How do snap-on induction charging systems not interfere with device functioning
Inductive equipment charging

Summary


A matching receiver is required.
The receiver uses a resonant circuit to greatly increase the voltage levels experienced. See below and Wikipedia resonance page re what resonance does.
Voltage increases of 100+ times may occur with resonance. 

Big voltages with resonance -> good power transfer.
Low voltages when non -resonant = low interference.
The field is near field magnetic.
Some induction into audio circuits may be noticed if the equipment is used while charging.  




They say:


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AND


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Simply place on any Powermat Mat to charge wirelessly, no plugging and unplugging  


Image: Phone held above charging base. 



The key point to note there is "and a receiver" - implemented in this case in the replacement case unit. 

The receiver has an inductive pickup which is tuned to resonate at the transmit frequency, which very greatly increases the available voltage levels. 



Graph: Increase of amplitude as damping decreases and frequency appro
aches resonant frequency of a damped simple harmonic oscillator. From Wikipedia resonance page



The fields generated are significant and MAY cause problems in some equipment but in most cases will not have any effect on untuned circuitry. The most likely effect would be audio pickup in audio circuits if the equipment is used while charging - but in almost all cases the equipment will not be used when on the charging "mat".  

Image: Phone with "receiver" attached.





Wireless Power Consortium


The Wireless Power Consortium is a cooperation of independent companies. The cooperation is governed by a Consortium Charter that defines rules for confidentiality, intellectual property and decision making,


From here on their site

Resonant Coupling




by: Eberhard Waffenschmidt, Philips Research

From the beginning of inductive power transmission, resonant circuits are used to enhance the inductive power transmission. Already Nicola Tesla used resonances in his first experiments about inductive power transmission more than hundred years ago. Especially for systems with a low coupling factor, a resonant receiver can improve the power transfer. Resonant power transmission is a special, but widely used method of inductive power transmission and is limited by the same constraints of magnetic fields emissions and efficiency.  

To understand the effect, it can be compared to mechanical resonances. Consider a string tuned to a certain tone as mechanical resonator. Even a far away and low level sound generator can excite the string to vibration, if the tone pitch is matched.  

Here, the resonator in the receiver consists of the receiver inductance and a capacitor. Also the transmitter can have a resonator. The general arrangement is illustrated in Figure 6a. The transmitter and receiver coils LTx and LRx can be considered as weakly coupled transformer. For this, an equivalent circuit diagram consisting of magnetizing and stray inductance can be derived, as shown in 6b. In this diagram, also the resistances of the windings are shown. The diagram shows clearly, that the resonant capacitors cancel out the stray inductance in the receiver and the magnetizing inductance in the transmitter. Now, the only remaining limit for the power transmission is the winding resistances of the coils, which impedance is one or two orders of magnitude lower than that of the inductances. Therefore, for a given generator source, much more power can be received.




Related - interest only:

I hold one US patent (now expired). It is for inductive power transfer between a long loop and "stations" which both receive power from the loop and which bidirectionally signal over the loop. Application was a warehouse picking system. Available power per station was a Watt or few. From memory the signalling rate was in the 100 kbps range. Quite a challenge overall. The system worked but the client did not proceed commercially with the overall system. 



The reason why:

Some apparently competent readers have queried WHY the description above describes what happens. Taken from the above text, the following summarises the key points. Sections in bold are comments/explanations on the text from above. :


The receiver uses resonant circuit to greatly increrase the voltage levels experienced.

It's generally found that big voltages have more effect than little voltages.
The receiver uses a resonant ciruit to greatly increrase the voltage levels experienced. 

Resonance causes voltages at the selected frequency to be much larger. This is a fundamental principle of tuned circuits. In fact, this is essentially what "tuned circuit" and "resonance" MEAN.
Some induction into audio circuits may be noticed if the equipment is used while charging.

Even the little voltages **MAY cause some effects - ie the induction system MAY in fact have SOME effect.**
The receiver has an inductive pickup which is tuned to resonate at the transmit frequency, which very greatly increases the available voltage levels.

What it said.
The fields generated are significant and MAY cause problems in some equipment but in most cases will not have any effect on untuned circuitry. 

And again
The most likely effect would be audio pickup in audio circuits if the equipment is used while charging - but in almost all cases the equipment will not be used when on the charging "mat".  

If you put your head against the phone and listen while it's on the charging mat you MAY hear some effects from the induced voltages - but that may just be the effect on your in-ear bluetooth earpiece :-).
The diagram shows clearly, that the resonant capacitors... which impedance is one or two orders of magnitude lower ... Therefore, for a given generator source, much more power can be received.

As shown in the diagram - the capacitors tune out the parts of the ircuit which are high impedance so you can get big voltages at the selected frequency when you use resonance. Without resonance you may get 100 times less current flow. 100 is a lot.