"Exciter coil concept"..... the way I determine resonance frequency of big aircore inductors (Tesla coil secondaries) is to use a single turn "exciter coil" in series with a 50 ohm resistor, hooked to the FG output. This is simply wrapped loosely around the big inductor. The FG is not hooked to the secondary at all, just to the single turn exciter. The scope probe is then hooked to the top of the big coil thru 1 megohm or even 10 megs, with the scope probe ground at the base of the big coil. Then one tunes the FG for peak response in the scope, as usual. I've used this technique for years.

@Gyula
@MileHigh
@TinselKoala

Gyula made me aware of the "decoupling" of the scope probe with a self made 1 pF cap. (My scope probe has an input capacitance of about 100 pF.)

See the attached measurement circuit where I put a 1 pF cap before the scope probe. The rest is as in the video.

Now, with this 1 pF in the measurement circuit the self resonance frequencies have gone up. It should be like that because the ~100 pF of the scope probe are now hidden behind the 1 pF cap. (Two caps in series results in a little bit less capacitance than the smaller cap.)

Self resonance frequency of the monofilar pan cake coil 9.6 MHz --> self capacitance ~ 8 pF (calculator http://www.1728.org/resfreq.htm )

Self resonance frequency of the bifilar pan cake coil 4.04 MHz --> self capacitance ~ 45 pF (calculator http://www.1728.org/resfreq.htm )

It is difficult for the scope to register the signal through the 1 pF cap, but after some fiddling it always works. The exciter coil has to be as close as possible to the centre of the pan cake coil to make it work (otherwise the induced signal is to weak to be picked up by my scope).

If you look at this post http://www.overunity.com/13460/teslas-coil-for-electro-magnets/msg382210/#msg382210 and the measurement I did then with the circuit proposed at http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm you will see a great agreement of the measurement I did just now (with the improve exciter coil circuit) and then:

Monofilar pan cake coil: self resonace 8.5 MHz and self capacitance 7 pF

Bifilar pan cake coil; self resonance 3.8 MHz and self capacitance 44 pF

So Gyula was quite right to believe in the web page http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm . It did not matter much that I used a ~3 pF decoupling cap then.

Since the self capacitance of the coil is ~7 pF (monofilar coil) and ~45 pF (bifilar coil) the input capacitance of ~100 pF of my probe falsifies the results if not hidden behind a 1 pF decoupling cap. Gyula, you did a great job of teaching me that, I did not catch on at first but now I learned a very important lesson.

Quote from: MileHigh on January 09, 2014, 09:51:23 PM

And here is my main point:  You are loading down the coil with the scope probe and it's associated capacitance and "disturbing" the measurement because of the extra capacitance.

However, you may see the possibilities for an interesting measurement trick that takes the scope probe load off of the coil completely.

The trick is to just measure the voltage across the series resistor, and leave the coil untouched and open-circuited.  When you do the test you look for the *minimum* voltage across the series resistor.  That is telling you that no current is flowing and the unloaded coil is being excited at its self-resonant frequency.

My first thought is that you need to do one final "small trick."  You put the series resistor in line with the signal generator ground and not the signal generator "hot."   Then monitor the voltage across the series resistor and when it is at a minimum, you know that the voltage across the coil is at a maximum.   And just to emphasize - the coil is *unloaded*!

MileHigh

@MileHigh: thank you for the explanation, I will try this "resistor trick". Also, thank you for the helpful explanations concerning the pick up coil dimensions which will help me to build the next pair of test coils (helical coils, monofilar and bifilar).

Quote from: TinselKoala on January 09, 2014, 11:21:53 PM
"Exciter coil concept"..... the way I determine resonance frequency of big aircore inductors (Tesla coil secondaries) is to use a single turn "exciter coil" in series with a 50 ohm resistor, hooked to the FG output. This is simply wrapped loosely around the big inductor. The FG is not hooked to the secondary at all, just to the single turn exciter. The scope probe is then hooked to the top of the big coil thru 1 megohm or even 10 megs, with the scope probe ground at the base of the big coil. Then one tunes the FG for peak response in the scope, as usual. I've used this technique for years.

@TinselKoala: I will try that as well.

After these initial tests I want to draw a first conclusion:

A measurable difference between a monofilar and a bifilar pan cake coil seems to be its self capacitance (monofilar has lower self capacitance than bifilar) which then results in different self resonance frequencies (monofilar has higher self resonance frequency than bifilar).

I will now try a few more measurments ("resistor trick" from MileHigh and "single turn Exciter coil" from TinselKoala) and then I will write up a little "document" with the measurement circuits that worked for me. May be I make a new video with the correct measurements. Also the tank circuit measurements have to be redone with the 1 pF decoupling cap (to hide the scope probe capacitance).

I regard the attached measurement circuit as one of the good ones. Also the circuit from  http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm seems to be good. And the results (self capacitance) agree very much.

It means that my progress will slow down, please be patient.

Greetings, Conrad

I just understood that the approach from http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm is indeed the classical approach (function generator -  resistor - coil to be tested - GND; scope over "coil to be tested") with the "Gyula improvement" in the circuit (namely the 1 pF cap to shade the 100 pF of my scope probe).

See the attached circuit diagram. And it worked just fine.

Monofilar pan cake coil, self resonance around 9 MHz.

Bifilar pan cake coil, self resonance around 4 MHz.

So, my mistake, I thought that the approach from http://www3.telus.net/chemelec/Calculators/Interwire-Coil-Capacitance-Calc.htm was something new, but it is the classical approach with a 1 pF cap to shade the scope probe. Might be nothing new for the experts, but it took a while till I got it.

Greetings, Conrad

P.S.: Working on MileHigh "resistor trick" in the classical approach.

Very strange effect with the MileHigh "resistor" trick:

The resistor trick works with the bifilar pan cake coil (Voltage over resistor is lowest at about 4 MHz) but it does not work with the monofilar pan cake coil.

May be the "resistor trick" only works with a certain self capacitance of the coil. The self capacitance of the monofilar coil is only 7 pF (self capacitance of bifilar coil 44 pF).

With the monofilar coil the Voltage over the resistor is higher at lower frequencies and continuously goes lower up to 20 MHz (the limit of my function generator).

May be someone has an idea what is going on. See the attached circuit diagram. The resistor has to be between coil and GND (in the classical set up it does not work even with the bifilar coil).

Greetings, Conrad

@Conradelektro,

This thread is entitled Tesla's "COIL FOR ELECTRO-MAGNETS". When do you you plan to test and compare the coils as electro-magnets?