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
>>
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