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Author Topic: Ferrite Core Resonance investigation.  (Read 20188 times)

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Ok,  i used both the 2x aaa batteries in series (3V) again and lateron a 12V Battery with a 3.3V regulator as power source.

A 0.1 non-inductive resistor on the emitter lead shows the current through the transistor (mje13005).
The 220pF variable (base to ground) and the 10nF (input to base) capacitors are removed.

The screenshot shows again the peak in the middle of the base opening window with the 3.3V regulator as power source:

Blue trace   = collector signal
yellow trace = base signal
purple trace = voltage across the 0.1 Ohm resistor in the emitter

Video here:   https://www.youtube.com/watch?v=EnfXLH-w3ko&feature=youtu.be
 
Regards Itsu
   
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Hi Itsu,

I am not a transistor expert, I learnt my trade before they existed so I can talk knowledgeably about vacuum tubes  :).

Looking at your waveforms I can see the rising current charging the inductor up to about 400mA and that is the point where it all goes wrong.  IMO it has got to be a characteristic of the transistor, i.e. the point where the emitter current has reached a value such that the base no longer drives sufficient current, the transistor switches off momentarily then recovers to a situation where the emitter current is just below that critical point.  The inductor is fully charged to that now constant current, its voltage drop is then due to its DC resistance hence we see a further period of constant current draw while the collector voltage is near its maximum DC value.  Then at the end of the base drive pulse the transistor switches off in the normal manner, the inductor discharges into the capacitance there giving the large resonant spike.  The middle spike sees the inductor losing energy to the capacitance as its current drops from the peak of 400mA to something slightly smaller.

Smudge
   

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Thanks Smudge,   much appreciated.

I can agree with your explanation, but as i have this behaviour with severall different type of transistors and even with a MOSFET,
i wonder if this behaviour is specific to this type of circuit then.

If not, then all these type of (simple) circuits should show this behaviour, meaning that the transistor/MOSFET shuts down premature.
I don't think this is the case unless it is a known behaviour and somehow avoided by adding some components somewhere (snubber circuit).

 
Regards Itsu
   
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It's turtles all the way down
For some reason I cannot view the video it is marked as private.

I agree to a large degree with Smudge's analysis.

If you still have your current probe handy, move it to the base drive and instead of looking at base voltage, look at base current.

I'm beginning to think the SG cannot supply enough current to keep the transistor saturated and at the peak point on the waveform, base current from the SG is insufficient to keep the ramp rising, so it turns off, then goes to the maximum current that the transistor can supply (it is now a current source limiting current to the level shown that is somewhat less than the peak current).

The current gain of the MJE13005 is a min of 10, max of 60. That is rather low, and if we consider that it may be as low as 10 then your SG must supply greater than 40 mA drive current to keep the transistor saturated. I suspect the SG is current limited to less than 40 mA output and the gain of the transistor may be a little higher than 10.

The transistor will certainly turn off or limit at some value when base current times transistor gain is exceeded by collector current.

At the low frequency you are driving, the core probably has insufficient inductance and is drawing a lot of current at the peak point on the current waveform.

A much higher gain transistor or higher inductance core would remove some limitations. If you must keep the present inductance, you could also go much higher with drive frequency, at least twice but three times f should remove the problem entirely.

I don't know if I have explained it clearly, but your transistor cannot supply any arbitrary current and stay saturated, it's collector current will be limited by base drive capability (multiplied by current gain) and exceeding that, it will become a current source and limit current at the collector.
« Last Edit: 2014-08-10, 14:11:24 by ION »


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For some reason I cannot view the video it is marked as private.

If you still have your current probe handy, move it to the base drive and instead of looking at base voltage, look at base current.


Oops,    sorry about that,  changed it to hidden, so you should be able to see it now.

I will probe the base current with the current probe additional to the above other traces later on as i have some visitors now.

Thanks so far,  regards Itsu
   
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I don't think the transistor actually turns off, the current continues to flow.  It switches to the new state as ION said.  The sharp spike is merely the inductor having to respond to a change from rising current (where it produces a voltage that almost backs off the supply) to a small drop down to a new current plateau, hence it has to produce that spike in voltage followed by zero emf.

Smudge
   
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I don't think the transistor actually turns off, the current continues to flow.  It switches to the new state as ION said.  The sharp spike is merely the inductor having to respond to a change from rising current (where it produces a voltage that almost backs off the supply) to a small drop down to a new current plateau, hence it has to produce that spike in voltage followed by zero emf.

Smudge

Yes you are correct, the transistor never turns off but goes to the current limited state, and the spike seen is the change in current. Since it takes some time for the change in current in the inductor to take place, there is a slight overshoot of the holding current level.

The ramp down to the new plateau exactly matches the total width of the spike.


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Ok,  guess you are right,  i put the collector signal (blue) in AC coupling and expanded the amplitude.
There is a difference of about 180mV between after the pulse and before opening the transistor, see first screenshot.

I am uploading a new short video with the base current included (green trace), see second screenshot.

https://www.youtube.com/watch?v=8vY3byFKsjg&feature=youtu.be      

Regards Itsu
   
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sorry for offtopic but I desparately need a solution how to cheaply cut ferrite Do you have experience with any hand tool able to do that ? what I need is to take yoke core from TV and cut the edge to obtain a ring
   
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sorry for offtopic but I desparately need a solution how to cheaply cut ferrite Do you have experience with any hand tool able to do that ? what I need is to take yoke core from TV and cut the edge to obtain a ring

I have also used small 1.25" diamond blades on my Dremel tool to successfully cut  small cross sections of ferrite material. A very thin blade at high speed seems to work nicely and wastes less material. Save the powder waste for future casting with a binder or stuffing in a tube to make your own  ferrites.


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Just because it has a patent application or is patented does not always mean it really works.
   
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I have also used small 1.25" diamond blades on my Dremel tool to successfully cut  small cross sections of ferrite material. A very thin blade at high speed seems to work nicely and wastes less material. Save the powder waste for future casting with a binder or stuffing in a tube to make your own  ferrites.
How much speed did you used ? Is there any range I should stick with ?
   
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It's turtles all the way down
Regarding my post #31,re: the bar magnet excitation method,  it seems the distributed capacitance and shunt resistance of the bar magnet creates a frequency filter that has a broad pass band around a particular frequency. This accounts for the transfer of voltage from one end to the other of the magnet.

The magnet with connections as shown is a complex network of resistors and capacitors, drawing the simplified network shows the reason it works the way it does.

So no special effect her, just an erroneous first assumption on my part of what I thought might be a possible new effect.

I invite others to try this as a backup check.

Forest:

Quote
How much speed did you used ? Is there any range I should stick with ?
I don't know what the speed was, never measured it. My Dremel tool from Harbor Freight has three settings, I used the highest, but lower will also work, just takes longer.
« Last Edit: 2014-08-13, 13:26:43 by ION »


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Just because it has a patent application or is patented does not always mean it really works.
   
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