Re: Tank protection: common mode vs differential mode transients
Subject: Re: Tank protection: common mode vs differential mode transients
From: Terry Fritz <twftesla-at-uswest-dot-net>
Date: Tue, 22 Feb 2000 12:05:34 -0700
At 02:02 PM 02/22/2000 +0200, you wrote:
>Within the THOR project, we are at the moment also struggling to get some
>with the current paths. As we are actually measuring the HV tank output, we
>to ground the negative pole of it. This adds a third way for the current
>supplied to the primary capacitor to circulate:
>1 - the positive pole cable
>2 - the negative pole cable
>3 - the common ground
>But I'll get to the point now. In series with the cables charging the
>we had two 3 mH inductors, to stop the HF coming back. As we were measuring
>(with a HF current probe) the above three currents, we noticed that:
>- the two inductors generated an unwanted high current ringing during the
You bet they will ;-))
>- there was an enormous difference between differential current (the one
>actually charging the capacitor) and common-mode current (the one flowing in
>same direction in both cables). Differential current was the expected 0.5 A,
>while common-mode current had large ringings and spikes up to 20 A!
Are you sure this isn't noise giving a false reading ?? There are some cap
to ground parasitics that by be unbalanced and there is pure RF emission too.
>Removing the two 3 mH inductors the unwanted ringing was gone, but the
>common-mode spikes were still there. This led us to think that a better
>would be a COMMON-MODE inductor inserted between the cables and the capacitor.
>We are now building that with a big double ferrite U core and two
Try big ferrite beads too. They work wonders.
>Terry and others:
>have you ever measured common-mode transients on the cables from the tank to
>capacitor? You can do that easily by passing both the cables through a current
>sensor. In theory you should always read zero, but we didn't.
I have not tried this.
>About the 3th current path, the ground:
>You may think that your primary winding + capacitor are floating, but you can
>still have that path through parasitics. For instance, we too should have it
>floating, but the capacitor cans are grounded and we probably have some
>of picofarads parasitic capacitance from the capacitor pole to its can. On the
>tank side, you'll have your NST case grounded with another winding-to-case
>capacitance: here is your third current path.
>Any comments or help on this?
When you are dealing with high voltages and fast dV/dT s all kinds of fun
noisy things go on. Keep lead paths short, avoid ferrous metal, and use
lots of big ferrites...