toroidal choke resonance?

[neazoi]

Hello I have a sinusoidal signal generator connected in series with a toroidal choke and the choke is connected to the oscilloscope to the ground.
As I vary the frequency of the generator I notice dips and peaks of the sinusoidal waveform on the scope. I find the maximum peak which is at a specific frequency. Then I reduce the number of turns in the choke and the maximum frequency where the peal appears rises.

What does it mean about the choke? That it is resonant at the frequency where the peak occurs?

 

[albbg]

I think your choke is resonating with the input port (probe) capacitance of the oscilloscope. If you know the inductance of your choke L (from number of turns and AL of ferrite) and the capacitance of the scope C (typically 10-15 pf) you can check the maximum falling at

f=1/[2*pi*sqrt(L*C)]

 

[neazoi]

I think your choke is resonating with the input port (probe) capacitance of the oscilloscope. If you know the inductance of your choke L (from number of turns and AL of ferrite) and the capacitance of the scope C (typically 10-15 pf) you can check the maximum falling at

f=1/[2*pi*sqrt(L*C)]

I am trying to make a saturable reactor. When I wind a second winding on the core and feed dc on it, about 1mhz above the frequency that the peak occurred, I notice that the peak disappear. Then when I remove DC again the peak is high.
If I connect a series diode The same behavior occurs, but the peak is at a lower level
I should expect the diode to make the saturable reactor have more gain (magnetic amplifier) but this is not the case.
I am trying to find out if the peak that occurs with the core saturated, is because the fact of the saturation or some strange resonance?

 

[albbg]

In my opinion there are two possible effects:

1. the added DC can saturate the core (depending from the concatenate DC current that generates the DC flux)
2. connecting a generator to a secondary of the transformer you short circuit it changing the primary impedance (depending from turn ratio)

In both case L is varied and the Q factor as well. Decreasing the Q factor the peak amplitude decreases.

I didn't understand the experiment with the diode. Where did you put it: in series with DC or in series with main choke ?

 

[neazoi]

In my opinion there are two possible effects:

1. the added DC can saturate the core (depending from the concatenate DC current that generates the DC flux)
2. connecting a generator to a secondary of the transformer you short circuit it changing the primary impedance (depending from turn ratio)

In both case L is varied and the Q factor as well. Decreasing the Q factor the peak amplitude decreases.

I didn't understand the experiment with the diode. Where did you put it: in series with DC or in series with main choke ?

I put the diode in series with the main choke.
So what do you propose for testing it? I am trying ti find out how many turns will the load winding require for a specific frequency of the reactor (amplifier).
How do you propose to connect the source of AC (PSU) to the load winding, I thought a signal generator would be sufficient and the oscilloscope acting like the load.
Nevertheless I notice strange behaviour when I alter the number of turns of the primary (DC) winding. The frequency of "resonance" of the load wiring changes a bit as well as the amplitude, so it may be the Q problem you refer to.

 

[FvM]

I think the high frequency resonances observed with small capacitances (oscilloscope input + winding capacitance) are effectively meaningless for saturable core operation.

 

[neazoi]

I think the high frequency resonances observed with small capacitances (oscilloscope input + winding capacitance) are effectively meaningless for saturable core operation.

So you say I do not have to worry too much about them?

 

[FvM]

Yes, that's the point. A change in resonance frequency indicates a variation of coil inductance, in so far it's interesting. But the coil impedance should be bettered measured with external load and source impedances similar to the intended application circuit.

 

[neazoi]

Yes, that's the point. A change in resonance frequency indicates a variation of coil inductance, in so far it's interesting. But the coil impedance should be bettered measured with external load and source impedances similar to the intended application circuit.

Currently there is not application circuit. The purpose is to build a simple audio amplifier, so it will be conencted to a bridge rectifier, and the load will be 80R, 32r or so (speaker or headphones).
How do you suggest to sonnect the "virtual load"? maybe just use a resistor for the load and connect the scope at it's two ends?

 

[FvM]

maybe just use a resistor for the load and connect the scope at it's two ends?

Sounds like a straightfoward idea.

 

[neazoi]

Sounds like a straightfoward idea.

The only thing I worry is the low ohms resistor may be presented like a short