Re: Connecting Transceivers in Vehicles - Change

[Bill - K6WLM]

FYI
 
Paul, sorry but I disagree.
 
 
For HF mobile installation, GM and others recommend that the radio NOT be grounded in the cabin.  Both battery leads should be fused at the battery.  The reason for the negitive side fuse  is in the event that the battery ground lead to the engine block should go open or have a high resistance there will be very high current trying to find ground when you start the engine.  Starter motor current can be in the hundreds of amps.  You do NOT want that amount of current flowing thru your rig!
 
73,  Bill - K6WLM
 
<home-run to the battery posts.  If the negative  fuse blows and the positive
<fuse remains closed -- and *if* the radio chassis is common to negative
<power with grounded connectors (e.g., SO-239) via a ground return through
<the vehicle chassis, then the coax braid and other supplemental grounded
<leads on the radio chassis will carry the full rig current.

<Seems like in all cases where the rig chassis is common to the negative
<power lead, the negative lead wiring should go un-fused at or very close to
<the negative battery post.

<Paul, W9AC



     
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[P.B. Christensen]

> K6WLM:

> "For HF mobile installation, GM and others recommend that the radio NOT be
> grounded in the cabin. Both battery leads should be fused at the battery.
> The reason for the negitive side fuse is in the event that the battery
> ground lead to the engine block should go open or have a high resistance
> there will be very high current trying to find ground when you start the
> engine. Starter motor current can be in the hundreds of amps. You do NOT
> want that amount of current flowing thru your rig!"

Fair enough concerning the grounding point, but the negative radio lead
should be *unfused* and then bond close to the point where the battery bonds
to the vehicle chassis.   In the case above, it would still be best to
unfuse the negative radio lead in instances where the radio chassis and
negative power lead are DC-common.

The danger in your proposal of fusing the negative lead is exactly as stated
in my last message:  That an opening of the radio's negative fuse line will
cause the radio's return current to complete through the radio's grounded
connectors where the chassis of the radio and the negative power leads are
common.  Now you have an electrical insulation burn issue to deal with in
your vehicle if the negative fuse blows and the positive radio fuse remains
intact.  For a 100W-class HF transceiver, that's roughly 20A of current
during TX in wiring not meant to carry a high DC current load.

Paul, W9AC


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[n7ws]

What you say about current through the radio is true, but is exactly why you *should not* run the radio negative to the battery.  The only thing connected to the battery negative post should be the cable that connects the battery to the chassis.  If you want to carry the radio negative lead to this point fine, but it should be un-fused.

For DC the negative lead is in parallel with the coax shield that is (should be) grounded at the antenna.  It is pointless (and hazardous) to fuse the negative return only to have the connection continue to be made through the coax.

--- On Wed, 3/17/10, Bill Miner <[hidden email]> wrote:

> From: Bill Miner <[hidden email]>
> Subject: Re: [Elecraft] Connecting Transceivers in Vehicles - Change
> To: [hidden email]
> Date: Wednesday, March 17, 2010, 10:55 AM
> FYI
>  
> Paul, sorry but I disagree.
>  
>  
> For HF mobile installation, GM and others recommend that
> the radio NOT be grounded in the cabin.  Both battery leads
> should be fused at the battery.  The reason for the
> negitive side fuse  is in the event that the battery
> ground lead to the engine block should go open or have a
> high resistance there will be very high current trying to
> find ground when you start the engine.  Starter motor
> current can be in the hundreds of amps.  You do NOT want
> that amount of current flowing thru your rig!
>  
> 73,  Bill - K6WLM
>  
> <home-run to the battery posts.  If the negative  fuse
> blows and the positive
> <fuse remains closed -- and *if* the radio chassis is
> common to negative
> <power with grounded connectors (e.g., SO-239) via a
> ground return through
> <the vehicle chassis, then the coax braid and other
> supplemental grounded
> <leads on the radio chassis will carry the full rig
> current.
>
> <Seems like in all cases where the rig chassis is common
> to the negative
> <power lead, the negative lead wiring should go un-fused
> at or very close to
> <the negative battery post.
>
> <Paul, W9AC
>
>
>
>      
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[W8JI]

For HF mobile installation, GM and others recommend that the
radio NOT be grounded in the cabin. Both battery leads
should be fused at the battery. The reason for the negitive
side fuse is in the event that the battery ground lead to
the engine block should go open or have a high resistance
there will be very high current trying to find ground when
you start the engine. Starter motor current can be in the
hundreds of amps. You do NOT want that amount of current
flowing thru your rig!>>

The problem actually comes from what is inside the radio,
not what GM thinks is inside our radios or how GM thinks the
system works. The main problem is connecting the radio
negative lead to the battery negative terminal. That's a bad
idea, and it applies to our station wiring as well as car
wiring. We can have similar destructive ground loops in
station wiring.

Our radios and accessories have small thin foil traces and
small components like RF chokes on the ground leads of many
accessories connectors. Blow a negative fuse or lose a
ground connection on the negative supply lead and you can
damage components inside the radio. The jacks and plugs
become the negative high current return path.

Our power supplies either need to be ground isolated, which
they often can't be, or they need to bond the negative into
a solid ground buss, and all the negative supply leads need
to connect solidly to that buss. Otherwise we risk damaging
our equipment from something as simple as a bad connection
or blown fuse.

In your car, the battery should have a heavy return to the
engine for alternator or starter current. It should have a
lighter, but still reasonably heavy, connection at the
battery to the chassis. The radio should be grounded on the
same sheet metal near the point, but not on the point, where
the battery is grounded to the chassis. There should be no
negative fuse. It should be a solid connection and NOT
common with the bolt grounding the battery. Then if the
battery negative comes loose, current will never flow
through the radio or radio wiring. It can't set the car on
fire, it can't blow out automotive electronics, and it can't
damage the radio. Every passenger area accessory connects to
the chassis as a return, and so do the lights and other
things. They would never run their stuff, other than the
starter and alternator back to a negative battery post, so
why should we do that with our radios?

It is foolish and unsafe to ground radio to the battery post
upstream of the vehicle chassis ground, just as it is unsafe
to ground equipment at home directly to the negative rail of
high current 12 V supplies instead of a ground buss or
common ground point, no matter who says otherwise. Without a
common reference point all it takes is a high current load
negative to come detached, or a positive lead low resistance
fault to ground, and we have equipment damage. I learned
this years ago when my radio lost the keying line ground
trace on a circuit board because the radio negative lead
fuse developed a bad connection, and the radio primarily
grounded through the keyer path.

73 Tom


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