[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: [TCML] High Voltage design questions and considerations
You have an interesting set of photos, Jim. Question: what kind of HV
transformer did you use for your 12-inch TC?
Asimov13647
Aka chuck
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Jim Mora
Sent: Wednesday, July 01, 2009 1:54 AM
To: 'Tesla Coil Mailing List'
Subject: RE: [TCML] High Voltage design questions and considerations
Hello Mike,
A typical OBIT (oil burner ignition transformer) is able to deliver 10kv @
23ma into a relative output short. It is a "shunted" transformer which
limits the amount of current it can deliver. When I was young, I made a
Jacobs ladder out of one and watched it for 10 to 20 minutes at a time and
don't remember the transformer getting hot. An arc is close to a direct
short on the output. Non current limiting transformers will pump every
electron it can get until the breaker pops or something bad happens. A neon
transformer or Obit is a good starting point.
As you discovered the output is 10Kv @ 23ma or 230 VA. If we divide 230/120v
the wall current should be around 1.9-2 amps for a theoretical "lossless",
100% efficient transformer. Not quite real world but good to know.
You mention charging the capacitor. The idea is to charge it until the
voltage reaches near the 10kv max at which point a set spark gap breaks down
and closes the "tank" circuit. It is "tuned to resonant at the frequency of
the secondary as you mentioned too. Since you only have 23ma to charge the
cap, a selection is made that allows for the cap to charge sufficiently to
breakdown the spark gap ideally at 120 breaks per second (the number of
peaks at 60 hz). Ok so we have E/I = R 10kv/.023=435k ohms of Cap reactance
max at 60hz so using your XC calc 1/2pi*fc is used - plugging in
XC=435kohms rearranging c=~ 6.1 *10^-9 or 6nf let me test that...Yup that is
the resonant cap to the transformer. It is better to go 1.5x resonant or
.009uf (9nf). A 4" coil wound with 26awg for 18" on a 22" form with 18"
winding length and a 14x3" dyer duct toroid top load and a .25" copper tube
primary spaced .25" with an inner diameter of 7" and tapped at 9.8 turns
will charge the cap to 86% with 120bps with this transformer. The resonant
freq is ~290Khz and should give a max of 2 feet of sparks. How do I know all
this? Go to this site...Java TC rules!!! http://www.classictesla.com/ It
does all the calcs, a great program!
We generally use CDE caps in a MMC $3.00-$4.00 each (look it up in the
archives). They are .15uf-2000v /.009= 16 caps in series rated 32kvDc a
great alternative to roll your own. You can use these toward your next coil
too. Someone can help you with beer bottle caps if you can't afford those.
I hope that gives you some direction. Start by reading all the safety info
in the archives and then the basic theory. We all got here wondering about
these things and will help you! Miraculously, I never got bit by my oil
burner transformer, see that you don't either! 10ma can kill. 10kv can
deliver that through you easily.
Jim Mora
Safety is always first.
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Mike Thompson
Sent: Tuesday, June 30, 2009 7:13 PM
To: tesla@xxxxxxxxxx
Subject: [TCML] High Voltage design questions and considerations
Hello All,
I am new to the list, and was wondering if I could get some advice and/or
opinions on Tesla Coil design. It?s been some time since I have played
around with high voltage. About 15 years ago a friend and I built a
ridiculously inefficient coil and enjoyed what fireworks it had to offer. It
was really just a collection of PVC, glass, tin foil, and wire. The
transformer was an oil burner igniter type. This is the only piece I
actually still have.
If the transformer is of a common type, the research I did states that it
has a 10KV output at 23mA. This would equate to 230 watts, meaning that the
primary of the transformer should not exceed 1.9 amps of current. The
transformer should also have a turn ratio of around 83:1. To prove that the
transformer truly puts out 10KV I did a little testing. Below is a rough
schematic of my test and some data follows the diagram.
_____________ ________
| | |||( |
_ _ |||( |
|~| |M| )||( -
| | )||( |M|
| | |||( |
| _ | |||( |
--|M|-------|||---------|
A variac was used to alter the AC voltage going into the transformer. I used
a total of three meters for this test. One to monitor the voltage of the
variac, one to monitor the voltage of the transformer?s secondary, and
finally one to monitor the current going from the variac into the primary of
the transformer.
