[RFI] Shielded wiring suggested on Heat Pump Controls

Jim Brown jim at audiosystemsgroup.com
Wed Feb 3 09:31:35 PST 2010

On Wed, 3 Feb 2010 10:19:02 -0500, Mike Wetzel wrote:

>Does that sound like a solution that would work?

Hi Mike, 

As usual, "it depends." You need to figure out where the (noise) 
current is flowing and prevent it from radiating. If you could 
force the noise current into a transmission line, you would 
essentially kill radiation from it. Something as simple as twisted 
pair would do it -- IF you could run the pair over the entire loop 
that carries the current. 

What often happens is that designers treat wiring like this as a 
DC circuit, with individual "hot" conductors and a common return. 
If hot and return are closely spaced, as in a transmission line, 
their fields cancel and there's little or no radiation. If they're 
separated, they create a magnetic field that's proportional both 
to the loop area and to the current, AND the wires act as 
antennas. At low frequencies,  source and victim are more likely 
to have a "near field" relationship (within wavelength/6), so the 
coupling is magnetic. Beyond that distance, antenna action 

It's also possible that some of this noise is coupling to the 
power line, and on control wiring (thermostat, digital readiout, 
etc), so I DO recommend a serious ferrite choke on those cables. 
Follow the winding guidelines in my RFI tutorial to achieve high 
choking impedance at the frequency(ies) where you have the 
greatest noise.


That's the wiring part of the solution. The other element is the 
relative strength of the RF component, which is essentially driven 
by the risetime of the pulse (and. like key clicks, can be 
"shaped" to soften the edges of the pulse). Your RFI is harmonics 
of square waves, so the second part of the solution is to reduce 
the strength of the harmonics. It MIGHT be possible to do that 
with some ferrite cores on a single conductor carrying the noise 
current. Depending on the strength of the current, there might be 
some saturation, but if you used a big enough core you might get 
enough rounding to reduce the noise. 

These are all "outside the box" solutions -- that is, no mods to 
the electronics -- but you may need to get inside at least a bit 
to get at the wiring. 


Jim K9YC

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