You are here:

Physics/How can I oscillate a grounded RLC circuit ?


Tesla Sender
Tesla Sender  
QUESTION: Hello Dr. Stephen Nelson,

Thank you very much for your recent answer.
While preparing my questions, I am using the most popular book “Fundamentals of Physics” of Halliday, Resnick and Walker, here now, chapter 33, “Electromagnetic Oscillations and Alternating Current”.
In this project study, I have an aerial capacitor (a ball shaped antenna) with a certain capacity of C, a coil with an inductance of L, and a ground connection.

The coil is attached to a metal plate which has a ground connection, and the aerial capacitor is attached to the other side of the coil, so here, we have an antenna like circuit.
It looks like the secondary circuit of a Tesla Coil, I am sending you a photo of this circuit.

My question is, is there a possibility to activate this grounded RLC circuit (Tesla Antenna) just with a function generator, (assume the resonance frequency is known) ?
If yes, how ?

Otherwise, we may need to discuss the possibility of mutual induction, by using an activator RLC circuit which forces the antenna circuit, am I right ?

Many thanks for your help,

ANSWER: Yes, you just connect the ground of the function generator to ground, and the other part to the other end of the oscillator.

---------- FOLLOW-UP ----------

QUESTION: Hello Dr. Nelson,

Thank you very much for your answer, it helped me much, so, this seems the simplest way as I thought, but Tesla used the principle of mutual induction, the first resonating system forces the secondary coil, so it seems possible to induce much higher potentials at the aerial capacitor by this way, am I right ?

The second system is a bit complex, lets start with the first system first, the values are given as :

A- Aerial capacitor, diamter of 15 cm, C would be equivalent to 8,34 pF

B- Resonance Frequency of the Tesla Antenna is chosen arbitrary as 300.000 Hz (later we may discuss the hit of the frequency decision)

C- We have a coil (without magnetic core) with an inductance of 33,50 mH

D- The function generator has an inner resistance of 50 Ohm, the resistance of the circuit is 20 Ohm additional (70 ohm totally).

E- The applied voltage has a maximum value of 20 volt p-p, which is equivalent to 7,071 volt-rms.

Under the given circumstances above, what is the potential which is induced on the ball shaped capacitor ?

Lets start with the calculations :

XC = 63611,09 ohm

XL = 63146,01 ohm

Z = 470,31 Ohm

i = 7,071/470,31

i = 0,01503 A

i = 15,03 mA

VC = XC x i

VC = 956,38 volt

Would you please discuss the potential which is induced on the ball shaped capacitor, it seems relative high, do you agree ?

b- Can I measure this potential with a voltmeter, when I connect the ground pole into the ground and the other side to the metal ball ?

Stephen, when I build such an oscillating system, can I really achieve such high potential values on the aerial capacitor ?

c- Is there any correlation between the theoretical calculated values and the measured values of this experiment ?

You may want to notice my E-mail Adress :

Many thanks for your help,

ANSWER: Higher voltage than your input, yes, but you've left our all the important things.  What is the frequency?  Where did you get that current?  Where is the resistance?  There's always resistance, you didn't build this with superconductors.  It won't be as high as you're calculating, there's too much time-dependence in all these things.

---------- FOLLOW-UP ----------

QUESTION: Hello Dr. Nelson,

In my question, I gave all the important parameters, would you please check them again.
The function generator has an maximum output of 20 volt p-p (peak to peak) which is equivalent to 7,071 volt rms, the function generator has an internal resistance of 50 ohm, the R value of the circuit (from the wires, etc....) is 20 ohm additional, which yields to 70 ohm.

The frequency of the circuit is 300.000 Hz (300 kHz), we have an aerial capacitor with a capacity of 8,34 pF, an inductor (air core) with an inductance of 33,50 mH, under this circumstances, the calculated Z value (impedance) would be equal to :

Z = ((XL-XC)^2+R^2)^(0,5) = 470,31 ohm, okay ?

Lets calculate the current,

i = Vrms/Z

i = 7,071/470,31

i = 0,01503 A

i = 15,03 mA

VC = XC x i

VC = 956,38 volt

Result : In this Tesla Antenna there would induce a potential on tha aerial capacitor, with a sinusoidal chance between the values of +956,38 volt and
–956,38 volt.

Is that so okay ?

Many thanks for your help,

Ah yes, I see you listed two resistances and I missed that sentence somehow.  However, if you check, you'll find that your function generator has a 50 Ohm impedance.  That's not actually a resistance on an instrument like that.  These instruments aren't built to directly drive things like resonators to above their rated output (20 V P-P) voltages.  The diagram you have is terribly unclear about the nature of the coil you have, but without multiple coils like you have in an actual Tesla coil (and I don't see the number of windings here) I don't think it will work the way you want it to.  Do you have a more clear schematic?   Are you trying to drive to to get 956 Volts across just the capacitor, relative to ground?  Because if that's the case, it looks like you hooked up the function generator just across the coil of wire itself.  If they're in resonance, they'll have similar peak voltages and this just won't work because a function generator can't drive that high a voltage across the inductor.  Do you have a better drawing, or is it really just a function generator hooked up across a coil which is then hooked to an open-ended sphere?


All Answers

Answers by Expert:

Ask Experts


Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

©2017 All rights reserved.