Re: Operating Frequency

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Mddeming@xxxxxxx

Hi Tiago,

    The secondary coil pretty well determines both the upper power 
handling capacity of a coil and the upper operating frequency. Since 
F~1/[2pi*srqt(LC)] The secondary's inductance and effective capacitance 
tends to lock in the upper limit. In general, small coils tend to operate 
at higher frequencies than larger ones. Adding the topload increases the 
capacitance and lowers the operating frequency even more. Most amateur 
coils operate in the 70-350 KHz range, although some mega-coils have gone 
as low as ~50KHz and some mini-coils operate at >1MHz. In general, the 
bigger the coil, the greater the power handling capacity. Some mini-coils 
operate <100W and some of the monsters are in the multi-megawatt range. =8^O
    To minimize RFI problems, it's a good idea to keep medium-power coils 
operating below 455 KHz.
    On a first attempt, secondaries are usually made with an H/D ratio of 
4-5:1, and a toroid whose major diameter is ~ winding height, so this 
pretty well locks in the operating frequency. Assuming that you are going 
with an AC power supply, you will want your tank capacitor to be at least 
1.5 times the size that will resonate with your power transformer at 60HZ. 
For your primary coil, use L pri X C pri = L sec X C sec as a first 
approximation.  A good rule of thumb is to make the inner diameter of your 
primary = diameter of secondary coil + 2 in. (+ 5 cm). Add about 50% to the 
calculated L pri to determine the size of your primary coil, because you 
will need "wiggle room" for tuning. Since streamer formation adds 
capacitance to the secondary, the primary will have to be tuned ~ 15% low 
for maximum effectiveness. Every time a coil is moved, or something is 
moved in the room with the coil, the optimum point changes slightly. Also, 
if you are like most coilers, over the following year or two, you WILL 
change your toroid size, you WILL change your cap size, and you WILL try a 
bigger power supply. It's all a balancing act: If L pri is small, more 
power will be dissipated in the primary circuit, if L pri is large, V sec 
will be lowered. For a beginner, you might consider this: 3 in coil <=1KW, 
4.5 in coil <=3KW, 6 in coil <=5KW until you have some experience. Coils 
over 6 in and/or over 5 KW are not recommended for a first project.
    You can use one of the many excellent Tesla programs mentioned in the 
list archives to work out the details closer, but this will usually get you 
in the ballpark for a working coil. Every experienced coiler has their own 
variation on these suggestions. Three coilers can agree completely if two 
of them are dead.  ;-)))

Hope this helps,

Matt D.
"If two engineers agree, fire one of them because the other guy's doing his 
thinking for him."
Old Bell Tel Labs poster ca. 1960


In a message dated 7/11/05 2:07:10 P.M. Eastern Daylight Time, 
tesla@xxxxxxxxxx writes:
Original poster: Tiago Ventura <tiago.ventura@xxxxxxxxx>

Hi All!

What´s the best operation frequency for a Tesla coil ?
Which factors should I to consider when choosing the operating frequency ?

Thanks,

Tiago Ventura
<http://www.pbase.com/tiagoventura>http://www.pbase.com/tiagoventura
PX9I2912