Charging a coil with less energy and get huge BackEMF energy pulses

[MeggerMan]

Hi Stefan,

Inductance test on a bifilar coil

I have just tried a simple coil wrapped around a small toroidal ceramic core and at 22 turns of 1mm plastic coated wire I get 450uH at 1KHz and 466uH at 120Hz.
The same wire, but wrapped as a bifilar coil around the same core gave me 0.3uH at 1KHz and 0uH at 120Hz.
So yes, provided there is a core it works.

However with a pancake coil wound around in a spiral on a CD the result was as follows.

120uH for the full coil.
27.6uH for one half
28.4uH for the other half.

I then reversed the centre to outer connections (clockwise centre to anti-clockwise outer).
24.1uH for full coil
56.2uH for one half
56.3uH for other half

I need to retest one of my experiments I did the other night now as I realise that the pancake was not connected as a bifilar arrangement now

Regards

Rob

[MeggerMan]

Hi Gyula,

One more notice: the ferrite core can be saturated by this huge current and this makes the switch on/off process nonlinear, so the repeatabilty/reconstruction may involve inherent differences in the measured data.  This is nicely shown in Naudin's another test here:

I am wondering if it is the inductance increases as the core is removed and therefore allows a larger back emf. See my tests above.
It would have been useful for JLN to have recorded the coils inductance at each ferrite core position.
As I understand it the inductance will be directly link to the back emf amplitude.
I would have a guess that a bifilar coil with an air core has more inductance than the same coil with a ferrite core or ferrous core.
Could it be the coil windings can each have a very tight localised inductance in air that is disrupted when you introduce a ferrite core, same as if you had lots of single turn coils all coupled in series?
Therefore placing a ferrite core into the coil suddenly cause these single coils to fight against each other and cancel themselves out, hence very little back emf.


Regards
Rob

[pese]

ja , das ist es.  in einer bifilaren spule ist es immer gegeben ,  das beide windungen sich gegenseitig  in einer  transformation entgegenlaufen .  Ein Eisen oder Ferritkern erh?ht diesen Effekt.  Deswegen sinkt der Induktionswert und vermutlich  auch der Wirkungsgrad      GP
yes, that is it. in a bifilaren reel is always given it, which both turns run toward themselves mutually in a transformation. An iron or a ferrite core increases this effect. Therefore the induction value and probably also the efficiency sinks
GP

[gyulasun]

Hi Gyula,
...I am wondering if it is the inductance increases as the core is removed and therefore allows a larger back emf. See my tests above.
...As I understand it the inductance will be directly link to the back emf amplitude.
I would have a guess that a bifilar coil with an air core has more inductance than the same coil with a ferrite core or ferrous core.
Could it be the coil windings can each have a very tight localised inductance in air that is disrupted when you introduce a ferrite core, same as if you had lots of single turn coils all coupled in series?
Therefore placing a ferrite core into the coil suddenly cause these single coils to fight against each other and cancel themselves out, hence very little back emf.
Regards
Rob

Hi Rob,

This morning I made a bifilar solenoid coil similar to that of Naudin's but with less number of turns. My ferrite rod is 9.5mm OD and 170mm long. I covered it with a 55mm long hard paper cylinder of 10mm ID so that it can easily slide on the surface of the rod.
I wound 2 x 23 turns onto the hard paper cylinder from 0.8mm OD enameled copper wire.

I measured the following self inductances:

Any one coil each, no ferrite rod:  L1=L2= 2.3uH
No ferrite rod, L1&L2 in bifilar cancel:       0.42uH
No ferrite rod, L1&L2 in series aiding:        9.1uH

Any one coil each, with ferrite rod, coils slided to any one edge of the rod: L1=L2=35uH
Any one coil each, with ferrite rod, coils slided to the center of the rod:     L1=L2=46uH
With ferrite rod, L1&L2 in bifilar cancel, coils slided to any one edge:                  0.44uH
With ferrite rod, L1&L2 in bifilar cancel, coils slided to the center of the rod:        0.44uH
With ferrite rod, L1&L2 in bifilar aiding, coils slided to any one edge:                   141uH
With ferrite rod, L1&L2 in bifilar aiding, coils slided to the center of the rod:         192uH

So the results show that it does not matter where the bifilar coil pair is placed onto the ferrite rod, its resultant and cancelled inductance (when connected in bifilar cancel mode like in the TEP) remains the same. I am a bit surprised too, that without the ferrite rod the same coils (when connected also in the bifilar cancel mode) show only slightly less self inductance than with the core, i.e. 0.42uH versus the cored 0.44uH.

This may justify the hint on severe core saturation due to the huge peak current and considering Naudin's non-linear time flow tests ( http://jnaudin.free.fr/html/tep61a5.htm )  these show that local, individual turns of wire do influence the core material under them and in the rod center this effect is the greatest (but mainly cancelled) and towards the rod edges and half way out of the coil this saturation effect reduces/diminishes, hence the local self inductances can increase, hence the back emf can also increase. 
Let's not forget that the back emf pulses we see in the scope shots they are there during the transistor switch off time when the coils L1&L2 are floating already, hence already mainly 'recovered' their self inductance. 
Let's also notice that the amplitude of the back emf pulse is very small, under 1V peak when Naudin loaded it with a 100 Ohms resistance (with a diode in series).  For me it may mean that without making further 'tricks' there is no free lunch yet...

Regards
Gyula

[pese]

Air Coils  , driven with higher frequencies as 1Mhz  will make the same knowledge
that induction will degress or add., the way of the clock (windings) direction.

look for "Variometer" that is an device , that an make this "variable induction"
with moving - to wind- air coil , that build in another -fixed-inside.

(old) RF and Radio-Man?s know that.

PS.  Have look for some pic?s ! Please:
http://www.alg.demon.co.uk/radio/136/pictures/rem_var1.jpg
http://www.historyofpa.co.uk/gfx/cw/crystal/Variometer.jpg
http://w5jgv.com/variometer/variometer.htm  f?r niedrige frequenzen (selbstbau)  low frequencies

http://www.vintageradio.info/images/vario-xtal.gif

Pese

@gezin,
think about that , to use fast switching diodes in the bridge rect. (also schottky)