Tesla's "COIL FOR ELECTRO-MAGNETS".

[conradelektro]

I made some more tests.

So far I get the impression that the classical circuit and a 1 pF decoupling capacitor for the scope probe works best, see my post http://www.overunity.com/13460/teslas-coil-for-electro-magnets/msg382419/#msg382419

The attached simplified "exciter coil" circuit also works well. It helps to put the second scope probe away to avoid clutter. Note that there is also the 1 pF decoupling capacitor for the scope probe. (This is TinselKoalas preferred way, may be with a smaller exciter coil of only two turns and a 50 Ohm resistor in series with the exciter coil. For lower frequencies, which means a coil with a higher self capacitance the 1 pF decoupling capacitor might not be necessary.)

I changed my video (because the one I posted yesterday did not feature the 1 pF decoupling capacitor and showed wrong measurements).


New video:  http://youtu.be/fC84W0PIZoE (might take some time till it is completely uploaded)

I would say that since I use the 1 pF decoupling capacitor for the scope probe all measurements started to match to a high degree:


Self resonance frequency of monofilar coil is about 9 MHz, self resonance frequency of bifilar coil is about 4 MHz.

Self capacitance of the monofilar coil is about 8 pF, self capacitance of the bifilar coil is about 45 pF.


My question to all bifilar coil magicians: how can the higher self capacitance of a bifilar coil do magic?

Greetings, Conrad

[conradelektro]

"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 tried that and it worked (bifilar coil self resonance frequency 3.9 MHz, monofilar self resonance 8.7 MHz). I had the feeling hat the method from my last post above worked better ( 1 pF cap instead of 1 M resistor). But that might have to do with my strange exciter coil.

After many tests, my favourite method has become the one from my video  http://youtu.be/fC84W0PIZoE  (also discussed in my post http://www.overunity.com/13460/teslas-coil-for-electro-magnets/msg382419/#msg382419 )

This is the classical method with a 1 pF decoupling capacitor in series with the scope probe over the "coil to be measured".

Greetings, Conrad

[gyulasun]

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

Hi Conrad,

I edited your circuit drawing to include and show the scope probes' and the pancake coil's  'parasitic'  capacitances and also included the generator's inner resistance, assuming 50 Ohm. So this is a kind of RLC network driven with the generator and possibly a Spice circuit simulator could surely show the frequency response, (the good old Bode diagram if you like),  taking the output across the 51 kOhm resistor.
The shunting effect of the 100pF or so probe capacitances (across the generator output and across the 51 kOhm) could explain that the output voltage continuously reduces as the frequency increases up to 20 MHz, shadowing the possible resonance of the monofilar coil around 9 MHz.  (A 100 pF capacitor has a reactance of cca 397 Ohm at 4 MHz and cca 176 Ohm at 9 MHz.)

A possible remedy to use MileHigh's suggested setup is to 'decouple' the scope input capacitances with a series low value coupling capacitor of 1-2 pF (the smaller the better).  Using the probe as a 10:1 divider (and not as 1:1) the probe's input capacitance can become as low as 10 to 16 pF, this could also help in this setup.

Nice and good work, thanks for all your efforts, pleased you have done it.

Greetings,  Gyula

[gyulasun]

....

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.
...

Hi Conrad,

You can of course call or refer to this measuring method as a 'classical' one but the use of the 1 pF coupling capacitor can only be useful up to the some MHz frequency range, it becomes a decreasing reactance value as you go towards the some 10 MHz range and higher  (at 10 MHz a 1 pF cap has 15.9 kOhm and at 20 MHz it has about 7.9 kOhm reactance).

The small difference between the results of using 1 MOhm and using 1 pF is explained by the fact that in the MHz frequency range resistors are not able to keep their original resistor value (measured at DC) because there is an equivalent shunting capacitor of a few pF (or maybe less) across their legs, (their equivalent circuit includes a parallel capacitor too) coming from their material and physical structure. Perhaps you could check your 1 MOhm resisor with your C meter to measure it, probably you would have to force the meter to measure C and not let it choose automatically the resistor range by itself.
Even a few kOHm resistor shows less resistance when you start increasing the working frequency towards the some hundred kHz and higher.  A possible solution to minimize this unwanted effect is to use several such resistors in series to reduce the resulting equivalent capacitance or using a parallel air core coil across one such resistor to form a parallel resonant LC circuit with the parasitic resistor capacitance i.e. to tune it out but this works at a single frequency of course.
The so called carbon resistors are the best frequency independent resistors, then the metal and the film resistors come.

Greetings,  Gyula

[MileHigh]

Conrad:

Another great clip!  You are in good hands with Gyula and TK for sure!  The home-made 1 pF capacitor solution really works well.  The "resistor trick" has it's limitations and the "improved classical" method seems to be the best.  More importantly, you are understanding the issues and the reasoning behind the different techniques.

Since we are dealing with such small capacitances, it's somewhat analogous to Heisenberg's Uncertainty Principle because you are affecting the system when you try to make a measurement on the system:

In quantum mechanics, the uncertainty principle is any of a variety of mathematical inequalities asserting a fundamental limit to the precision with which certain pairs of physical properties of a particle known as complementary variables, such as position x and momentum p, can be known simultaneously. For instance, in 1927, Werner Heisenberg stated that the more precisely the position of some particle is determined, the less precisely its momentum can be known, and vice versa.

TK:  Hey!  I "discovered" something that is actually a well-known technique.  So now I am officially part of the OU.com club!  lol

MileHigh