[Prev][Next][Index][Thread]
Many spark gap tests !
Hi all,
I have some serious questions about spark gaps.
The story goes like this:-
I have only built one tesla coil operaing at around 1kW, and I posted
about this many weeks ago with the title "First coil I built worked
great !", if anyone is interested in the spec.
I just recently demonstrated this coil to a local University who were
interested and had never seen anything like it before. In a short test
before demonstrating to many people a neon transformer failed short-
circuit after only one or two seconds of normal operation. I had to
do the demo with only one transformer. Otherwise it went well.
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 ?
I suspected that the Breakdown Voltage of my TCBOR gap may have changed
since it was originally constructed, tested and GLUED,
So I re-tested it.
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.
(hope this ASCII chart comes out ok!)
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.
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 ?
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.
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.
All tests were carried out in the same afternoon, at room temperature,
with the same apparatus. I have all the results, and graphs on Excel
if anyone is interested and wants to examine them clearly.
Now back to making some _REAL_ sparks !!!
Richie,
- watching spark gaps snap in
sunny Newcastle.