[Elecraft] New Output Power Issue, and a new question

[Richard Fjeld]

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: elecraft at mailman.qth.net 
  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