Good advice! (And a new question) I haven't followed this too closely until I saw Don mention blade-type automotive fuses. (Not to be confused with the heavier blade-type fuses.)
We use a blade-type automotive fuse at our repeater site. I found the fuse holder melted one day. The fuse blade had slipped to one side of the internal terminal in the molded fuse holder. This caused a resistance connection. It is nearly impossible to tell when the fuse blades are properly inserted into the two terminals. As the current increased across the resistance the voltage drop increased. The Voltage times the current, produced enough wattage to melt the fuse holder, nearly creating a fire. Thankfully, it burned open. For Ian's benefit, I made some observations of my K3 for comparison, using my homebrew power distribution lash-up, and the wire used. (Emphasis on the latter) My Astron 35M is a bit light on voltage. Two DVMs show 13.66, and 13.72 Volts at the power supply. The K3 shows 13.6 V at the radio during key up. With key down, the K3 shows 11.9 V, and 15.4+ Amps. The Astron ammeter indicates at least 14 Amps. (I don't see the volt meter vary on the Astron) ***Considering that I have a P3 attached, Does that amperage track with other users?*** (Using simple math, the K3 seems to be very efficient on CW.) So, in summary, with my PS voltage and wiring lash-up, I am losing 1.7 Volts on key down. It indicates a full 100 Watts at the K3, at my in-line power meter to the dummy load, and also into a service monitor. All tests were done on 75 meters. Rich, n0ce P.S. Don mentioned low voltage, high current applications. Accidents can also weld firmly together in those situations. Planning good fuse placement is important. ----- Original Message ----- From: Don Wilhelm To: [email protected] Sent: Monday, December 24, 2012 6:43 PM Subject: Re: [Elecraft] New Output Power Issue Despite the potential for voltage drop, I would not forget the fuse at the power supply end of the wire unless you are certain the power source has adequate overcurrent protection built in. The blade type fuses with tight fitting sockets will cause very little voltage drop. If the connection is not in the milliohm range, connection tightness is to blame. That fuse is not intended to protect the equipment connected, but is instead to protect the wire from a short circuit. Many power supplies do have adequate overcurrent protection - however --- I hear of instances where this was not in place when the power source is a battery. In the event of a short at the end of the power cable (or internal to the cable), a LOT of energy can be available which will have current sufficient to melt copper conductors of any size and spew molten metal all over the place - a personnel and a fire hazard. Low voltage high current power sources can be just as maiming as high voltage supplies. Please be safe! Merry Christmas and Happy New Year. Keep all hams healthy and free of injury in the coming New Year. 73, Don W3FPR ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
