Re: [TCML] RFI EMI Filter question

[Jim Lux <jimlux@xxxxxxxxxxxxx>]

-----Original Message-----
>From: BunnyKiller <bunnikillr@xxxxxxx>
>Sent: Jan 28, 2009 8:29 AM
>To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
>Subject: Re: [TCML] RFI EMI Filter question
>
>Hey Thomas
>
>probably should also do 
>Line=Filter=Computer
>
>Scot D
>

You need to think about where the interference is coming from and going to. The usual line EMI filter is designed to keep noise *inside* the box it's attached to, and the noise they're concerned about is usually fairly high frequency stuff (digital clocks and such).

If you're trying to keep tesla coil transients out of other stuff, it's down in the hundreds of kHz range, and the coupling mechanisms are different than for the usual consumer electronics EMI/EMC.


First, consider the TC without breakout.  It's basically a big RF tuned circuit, and your stuff is in the field of the capacitor.  So, you want to make sure the RF current in that capacitor stays near that capacitor, and isn't coupled out somewhere else. The coupling happens either by capacitance (i.e. you've got some sort of "antenna" that couples to the electric field) or by inductive coupling (you've got a wire that picks up the magnetic field from the current flow)

If the current flow (from top of secondary to bottom of secondary) is via some wire, then that wire radiates RF, which will be picked up by other wires that are approximately parallel to it. So, the take home is: no long wires carrying significant RF current.  Short wire from bottom of secondary to ground plane, which is right under the coil.


Now consider sparks and breakout.   This is a bit different.  You have a spark which connects the top load to the ground, and the rise time of the spark is a LOT faster than the hundreds of kHz.  What you now look at is a loop carrying a fast rise time pulse, radiating a magnetic field. (and, of course, the field is proportional to the 1/rise time and the current being carried).  That pulse gets picked up by other conductors, and because it's fast rise time, it can induce bigger voltages (V is proportional to d(field)/dt).  

But those pulses probably won't be stopped by a EMI filter.  Why? because it's the ground wire picking up the pulse. Say you've got a PC connected to the wall socket and then connected with a cable to a monitor, which also is connected to the wall socket.  There's a loop formed by wall socket: "ground wire in the power cord of PC": "chassis of pc": ground wire in video cable: chassis of monitor: ground wire in power cord of monitor, wall socket, ground wire between wall sockets.

That loop picks up the transient field from the TC spark (or Marx bank spark.. a real equipment killer) and a voltage is induced around the loop.  Not only that, but the current flow in the loop can couple the transient into parallel wires in cables.  The power cords aren't a big deal (after all, there's a filter at the equipment end), but the monitor cable probably isn't filtered, so now, the transient couples back into your video adapter. Ooops, dead ICs.

And, the line filter on the PC might not have very good isolation for a 100 MHz pulse.  The fast rise time pulse from the spark (we're talking nanoseconds, here) might couple by leakage capacitance right over the filter into the PC.  Remember, the filter on the PC was designed to keep PC noise inside, not 100 MHz transients out.

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