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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: A current input MC phono preamplifier project posted by RJM on December 4, 2002 at 06:37:01:
Dear friends,Regarding induction, & time-changing voltages & currents, as well as time-changing electric & magnetic fields, James Clerk Maxwell laid this issue to rest in 1873. Most EE's still don't fully comprehend the whole truth, although most have a partial understanding.
In one of his posts, Jon Risch stated that the voltage/current issue with regard to induction, is analogous to the "chicken/egg" issue. This is positively correct, & is the paradigm we must adopt. In all of his threads, "jcox" stated that induction type transducers, namely a phono cartridge in this case, operate on the principle that the voltage is the induced quantity or the "cause", if you will, & that the current is the **result** of voltage & resistance, or the "effect", so to speak. This is dead wrong. A very common, & even popular fallacy it is, but nonetheless, cannot withstand scrutiny. This is, essentially, the pre-1873, pre-Maxwell paradigm, long since discarded.
Any reference text on E&M fields, motors, generators, transformers, antennae, transmision lines, etc. is quick to point out the following irrefutable property:
Time-changing electric fields & time-changing magnetic fields are mutually inclusive. The presence of one mandates the presence of the other. They cannot exist independently. Likewise, time-changing voltage and time-changing current, are also mutually inclusive. One cannot exist without the other. Let me emphasize the "time-changing" aspect.
In the case of the phonograph cartridge, or any other induction-principle based generator, mechanical energy, or power, is being converted to electrical energy or power. When a stationary conductor is subjected to a time-varying magnetic field, or when a conductor is moving through a stationary magnetic field, or both, induction takes place. Michael Faraday studied this, as well as Joseph Henry, and later, Maxwell. Keep in mind, that Faraday got it only partially right. His original findings covered conduction current, but not displacement current. Electric fields, magnetic fields, electrostatics, electrodynamics, wave propogation, etc. did not become unified until Maxwell fully developed the concept of displacement current.
When the changing magnetic field induces energy/power into the phono cartridge coil, both current & voltage are simultaneously present. This is true even if the coil is open-circuited (terminated in infinite impedance). The notion that **voltage** is independent of current is too often cited, & is easily demolished. Let's say that the cartridge is open-circuited. The time changing magnetic field induces a time-changing voltage across the coil. Let's use "ac" from here on to denote time-changing. The coil does indeed posess some finite amount of inductance. An ac voltage is impressed across an inductance (with some resistance as well). There must, therefore, exist, an ac current in accordance with the familiar circuit relation " V = L (dI/dt) ". But how, some may ask, can this current exist without a closed path? After all, the circuit is open! This is where Maxwell & dispacement current come in. Since an ac voltage exists across the whole coil, a fraction of that ac voltage exists across any single turn & the next turn of wire. An ac electric field is present across any two adjacent turns. Electric fields are produced by charges, & ac (time-changing) electric fields are produced by charges in motion, or time varying charges. "Charges in motion" or "time-varying charges" precisely define the quantity known as "current". Adjacent turns in a coil store energy in an electric field, in the same manner that a parallel plate capacitor does. This is the quantity known as "displacement current". In a resistor with direct current, conduction current exists. In a capacitor, no conduction current exists. If an ac voltage is impressed across the plates, then displacement current exists. This (ac) displacement current results in an (ac) magnetic field around the wire, in accordance with the right hand rule. Likewise, if current is induced into the coil, the ac current through the coil produces a voltage per " V = L (dI/dt) ". In the frequency domain, an inductor's reactance X, is equal to the radian frequency multiplied by the inductance value. If an ac voltage is present, then an ac current, equal to V/X must exist. If ac current exists, an ac voltage equal to I*X must exist.
Even if the coil is wound with superconductors, inductance & capacitance must be present, even in the absence of resistance. The displacement current exists with or without the conduction current which flows when the loop is closed (coil terminated in a finite resistance). The ac electric & magnetic fields induce both ac voltage & ac current into the coil simulataneously. Arguing which one is the cause, & which one is the effect, is indeed as futile as the chicken/egg issue.
Displacement current is the only explanation for the existance of current in an antenna. Take a twin-lead transmission line & form a dipole from it. At both ends, we have wire abruptly ending in mid-air. Yet current flows. Both ac voltage & current are present, as well as ac electric & magnetic fields.
