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Neon filtering (long post warning)
>Date: Thu, 24 Oct 1996 08:58:16 -0500
>From: huffman <huffman-at-fnal.gov>
>To: List Tesla <tesla-at-pupman-dot-com>
>Does someone have a working 'filter' for a neon setup that they believe
>works? I'm still puzzled by the post that bypass caps in the 100s of pF
>caused neon failure.
>Dave Huffman
>
Dave,
Don't forget to include the effect of the leakage inductance
of the neon. It will resonate with the TC's primary cap. Robert
Stephans has had good results using 300mH filter inductors. This may
well be due to both it's RFI filtering properties and it's inductance
"killing" the resonant circuit that would have been created by the
neon's leakage inductance and the primary cap. Malcolm Watts talked
about this effect earlier this year:
jim
---------include of Malcolm's prior post on neon filtering -----------
To: Tesla-List-Subscribers-at-grendel.objinc-dot-com
Subject: Neon Filtering
From: tesla-at-grendel.objinc-dot-com
Date: Mon, 22 Apr 1996 09:00:36 +0700
>From MALCOLM-at-directorate.wnp.ac.nz Mon Apr 22 01:12 MDT 1996
>Received: from rata.vuw.ac.nz (root-at-rata.vuw.ac.nz [130.195.2.11]) by uucp-1.csn-dot-net (8.6.12/8.6.12) with ESMTP id TAA08813 for <tesla-at-grendel.objinc-dot-com>; Sun, 21 Apr 1996 19:42:41 -0600
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
To: tesla-at-grendel.objinc-dot-com
Date: Mon, 22 Apr 1996 13:42:13 +1200
Subject: Neon Filtering
Hi all,
I spent half a day having a further look at all this. I in-
vestigated the properties of three circuits. Before describing them,
I'd welcome input from others on any of these schemes.
(1)
o-----\/\/\/\------UUUUUU-----+-----------o
|
------
------
|
|
o-----------------------------+-----------o
Characteristics : with R set so that damping was critical
(Butterworth response), the response fell off gradually and definitely
towards a zero.
Advantages : Simplicity, damping resistor will also help damp the
tuned circuit formed by the transformer inductance
and primary capacitor.
Disadvantages : Transformer power is lost in the resistor as well as
the RF. Also, no damping between the filter cap and
transformer reactances.
(2)
o----+-----UUUUUUU----+--------+-----o
| | |
| | -----
-----\/\/\/\/----- -----
|
o------------------------------+-----o
Chracteristics : At high frequencies, response falls off more
gradually than circuit one and tends to give
an R-C response (single pole) as the choke
becomes effectively left out of the picture
Advantages : No transformer power is wasted in the resistor
Disadvantages : Not as effective as (1) at suppressing RF having
a response more akin to an R-C filter. Also, no
damping applied to the Cp/transformer circuit.
(This is a distinct disadvantage when used with a neon transformer.
I think this circuit would be almost ideal for a pole-pig system
where high primary powers are being used).
(3)
o---------\/\/\/\-----UUUUUU-----+------o
|
\
/
\
/
\
|
-------
-------
|
o--------------------------------+-------o
Characteristics : Standard 2 pole response with resistors adjusted
for critical damping.
Advantages : Some damping for power components as well as RF
Resistor in series with filter cap also helps damp any
reaction between this component and the transformer
secondary at high frequencies.
Disadvantages : More components than the other two circuits.
Some 50/60 Hz power wasted in the series resistor.
There are other variations on these (e.g. resistor across the
capacitor). I didn't test them all because some disadvantages were
immediately obvious. For example, a resistor in parallel with the
filter capacitor will have the transformer voltage impressed across
it continuously and will need suitable voltage ratings. Resistors
in series with chokes suffer the same thing. All circuits can roll-
off much faster but at the expense of peaking (ringing, Tchebyshev
response) which is what we don't want. Personally I'd pick circuit 3
as being most useful for a neon supply, but I'd like to hear of
other's experiments/experiences with this.
Finally, I found that the transformer secondary had little effect
on the lower RF filter response in general.
Malcolm
---------include of Malcolm's prior post on neon/cap resonance -------
Hi all coilers,
I've decided to elaborate on this little snippet I
wrote earlier on the subject of series-resonance in inductively
limited transformers for the benefit of those running neons. I
have done a number of measurements on my 12kV 60mA unit and have
come up with some interesting facts....
> That impedance is valid only across the individual components (L or
> C). If you measure circuit current, you will find it is only limited
> by the resistance present in such a circuit.
Firstly, some circuit values. The secondary resistance of my unit is
approximately 32kOhms. The inductive reactance of the secondary is
approx. 200kOhms. By vectorially adding the reactances it is seen
that inductance dominates heavily. If I now connect a capacitor
across the transformer such that Xl = Xc at 50Hz (the mains freq.
here) the circuit is series resonant at 50Hz. (Xl = Xc = 200kOhms)
Other values of C make the circuit series resonant at some other
frequency such that Xl = Xc.
I have measured the Q of this particular circuit at somewhere
about 6, which happens to approximate Xl/R. In this condition, the
effective circuit elements are a resistance of 32kOhms and a voltage
source. So my transformer can pump a current of :
12k/(32k+Rextra) Amps through all the circuit components where
Rextra is the resistance of anything else I might put in the circuit.
(If Rextra is non-zero, Q will of course drop). So assuming Rextra is
zero, I have a resonant charging current available for my capacitor
of 12/32 Amps = 375mA which also happens to = (approx) Q x the rated
transformer current. Given this, I can charge the capacitor WAY beyond
the open-circuit output voltage of the transformer (current-limiting
huh!). Well, the transformer was never designed to run in a resonant
circuit like this.
I guess there are two morals to this story. (1) Your poor old
transformer wire is probably doing rather well to last under this
current burden, and (2) the spark gap across the transformer should
never be opened up beyond the open circuit peak voltage output of
the transformer.
Am I right/wrong? Comments welcome as always.
A couple of cents more,
Malcolm