Author Topic: help with finding resonant frequency  (Read 1860 times)

Offline HVuser

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help with finding resonant frequency
« on: November 18, 2021, 01:28:01 AM »
I am having trouble with finding the resonant frequency on my primary coil. I am using the method in the book The Ultimate Tesla Coil Design and Construction Guide. It uses signal generator with 1k ohm resistor to the coil. I have tried series and parallel capacitor to lower the theoretical resonant frequency, although I still can't find it. I even added the LEDs to visually see if the coil is in resonance. See the included image to see the setup. I am using function generator producing sine-wave at 2 volts. The highest capacitor I have is a 10 uF ceramic. The primary is 4.4 uH measured with cheap LCR meter. Together I would expect the theoretical resonant frequency to be around 23.99 kHz. What I am finding is I have not reached the resonant frequency at 12 MHz (maximum of my function generator). I found a video that was the same setup here https://www.youtube.com/watch?v=DF6WdTM8s4U. I have used lower capacitors, tried my secondary coil instead of primary, verified continuity/connecting wires and all the same results. What else can I try?
« Last Edit: December 03, 2021, 04:30:28 AM by HVuser »

Offline thedoc298

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Re: help with finding resonant frequency
« Reply #1 on: November 18, 2021, 05:10:43 AM »
I am having trouble with finding the resonant frequency on my primary coil. I am using the method in the book The Ultimate Tesla Coil Design and Construction Guide. It uses signal generator with 1k ohm resistor to the coil. I have tried series and parallel capacitor to lower the theoretical resonant frequency, although I still can't find it. I even added the LEDs to visually see if the coil is in resonance. See the included image to see the setup. I am using function generator producing sine-wave at 2 volts. The highest capacitor I have is a 10 uF ceramic. The primary is 4.4 uH measured with cheap LCR meter. Together I would expect the theoretical resonant frequency to be around 23.99 kHz. What I am finding is I have not reached the resonant frequency at 12 MHz (maximum of my function generator). I found a video that was the same setup here https://www.youtube.com/watch?v=DF6WdTM8s4U. I have used lower capacitors, tried my secondary coil instead of primary, verified continuity/connecting wires and all the same results. What else can I try?

This works for me.

Offline davekni

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Re: help with finding resonant frequency
« Reply #2 on: November 18, 2021, 06:44:13 AM »
Circuit shown in first post is not complete.  One side of the capacitor has no connection.  Posting a complete circuit would help.

LEDs in series with a 2V signal generator output may block any signal.  Depending on color, LEDs typically take more than 2V to conduct anything.

Don't waste time checking way beyond expected frequency.  Calculate frequency based on measured values, then sweep carefully and slowly through that frequency, at most from 0.5x to 2x frequency.  Issue is more likely missing resonance by sweeping too fast through it.
David Knierim

Offline HVuser

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Re: help with finding resonant frequency
« Reply #3 on: December 03, 2021, 04:43:39 AM »
Thank you thedoc298 and davekni for all the suggestions, and my apologies for not updating sooner. I increased the voltage and the LEDs did light up.  I updated the photo to include the steps I tried. After using the formula for the coil dimensions I was able to find the close to the resonant frequency. The LCR meter must be way off as the coil is very low inductance. The primary coil is way off what I need with my capacitor as I assumed the LCR meter was close (whoops). My plan is to make a new primary coil and if I did it correctly the coil will resonate.

Offline davekni

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Re: help with finding resonant frequency
« Reply #4 on: December 09, 2022, 07:00:22 PM »
Quote
I updated the photo to include the steps I tried.
Any of your three tries would work IF you add a ground connection to the top node (top of coil to ground).
BTW, secondary resonant tests don't have an explicit ground connection there.  The top load capacitance is inherently to ground.  Primary resonant tests always need some explicit way for current to complete a circuit back to signal generator ground.

Quote
Me too have no clue what is resonant coil. I only understand resonant circuit.
A "resonant coil" just refers to a coil that is used in a resonant circuit.  Same for "resonant capacitor", a capacitor used in a resonant circuit.

Coils do have a "self-resonant" frequency.  That is the frequency where the coil resonates with its own internal parasitic capacitance between windings.
Capacitors similarly have a "self-resonant" frequency.  This is where the capacitor resonates with its parasitic series inductance (lead wire inductance and inductance of capacitor size itself).
David Knierim

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Re: help with finding resonant frequency
« Reply #4 on: December 09, 2022, 07:00:22 PM »

 


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