# RL circuit practice

Discussion in 'General Electronics Chat' started by Elerion, Dec 19, 2015.

1. ### Tony StewartWell-Known MemberMost Helpful Member

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Again your assumptions are false, and thus your conclusion is false of Caps in parallel .
As we both agree uses the series C formula rather the false formula as you imply with // Caps

2. ### RatchitWell-Known Member

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You are being seduced by the force (formula). One can write anything in a formula, but it does not necessarily mean that it is physically implementable. It is just a curious coincidence that the term for the series capacitor shows up. If there was truly a pair of series caps, then they could be combined and placed somewhere in the network. Where would they be placed? I do not say the caps are in direct parallel either.

Ratch

3. ### Tony StewartWell-Known MemberMost Helpful Member

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Another false claim you cannot backup which is basic 1st year stuff.

The order of a filter can shift 90° per reactive element unless equivalent.
A passive network can shift thousands of degrees in high order filters not just 90°

Regardless of your lack of awareness, I had already showed simulation of the filter's 180° phase shift.

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5. ### RatchitWell-Known Member

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OK, I will concede that point to you. I should have run a Bode plot on it. But tell me, if a positive voltage is applied to the inverter input, the output will be a negative voltage. Then the network will change it to positive again for the input. So why doesn't the inverter lock up to its positive input value?

Ratch

6. ### The ElectricianActive Member

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This inductor is not suitable for an AM radio. If you want to build an AM radio, consult this board: http://theradioboard.com/rb/

You'll find everything you could possibly want to know about inductors for AM radios.

7. ### Tony StewartWell-Known MemberMost Helpful Member

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Good question.

At DC zero filter phase shift, you have just negative feedback to self bias at the input threshold , typically Vcc/2.

At AC resonance, phase shift is 180 deg. + inverted out gives positive feedback and as long as gain >=1 it will oscillate.... Gain >1 grows to saturation at a rate depending on gain

Eratta
If you have hysteresis with an inverter then with R feedback and C input, it oscillates as per your description..or . Astable Multivibrator., the term given to RC types with delay induced positive AC feedback with logic level dependancy and sensitivity to R feedback current

This Choke is suitable for any linear RF, LF or LPF application below the self parallel resonant frequency (PRF) , typically caused by interwinding capacitance and L value. In this case size,~10pf. ... So F decreases with rising L. For VHF/UHF, they are usually tiny air coils precision wound, such as in tuners or SMPS when small values are needed, or strip line... Or...

The RLC Pi filter debated in this thread, is high Q BPF but with a large series R also has a LPF superimposed, which is very high Q Shape . Remember this when doing XTALS. The advantage being to make the harmonics of ALL crystals attenutated to avoid spurious resonances at harmonics, unless desired, then you want LOW ESR series resonant Crystals. So you will always see a Parallel resonant XO or xtal oscillator with ~1k R in series, or from driver ESR internals.

~fini~

Last edited: Jan 2, 2016
8. ### The ElectricianActive Member

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I don't have a 1 mH inductor of this style, but I do have a 1/2 mH one. Here's a sweep of the inductance and AC resistance of mine from 100 Hz to 5 MHz:

The SRF is plainly visible at about 4.5 MHz. The OP's inductor SRF will be somewhat lower. The DC resistance of my inductor is 7 ohms, and the AC resistance begins to rise a little below 10 kHz. The OP's inductor probably begins to experience a rise in AC resistance at a somewhat lower frequency, but with a DC resistance of 13 ohms, its AC resistance is probably still well below 100 ohms at 16 kHz.

His original method of using a 100 ohm resistance in series with the inductor would work ok with the technique of measuring the voltage across the 100 ohm resistor and across the inductor (with its series AC resistance) differentially, adjusting the frequency until they're equal. Since the reactance of the inductor and its AC resistance add vectorially to give the impedance, as long as the AC resistance is below 20 ohms, the error is only about 2%. With a much higher frequency, the unknown rise in AC resistance will begin to become a significant part of the impedance, introducing substantial error in the inductance measurement.

Worse for his stated purpose, making a tank for an AM radio, the AC resistance will be quite large in the neighborhood of 1 MHz. As I told him in an earlier post, this inductor is not suitable for that purpose.

9. ### Tony StewartWell-Known MemberMost Helpful Member

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AC core loss is significant in ferrite beads where DCR is near zero, which is a useful feature.
But AC core loss in inductors is not significant compared to ωL.
We know this since Q is high in this class of inductors.

Thus I stand by my previous comments. Try plotting Z vs f. Not L & R

10. ### The ElectricianActive Member

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I didn't say anything about whether or not you should stand by your previous comments. Plots of Z vs f are in the manufacturers data you linked to in an earlier post: http://en.tdk.eu/inf/30/db/ind_2008/b78108_148s.pdf

My plot is just showing the parameters in a form more directly relevant to the OP's problem.

11. ### Tony StewartWell-Known MemberMost Helpful Member

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ok , thanks for clarifying "AC resistance", which we know contributes errors of this unfavorable tuning method, when small series R is used.

It would not make the best AM tuning filter, but could be made to work easily for proof of concept .

Using a low impedance a // tuning cap from 10~100pF into a high impedance amp. This would resonate with an reactance of 5k~10k and thus a gain of 100 might possibly be achieved from a low impedance driver.

Last edited: Jan 9, 2016