[sdiy] EM radiation -- current or power ?

[Bob Weigel]

On 12/18/2010 12:31 AM, Andre Majorel wrote:
> Does the amount of EM radiation depend on current or power ?
>
> In other words, will two wires carrying 1 W at 18 VAC induce
> more 50 Hz hum in neighbouring audio cables and circuits than
> two wires carrying 1 W at 230 VAC ?
>
> In other other words, are there any drawbacks to putting the
> power transformer and the rest of the power supply in separate
> boxes ?
>
> Thanks in advance.
>
>    
Hi Andre!

It's current.  coulumbs/second.  The current is a result of voltage and 
load factors.  Say you had only a 12V system with huge wires that would 
support the current :-).   Ideally of course a household needs a certain 
amount of *power*.  They could locally convert the voltage with very 
efficient transformers to whatever they need it to be...but 100W light 
bulb needs 100W of power to be the right intensity. So you have to 
supply enough voltage so that it's impedence will flow enough current so 
that I x E = 100W.  (I being current, and E voltage).

Anyway so let's say a houshold needs 1200W average.  The 12V supply 
would have to flow 100Amp to accomplish this.  Were we to up the voltage 
to 120V then of course only 10A would be required.  And thus there is a 
lot smaller magnetic field component generated in the process.  Wires 
usually are higher voltage than this when they travel any distance so 
that currents are scaled down even more.  This results in needing 
smaller wire diameter as well of course.

But anyway..the closer wires are running on a push/pull pair, the less 
field they will generate at a distance.  Electric fields are easy to 
shield but magnetic fields require high permeability materials.  It 
always helps some to keep the currents as low for as much of the run as 
you can of course.  So if you're talking putting a transformer that 
steps down far away, then yes that's more wire with higher current.  As 
to whether it will be audible somewhere is another question though.  
Depends how much current and how close the wires are to each other and 
how well shielded they are.

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