> > > ______________________________________________________________ > 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 > > ______________________________________________________________ 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
