[RFI] Ferrite Beads from DX Engineering

Jim Brown jim at audiosystemsgroup.com
Mon Jan 11 15:10:46 PST 2010


On Mon, 11 Jan 2010 16:45:22 -0600, Peter Laws wrote:

>Well, I mean not "really" as in "you don't know what you're talking
>about" but as in "wow, the SAE types spend a lot of time getting those
>rise-fall times just right so that the car meets the applicable
>standards for fuel consumption and emissions" ...  Although not
>mandated, there are also drivability concerns as well ...

I can't comment on the automotive aspect of the design problem, but the 
electronics and EMC part is solid and well known. 

>Just how much would chokes change that?  

That's hard to predict. It will certainly vary with the plug circuitry 
and the ferrite part(s) being used. HOWEVER -- it is also well known 
that rather small changes in the switching waveform can make a large 
difference in harmonic content. If I were working on the problem in a 
vehicle, I'd have a scope with a piece of wire attached to its input 
set up next to the engine and watch the waveshape as I worked on the 
plug wires. Even better, a spectrum analyzer. 

>And how does this compare to the old "resistor plugs" that were sold
>to make the ticking on the car radio go away?

Same fundamental idea, except that a ferrite part has much less effect 
at DC and low frequencies than does a resistor, and has more effect at 
high frequencies. In other words, ferrites can probably provide more 
suppression with less effect on motor performance. 

73,

Jim Brown K9YC






More information about the RFI mailing list