Some tests on mono and bifilar coils

[conradelektro]

As far as filters go, a series LC circuit is a what's called a band-pass filter.   It lets a certain range of frequencies pass power from the input to the output:

(signal source) -> (series LC) -> load resistor -> Gnd.

A parallel LC circuit in the same setup is called a notch filter because it prevents a certain range of frequencies from passing power from the input to the output, i.e."the notch."

(signal source) -> (parallel LC) -> load resistor -> Gnd.

If you know the impedances of series LC and parallel LC circuits at resonance it should all make sense.

I do not understand how to test "speed up under load" with a series LC circuit. Please see the attached drawing.

It is assumed that the coil is excited by a spinning magnet or by help of a "few turn exciter coil".


- The parallel LC circuit in the drawing can swing without a load, just the coil and the parallel cap.

- The series LC circuit in the drawing needs a load to swing, or it would be "open"? (And if the load is just a wire one is back at the parallel LC circuit? But even with a resistor as a load, one could imagine the resistor as being a big resistive component of the cap and one is again back at a parallel LC circuit?)


The only way I can imagine to test "speed up under load" with a series LC circuit is to test with different loads, e.g. a very high resistor (e.g. 10 K) and a very low resistor (e.g. 10 Ohm)?

Greetings, Conrad

[Farmhand]

I do not understand how to test "speed up under load" with a series LC circuit. Please see the attached drawing.

It is assumed that the coil is excited by a spinning magnet or by help of a "few turn exciter coil".


- The parallel LC circuit in the drawing can swing without a load, just the coil and the parallel cap.

- The series LC circuit in the drawing needs a load to swing, or it would be "open"? (And if the load is just a wire one is back at the parallel LC circuit? But even with a resistor as a load, one could imagine the resistor as being a big resistive component of the cap and one is again back at a parallel LC circuit?)


The only way I can imagine to test "speed up under load" with a series LC circuit is to test with different loads, e.g. a very high resistor (e.g. 10 K) and a very low resistor (e.g. 10 Ohm)?

Greetings, Conrad

You're right Conrad, but in my opinion if there is no (parallel) load then the parallel capacitor is also a series capacitor as it is the only thing in series with the coil leads just like if there was no load resistor in a series output circuit. Going by one train of thought you could use a 0.1 Ohm resistor in series with the coil and the capacitor and it is a series circuit or you could put a 0.1 Ohm resistor in parallel with the capacitor as a load and you have a parallel circuit. Blurred lines or what, Just adding a CSR in series makes a parallel tank into a series loaded tank. 

To see some difference just change the CSR to a higher value and you have more of a series load.

My question would be with series resonance what is the current/voltage phase relationship as compared to the voltage/current phase relationship in a parallel resonant circuit.

Also once the output is heavily load there is no real resonance if no energy is sloshing back and forth. Technical point or Perspective point is all.

Your experiments are very interesting Conrad, congrats on a good job.

Cheers

P.S. On supply side resonance of a supply to a  transformer I think it's different as series resonance is done by a capacitor in series between the supply and the transformer primary and parallel resonance is done by a capacitor in parallel to both, while the output or the secondary of the transformer can have no resonance.

..

[MileHigh]

Conrad:

I was not clear enough, for these tests there is no motor, just your signal generator and related components:

(signal generator) -> (series LC) -> load resistor -> Gnd.
(signal generator) -> (parallel LC) -> load resistor -> Gnd.

Supposing your LC resonance is 10 KHz.   So if you connect up the two circuits above and you sweep the sine wave output frequency on the signal generator, what will you see on the load resistor at 10 KHz?   What will you see above and below 10 Khz.  Observe the band-pass filter characteristic and then observe the notch filter characteristic by putting your scope probe across the load resistor.  Then switch from the monofilar coil to the bifilar coil and repeat the tests.  Do you see any difference?

Imagine channel 1 of your scope monitoring the signal generator output and channel 2 monitoring the voltage across the load resistor.  That will allow you to monitor the amplitude of the load voltage and also allow you to observe if there are any phase differences between the signal generator and the load resistor voltage.

Let's imagine that you are powering your scope from an isolation transformer so that the scope's ground is independent of the signal generator ground.  We know that at for a series LC resonator at resonance the voltage across the capacitor and the voltage across the inductor cancel each other out.  If you put your scope probe grounds at the junction between the capacitor and the coil, then with one channel you can monitor the voltage across the capacitor and with the other channel you can monitor the voltage across the coil.  You may have to invert one of the channel polarities to make the display "make sense."

At series LC resonance, do you observe how the voltages cancel each other out?  What happens when you are just below the resonance frequency?   What happens when you are just above the resonance frequency?

I am just throwing some ideas at you for fun but by all means, do your own thing!

MileHigh

[Magluvin]

You're right Conrad, but in my opinion if there is no (parallel) load then the parallel capacitor is also a series capacitor as it is the only thing in series with the coil leads just like if there was no load resistor in a series output circuit.

I agree. They are all series, with what we are looking at here The only difference is perspective of where you apply the load, or even just a measuring device.

But once you introduce more than the load in the loop, then the difference is clear as we have to look at the circuit as a whole. Like if we have a parallel LC in the loop, and say a switch in the loop were to open, a series LC cant ring together, but the parallel LC would. So they each have their place in different circuits and are not always in a simple loop.

But here, being that the bifi can ring on its own, even open ended leads, then the comparison would be a simple parallel LC and no need to look at series, when it comes to the goal of the thread really. Not saying a series LC using a coil and cap shouldnt be part of the testing, as that data may as well be included. All in a nice little box. But Im not sure if it will help in the distinction between a normal coil and a bifi.



Mags

[conradelektro]

Conrad:

I was not clear enough, for these tests there is no motor, just your signal generator and related components:

(signal generator) -> (series LC) -> load resistor -> Gnd.
(signal generator) -> (parallel LC) -> load resistor -> Gnd.

Supposing your LC resonance is 10 KHz.   So if you connect up the two circuits above and you sweep the sine wave output frequency on the signal generator, what will you see on the load resistor at 10 KHz?   What will you see above and below 10 Khz.  Observe the band-pass filter characteristic and then observe the notch filter characteristic by putting your scope probe across the load resistor.  Then switch from the monofilar coil to the bifilar coil and repeat the tests.  Do you see any difference?

Imagine channel 1 of your scope monitoring the signal generator output and channel 2 monitoring the voltage across the load resistor.  That will allow you to monitor the amplitude of the load voltage and also allow you to observe if there are any phase differences between the signal generator and the load resistor voltage.

Let's imagine that you are powering your scope from an isolation transformer so that the scope's ground is independent of the signal generator ground.  We know that at for a series LC resonator at resonance the voltage across the capacitor and the voltage across the inductor cancel each other out.  If you put your scope probe grounds at the junction between the capacitor and the coil, then with one channel you can monitor the voltage across the capacitor and with the other channel you can monitor the voltage across the coil.  You may have to invert one of the channel polarities to make the display "make sense."

At series LC resonance, do you observe how the voltages cancel each other out?  What happens when you are just below the resonance frequency?   What happens when you are just above the resonance frequency?

I am just throwing some ideas at you for fun but by all means, do your own thing!

MileHigh

@MileHigh: I am grateful for ideas about how to look for differences. Thank you for giving it so much thought.

Attached please find two circuit diagrams outlining the tests you wrote about. I guess it depicts what you suggest?

Would it be useful to separate the signal generator output and the test circuit with a 1:1 transformer or coil?

Instead of powering my scope from an isolation transformer, which I do not have. Several times since I play with electronics the isolation transformer came up as a necessary item for useful measurements but it would cost around 300.-- EUR which I always put off because of other acquisitions. Sometimes I also wanted a variable transformer but it usually needs an isolation transformer in addition (to be safe). A simple variable transformer would cost around 150.--.

Talking about toys, I am having my sight on a 3D printer. My impression is that a good one still costs up to 3000.-- EUR. So I am waiting till a useful one can be bought for less than 2000.--. The ones available right now for around 1000.-- seem to still have many flaws. I am always reading the reviews.

Greetings, Conrad