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VK3API > ALL 08.09.13 08:22l 52 Lines 2392 Bytes #999 (0) @ WW
BID : CE0588VK3API
Read: GUEST
Subj: Re: Radio Power Leads Issues
Path: IZ3LSV<I0OJJ<UA6ADV<GB7CIP<VK3API
Sent: 130908/0411Z @:VK3API.#MEL.VIC.AUS.OC #:62263 [Lilydale] FBB7.00g
From: VK3API@VK3API.#MEL.VIC.AUS.OC
To : ALL@WW
Hi John and others,
The reason I bypass the fuse in the negative lead is that all the cars I
have are positive 12volt supply with negative ground.
I always have the plus 12volt power lead directly fed from the positive
battery terminal with it's own fuse close to the connection point. This
protects the car and radio from any shorts that occur anywhere along the
12 volt feed cable.
It is also good practice to take the earth lead from the radio directly
back to the battery terminal and avoid as much other earthing of the radio
as possible.
In modern cars the ground strap between the car chassis and battery and
the strap between the engine and ground are so thin that you wouldn't
believe they were capable of carrying sufficient current to start the car.
What actually happens of course is that you end up with big voltage drops
during starting and lots of alternator whine across the straps. All of the
whine seems to induce itself into your receiver and transmission.
I take your point on the possible problems with fault conditions causing
the radio earth lead to be the alternate path if there is a failure in the
earth strap between the engine and the battery. I do all my own vehicle
maintenance and I check the condition of the ground straps as part of the
normal maintenance cycle and hence have not experienced your particular
problem.
On one occasion before I started this unfused earth practice I had an
occurrence where the fuse in the negative lead became high resistance due
to the cheap 3AG type fuse holder which was the original fitting on the
radio. This meant all the ground current for the radio was going, as you
say, through the coax. Whilst this did not cause any heating it did cause
some very erratic radio behavior with the high resistance in the ground
lead.
Failure of fuse holders seems to be a problem in the Australian climate
with the usual "white plastic" fuse holders becoming soft enough in the
normal daytime heat to deform and cause problems. Even if they don't
deform they quickly become brittle and lose their strength.
All fuses have resistance and cause voltage drops particularly when
handling high currents. Remember that a fuse works using I2 x R (that's
current squared multiplied by Resistance) so they always suck up some
power and cause voltage drops.
Regards Tony VK3API
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