V In Amps In Calc Res in Ohms V Out Calc Turns Ratio
1.3 0.0085 152.941176470588 119.3 91
2.1 0.0116 181.034482758621 187.2 89
3.2 0.0153 209.150326797386 277.8 86
4.1 0.0177 231.638418079096 346.5 84
5.2 0.0208 250 434 83
6 0.0229 262.008733624454 498 83
7.1 0.0258 275.193798449612 590 83
8.3 0.0286 290.20979020979 683 82
I took as many readings as I could until I reached the upper limit of the
meter hooked up to the transformer's secondary (750 volts). Then I simply
calculated the ratio of the input to the output voltage. As can be observed
it appeared to be around 83:1. I am assuming that the current consumption is
the result of inductive reactance or what I like to term as the "Angel's
share" of the power.
I guess my questions in regards to the transformer are. What is the maximum
current that I can have going through the primary before the transformer
really starts to have problems? Also why does the inductance seem to
increase when I apply more voltage?
Ok enough about the transformer.
Now I have some questions about capacitors. Because I am on a budget I am
not able to afford fancy capacitors. In fact, in the past I used patio door
glass and tin foil. This tended to be bulky and dangerous in a limited
space.
Recently I read about saltwater capacitors and decided to give that a try.
My capacitor is simply a coffee can filed with saltwater. Inside the can I
placed three water bottles containing salt water. Electrodes were inserted
in each bottle and tied together. All vessels that contained water were
topped off with vegetable oil. Picture available upon request.
I used a meter to read the capacitance. The meter used 12 KHz as the
frequency to test with. The reading came up to be 19.2nF. At 60Hz this would
equate to be 138,155 ohms of reactance. So at 170 Volts (The limit of my
variac) I should get a reading of 1.23mA of current. This too I thought I
needed to test. Below is the schematic along with the data that I logged.
_____________
| | |
_ _ |
|~| |M| _
| | ^
| | |
| _ | |
--|M|-------|
Again a variac was used to alter the AC voltage. Two meters were put to use.
One to monitor voltage and the other to monitor current. I took a reading
every ten volts.
Volts Amps Calc Xc in Ohms Capacitance in Farads
10 0.000072 138888.888888889 1.90989E-08
20 0.000143 139860.13986014 1.89662E-08
30 0.000208 144230.769230769 1.83915E-08
40 0.00028 142857.142857143 1.85683E-08
50 0.000349 143266.475644699 1.85153E-08
60 0.000418 143540.669856459 1.84799E-08
70 0.000489 143149.284253579 1.85304E-08
80 0.000559 143112.701252236 1.85352E-08
90 0.00063 142857.142857143 1.85683E-08
100 0.000698 143266.475644699 1.85153E-08
110 0.000767 143415.906127771 1.8496E-08
120 0.000837 143369.17562724 1.8502E-08
130 0.000909 143014.301430143 1.85479E-08
140 0.000981 142711.518858308 1.85873E-08
150 0.001051 142721.217887726 1.8586E-08
160 0.001112 143884.892086331 1.84357E-08
170 0.001192 142617.44966443 1.85995E-08
Using Ohm?s Law, and the formula for capacitive reactance (Xc = 1/2piFC) I
was able to calculate capacitance. I was pleasantly surprised to see that
the values came out close to what the meter was reading at 12 KHz.
Furthermore there was a general sense of satisfaction when the math actually
came within the ballpark of what I was reading.
All pleasantness aside I started to have some concerns. First off, the
reactance at 60Hz produced a resistance of approx 140KOhms. At 10KV the
capacitor would draw 71mA. Well above the rating of 23mA of the transformer.
This also explained the loud hum and 4 Amps of current being drawn by the
transformer?s primary. Can the standard oil ignition transformer take this
sort of punishment for very long? Next I started to wonder where this 71mA
was going. Was this purely reactance?
I am aware of the dual purpose of the capacitor in a Tesla coil. It needs to
be able to not draw more current then the power supply can handle at 60Hz
yet be of a decent value to properly tune the coil's primary to resonate
with the secondary. BUT, does the 71mA in anyway go to charging the primary
of the coil? Or is it merely burned off in reactance? This also got me to
thinking. If indeed the 71mA is just the "Angel?s share" and has nothing to
do with charging the Tesla Coil's primary, do we then need to insure that
the power supply can deal with the reactance of the capacitor, the reactance
of the primary, and only then use the rest of the power to transfer to the
secondary of the Tesla Coil?
Perhaps I am missing something. Is my line of thinking as least in the right
direction? I have no engineering training, so to someone in the known I
probably look a bit silly, but can anyone help me with these questions? As
freaky as it sounds, since I have started playing with all this again I have
been lying awake at night ponder these questions. Can anyone help?
Thanks
Mike T.
Homepage
http://home.comcast.net/~mikethompson236/index.htm
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla