Role of Capacitors in Automotive Ignition, Power Supply & Radio Tuning

[negitron]

No. It is not.

Ever.

 

[fleem]

No. It is not.

Ever.

No, an (ideal) current source has infinite impedance, a voltage source has zero impedance. Practically speaking the impedance of a 12V car battery, at a couple hundred kilohertz, is in the milliohms.

 

[negitron]

That's internal impedance. It's not considered in circuit analysis when you're trying work out the impedances of various current loops.

 

[Bob S]

You seem to have neglected the impedance of the battery. Hint: its zero.

Please look at the impedance equation for a series resonant circuit in
http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/serres.html
It does not include the impedance of the voltage source on the left side.

 

[negitron]

It does not include the impedance of the voltage source on the left side.

Or anywhere else, for that matter.

 

[fleem]

That's internal impedance. It's not considered in circuit analysis when you're trying work out the impedances of various current loops.

And what is the external impedance of a voltage source, then? Infinity?

 

[negitron]

Well, why don't you draw for me what you think a series AC circuit looks like?

 

[fleem]

Well, why don't you draw for me what you think a series AC circuit looks like?

I'll wait until you give me some proof that the impedance of a voltage source (i.e. a battery) is anything but zero (practically very low). By the way, if you google "impedance" with "voltage source", you might find the answer. HINT: its very low--zero for an ideal voltage source. yet you claim it isn't. Why?

 

[negitron]

When I said it was not zero, I meant it was not zero in terms of the way the circuit is connecting when analyzing a circuit loop and determining if it is a series- or parallel-resonant circuit. Your claim that the top of the inductor is connected to the bottom of the capacitor through this so-called zero impedance is laughable.

Batteries and other voltage sources do have an internal impedance (which is ideally zero, but never really is; and sometimes it's not even low), and this fact is important when attempting to transfer maximum power from a voltage source to a load, but it is not considered as a part of such an analysis as this.

 

[fleem]

Well, why don't you draw for me what you think a series AC circuit looks like?

Why should we argue about the definition of "series" and "parallel" before we even agree on whether the impedance of that battery in your circuit is zero or infinity? Or are you trying to avoid the fact that a simple google search shows the impedance of a battery as nothing but very low (ideally zero), rather than high, since that is the question that must be answered in order to even decide whether we want to call that circuit series or parallel?

 

[Bob S]

Why should we argue about the definition of "series" and "parallel" before we even agree on whether the impedance of that battery in your circuit is zero or infinity? Or are you trying to avoid the fact that a simple google search shows the impedance of a battery as nothing but very low (ideally zero), rather than high, since that is the question that must be answered in order to even decide whether we want to call that circuit series or parallel?

Hi Fleem-
Why don't you post a sketch of what the ignition circuit looks like, and run it with SPICE like I did in post #3. or I will run it in SPICE myself. Include the breaker points in the circuit.
Bob S

 

[negitron]

since that is the question that must be answered in order to even decide whether we want to call that circuit series or parallel?

No, it doesn't. There's nothing to argue about concerning the definitions of "series" and "parallel" as they are rigorously defined in the link that both Bob and I provided for you.

 

[fleem]

When I said it was not zero, I meant it was not zero in terms of the way the circuit is connecting when analyzing a circuit loop and determining if it is a series- or parallel-resonant circuit. Your claim that the top of the inductor is connected to the bottom of the capacitor through this so-called zero impedance is laughable.

Batteries and other voltage sources do have an internal impedance (which is ideally zero, but never really is; and sometimes it's not even low), and this fact is important when attempting to transfer maximum power from a voltage source to a load, but it is not considered as a part of such an analysis as this.

Guys, the fact that you are in agreement that the battery is a high impedance in an analysis of that circuit, is giving you both a confidence that is going to result in some notable embarrassment when you realize that it really is a short at any non-zero frequency. Specifically, the battery "connects" any voltage changes on one electrode to the other, just like a wire but for alternating voltages. I suggest you both offline think some more on exactly how you would calculate the waveform on those components, and ask yourself where the AC loops are in that circuit (when the switch is open). I've got to move on now, but my silence only indicates I've given up, not that I concede that batteries are like opens at non-zero frequencies.