Re: A few questions...
Subject: Re: A few questions...
From: Terry Fritz <twftesla-at-uswest-dot-net>
Date: Sat, 24 Jul 1999 17:53:57 -0600
At 12:41 PM 7/24/99 -0400, you wrote:
>I think it's been awhile since the last time I advertised my ignorance so I'm
>about due for another one of those posts. I've been reading and re-reading
>many of the posts lately and I just can't understand some of the terminology,
>probably a direct result of my lack of a college education but such is life
Please ask if you don't understand something. 90% of the people on this
list never post and I am sure there are many with the exact same questions.
>1) What is RMS? I have a vague idea that the letters stand for Root Mean
>Squared but what does this mean and what are the applications for it?
The RMS value is used with sine and other none DC waveforms. It is the
equivalent value of current or voltage that will have the same heating
effect as a DC signal. So, if some messy current waveform has an RMS
current value of 10 amps, it will heat a resistor just the same as 10 amps
of DC would despite the far from DC waveform. Finding the RMS levels
theoretically and the engineering definitions of RMS can be really involved
but just remembering that the RMS value is the same as an equivalent DC
value will get you pretty far.
>2) I've seen the number 555 several times, what does this mean?
There is a super common little integrated circuit sold at Radio Shack and
everywhere else that is a simple and real nice square wave oscillator. The
LM555C and the new CMOS versions like LMC555 are used in all kinds of
things. The frequency and duty cycle are determined with just a few extra
resistors and capacitors. Hobbyists use them for all kinds of things (like
driving ignitions coils to charge Tesla coil caps). When someone needs a
nice oscillator, the LM555 IC will fit the bill 95% of the time.
>3) I've heard several people reffering to TC simulators, where can I get one
These are computer programs that simulate the electrical signals inside a
Tesla coil. You may want the check out my paper at:
This shows how a computer can simulate the electrical signals inside a
Tesla coil. many of us use MicroSim's demo program. This is a 16 Meg
program that can be downloaded from the Orcad site (Orcad bought MicroSim).
Of course, your are a student of Tesla coils... MicroSim (Orcad) gives out
this demo to get students hooked on MicroSim so they buy the real version
when they get to the real world. This works VERY well by the way...
Fortunately, the demo version is easily powerful enough to model Tesla
coils. I have a number of TC models at my web site in the "misc" area as
MicroSim is not the easiest to use program in the world but it is extremely
powerful and the best out there by far IMHO.
>4) If I have a choice between a single capacitor with the correct uF and
>voltage ratings or an MMC what are the advantages/disadvantages to one over
>the other, besides cost, adjustability and availability does the MMC have
>anything going for it?
Even though a cap may say 30kV at 10nF it may blow up in a Tesla coil.
Remember the RMS thing. The RMS current in a primary circuit of a Tesla
coil is around 15 amps. So the cap needs to pass 15 amps through itself
without overheating, failing, exploding, etc. Many high voltage caps made
from paper, mylar... will quickly die with 15 amps running through them.
Some big commercial polypropylene caps will be fine but others that are
meant for DC filtering and other low current applications will fry. MMCs
are made from the best poly caps we can find and stacked up to take both
the high voltage AND the high current. Unless the "single" cap is very
special and weighs several pounds, it will probably be very hard pressed to
handle the primary current in a Tesla coil.
A Tesla coil is pulsed with giant spikes that can reach 1000 amps many
times every second. Even though the spikes may get to 1000 amps, the RMS
or equivalent DC current level is around 15 amps. Thus, the cap has to
dissipate the heat internally that a 15 amp current would create. Since a
MMC has an internal resistance of say 0.06 ohms, the power dissipated would
be 15^2 x 0.06 = 13.5 watts. That may not sound like much power but to a
plastic cap with 20kV across it can be a lot. If you change the cap to a
paper dielectric, the internal resistance may go up to 2 ohms. 15^2 x 2 =
>Left, left, I hadda good brain but it left...