Topband: Chassis Bonding

[Tom W8JI]

>> 2.) #10, ignoring connection imperfections, has about .01 ohms per 10 ft. 
>> That's up to 0.2volts with 20 amps of power supply loop current.
>
> Not a problem unless you have bonded DC- to the power supply chassis.
>
Bingo.

Virtually EVERY piece of gear has a bonded negative to chassis. The whole 
concept is screwed up, especially negative lead fuses, but it is what it is. 
No one can force the industry to float negative grounds from chassis. Even 
RF transistors have the metal bases tied to emitters, which forces a 
negative ground to heatsinks unless the designer can seriously compromise 
cooling.

>> Also, almost every Ham radio manufacturer in the world isolates the low 
>> level audio shields inside radios except at one point because they are 
>> well aware of issues with the large currents in 12V solid state high 
>> power systems.
>
> Yep. It's called "The Pin One Problem," and is a well documented cause of 
> hum, buzz, and RFI.

...and no matter how much we idealize the situation in our minds, we cannot 
change the population and things we cannot change just because we dislike a 
$2 transformer for some reason.

 >> Ham stations have grounds, and those grounds cannot be bonded
>> perfectly to the mains, nor will many people bond them at all.
>
> WRONG!  We MUST bond all grounds in a facility for lightning safety, and 
> for safety of personnel.  And when done completely and properly, it SOLVES 
> problems.

We cannot force the world to do things that are difficult, expensive, and/or 
inconvenient.

Because of that, we have to move away from the theoretically ideal world of 
zero ground potential difference and face the way the world really works.

Very few people are going to bond grounds properly, and fewer still will 
drill into a computer or other equipment to add a ground lugs or modify 
equipment to float negative returns from chassis.

The only thing eliminating the isolation transformer does, other than saving 
$2, is ruin systems in the real world. It is a terrible suggestion. No 
accessory manufacturer will ever follow your directive.

As an example, when MFJ designed the voice keyer they "cheaped out" or 
neglected the isolation transformer. About 80%-90% of the units came back 
from audio distortion and "RF" problems. They came to me, and I redid the 
layout with a simple $1.10 audio transformer and low frequency floating 
shield system. Complaints vanished.

Now I could have written a letter like this:

"Dear Customer,

Because we have some opposition to using $1.10 isolation transformers, we 
would like you to rewire your station. Please do the following:

1.) Never ground the case of the VK to anything except a perfect station 
ground buss that is not common with the main supply

2.) Never power the VK from the station supply. Always use a DC filtered 
ground independent wall wart

3.) Never allow the case to contact any other metal or conductors in the 
shack

4.) MFJ is not liable for any damage to the radio mic input circuit by 
accidental grounding of the + power supply lead

Sincerely, MFJ"

....but silly me, I went ahead and added the transformer so they could 
pretty much connect it any way they like without blowing up their radio or 
having distortion.

73 Tom