[Prev][Next][Index][Thread]
Re. Many spark gap tests !
>Original Poster: "R.E.Burnett" <R.E.Burnett-at-newcastle.ac.uk>
><snip>
>Afterwards I thought that this would eventually happen without any
>protection on the neons. Then I thought that my TCBOR (RQ) spark-gap
>is accross the HV secondary so it should short-circuit the HV sec
>during primary tank ringdown, and SHOULD ITSELF LIMIT the maximum
>voltage accross the secondary of the transformers. So why should I
>put a second (single gap) SAFETY GAP in parallel with the main gap ?
The main gap is not an absolute clamp. During the time when gap current
passes through zero and it stops conducting, brief VHF oscillations at
twice Vgap appear across the gap. But this is still not the whole story,
there are still some mechanisms yet to be identified that determine why
and when safety gaps fire.
>I set up a test with a tesla coil primary coil, capacitor, and spark
>gap operating from a low current HVDC flyback power supply. The cap
>would charge to around 8 Kv then the gap-would breakdown with a snap
>and charging would begin again. This resulted in a rather erratic
>series of snapping sounds. Investigation with a high voltage scope
>revealed that the breakdown voltage was not CONSISTENT on every spark.
>The breakdown voltage for 1000 sparks was logged by computer and found
>to be centred around 8Kv but fluctuated randomly by several Kv !
>
>After taking the time to setup this test, I took full advantage and
>recorded all the Breakdown Voltages for 1000 sparks accross 1, 2, 3, 4,
>5 and 6 gaps of my RQ gap. The likelihood of breakdown at a particular
>voltage is shown on the distribution chart below.
>
>Cumulative
>Frequency
> ! This shows that a single gap
> ! 1Gap fires consistently at 3.6Kv,
> ! _ Many gaps in series fire at
> ! I I higher voltages but become
> ! I I _MUCH_ less consistent !
> ! I I 2Gaps
> ! I I _
> ! I I / \
> ! I I I \ 3Gaps
> ! I I I I __ 4Gaps
> ! I I / I / \ _____ 5Gaps
> ! I I I \ / \ / \________
> ! I I / I / \ / /\ \
> ! I I / \ / \/ / \ \
> --+-----+---------+----------+---------+-------+---------->
> 0 3.6Kv 5.8Kv 6.9Kv 8.3Kv 9.7Kv Volts
>
>
>With my choice of 4 Gaps, my average firing voltage would be 8.3Kv,
>(just below the peak secondary voltage of my neon transformer.)
>However this test shows me that the voltage could OCCASIONALY far
>exceed 8.3Kv because the firing voltage is not very consistent, causing
>damage to insulation in transformers and capacitors.>
This is interesting, I've not seen such data before. However, a
deviation of just a KV or so should not be cause for concern so far as
keeping tank voltages to non-hazardous levels.
>I repeated the same test with one single set of copper tubes to form
>a single static gap. 1000 sparks were logged at many different gap
>spacings, and the snapping sound form the gap was always "regular"
>as the cap charged to almost the same voltage every time.
>
>The results are shown below and indicate that a single gap
>has a far more consistent breakdown voltage.
>
>Cumulative
>Frequency
> !
> ! Small
> ! Gap
> ! _
> ! II
> ! II Big
> ! II _ _ Gap
> ! II I I I I _
> ! II I I I I I I _
> ! II I I I I I I I I
> ! II I I I I I I I I
> ! II I I I I I I I I
> ! II I I I I I I / I
> ! II I I I I / I / I
> --+-----+---------+----------+---------+-------+---------->
> 0 3.6Kv 5.8Kv 6.9Kv 8.3Kv 9.7Kv Volts
>
>
>This seems odd because it goes against what I have seen several times
>and have been told by several people, and that is:-
>
>"A series of small static gaps in series will breakdown at a more
>consistent voltage than one big gap"
>
>Is this true ? If not then why is the multiple gap _SO_ popular ?
>Is quenching and "spreading the heating effect" more important than
>consistency of breakdown voltage ? Is the resistance lower ? Or am
>I missing someting totally ?
I've not heard such advice concerning consistent breakdown voltages, just
that multiple gaps will quench better, and this is the reason for their
popularity. It's also very important to keep the electrodes cool, as the
presence of hot metal ions impairs quenching. I myself use a well-cooled
single gap, as this appears to have less loss and produce better output,
despite poorer quenching (and more strain on my caps).
>If I try a single static gap (set correctly) on my TC will I really
>get a more consistent firing voltage and less chance of the gap voltage
>going too high.
It may well result in more consistent firing, but I doubt 1-2KV will mean
the difference between life and death to the NST and caps. Remember,
just running an NST with NOTHING connected to it's secondary puts a
greater HV stress on it's insulation that it's maker had intended.
>I have put a lot of effort into this, and am getting a bit confused !
>As you have probably guessed I like to get to the bottom of things,
>so please let me know if you have any thoughts.
>In the meantime I will probably set the single gap and then try it on
>my coil anyway, I will let you all know what happens.
>
>PS. I also noticed a little TV and Radio interference when testing the
> RQ style gap but none at all when testing the single gap, even at
> higher voltages.
> Richie,
Interesting!
Regards. Gary Lau
Waltham, MA USA