Also, "jcox" used the following transformer analogy to illustrate the (flawed) concept of voltage independence:
** "try a transformer example: what is the open circuit voltage on bifilar wound secondaries with different wire sizes (hint there're equal because they link the same dB/dt)- the emf of a coil moving in a magnet gap has an open circuit voltage proportional to the dB/dt from its motion for exactly the same reason that the transformer works, there is no current source that magically sizes itself to the wire resistance in order to give the result predicted by Farady's Law" **
Earlier in my EE career, & recently, I designed transformers. This analogy has two major fallacies. First of all, the secondary voltage & current are both zero, i.e. no induction at all, unless a power source is attached to the transformer primary. Whether the transformer secondary outputs constant current or constant voltage will be determined solely by the type of source driving the primary. I've designed & used current transformers, where the secondary current is a multiple of the primary, determined by the turns ratio. If the secondary terminating impedance changes, so does the secondary voltage, current remaining constant. In this analogy, "jcox" simply assumed a priori, that the primary is being driven by a constant voltage source. It is ** this constant-voltage source ** that accounts for the sysem's constant-voltage behaviour, not the transformer or induction principle. The second fallacy is that the ** "open circuit voltage proportional to the dB/dt from its motion for exactly the same reason that the transformer works, there is no current source that magically sizes itself" **
On the contrary, yes there is a current source (not magical, of course). The dB/dt, which induces the open circuit voltage, could not exist without the magnetizing current which must flow in the primary in order to establish the magnetic flux present in the core. The B-H curve of the core determines how much magnetizing current is needed for a given flux density. With no secondary load current (conduction current), the primary current is simply the magnetizing current, plus the core loss current due to hysteresis & eddy currents. Together they add to form the "exciting current". When secondary load current is drawn by placing a load across the secondary, that secondary current produces an mmf (magnetomotive force) which is counteracted by an equal mmf in the primary. The ampere-turns product for primary & secondary must balance, except for the exciting current. Hence, the net mmf, or primary current is still the exciting current, necessary to maintain the core flux. Should a large enough load current be drawn, the primary current times the primary wire resistance produces a voltage drop, I*R, which results in slightly less emf (electromotive force) to the core, reducing the core flux density, and as a result, the magnetizing current decreases in order to support a lower level of core flux. The magnetizing current "sizes itself" so to speak! As far as the bifilar wound secondary with different gauges goes, the smaller wire will produce less displacement current than the larger one. With a smaller conductor area, less energy per unit voltage can be stored in the electric field between adjacent turns. Remember, even with open circuited secondaries, a small displacement current consistent with " V = L dI/dt " or " V = 2*pi*f*L " must exist. Likewise with the primary winding. Once again, the ac magnetic & electrical fields in the core exist simultaneously. Even with no secondary load, both secondary current & voltage exist, as well as primary V & I.
I have, in my 25-year EE career designed numerous transducer related amplifiers & associated drivers. Among them are voltage to-current converters (V to I), I to V, transimpedance amps, transadmittance amps, voltage amps, current amps, voltage transformers, current transformers, impedance matching transformers, as well as motor drives, & switch mode power supplies. Sometimes, it is more desirable to control current instead of voltage, With LED lamps, light output is roughly proportional to forward current. With photodiodes, current is very precisely proportional to incident light, hence the photodiode is placed across the op amp's input terminals & operated in the short circuit mode (transimpedance amp). When driving motors, if you wish to control torque, the best variable to control is current, since the two are closely related. I've never worked with phono cartridges, & can't really comment on which is optimum, voltage-, or current-mode operation. This post is intended to address & clear up the current/voltage confusion regarding induction.
Every E&M expert I've worked with from universities to aerospace & defense facilities, including PhD's has confirmed positively that whenever induction takes place, no one knows, whether voltage, or current is the cause or the effect. Every one of my reference texts concur. Since 1873, all experimentation has concurred with this important fundamental property put forth by James Clerk Maxwell. You can take it to the bank.
My, my, I do run on. Best regards to all.
Claude
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Follow Ups
- Induction & voltage/current debate rebuttal - Claude Abraham 12/10/0212:26:56 12/10/02 (0)
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