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I know that there is a way to induct electricity with an electromagnetic field. I also know that is the part of physics used by wireless phone chargers. I have a basic understanding of physics and some electronics.

I wonder why there is no product on the market (I cannot find one) that would be an extension cord with a wireless section with two suckers for use through a window (glass.) Are there physics-based limitations, like

  • Windows being too thick
  • Windows are some kind of isolators
  • Current/voltage out of viable intervals
  • Too much inefficiency/losses

If so, which one is a choke-point?

brief schema of item I am trying to inspect

Please don't hesitate to give me some math, but please, provide explanations.

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  • \$\begingroup\$ There's about 30% glass fill in a transformer coil former and that separates the coils by up to a few mm. \$\endgroup\$
    – Andy aka
    Oct 1, 2022 at 19:54
  • \$\begingroup\$ What you are proposing is a transformer with an air gap of several mm (or much more for double-paned windows). I think the biggest problem is that the transformer would need to be very large and heavy. But it could be done some other way, similar to wireless charging, for example, with planar coils on both sides of the window. It wouldn't just be like an extension cord. \$\endgroup\$
    – user57037
    Oct 1, 2022 at 19:58
  • \$\begingroup\$ @mkeith Are we able to calculate some of these considerations? Is there a formula which binds Vin Iin with Vout Iout, some properties of planar coils and window thickness? \$\endgroup\$
    – Przemek B
    Oct 1, 2022 at 20:05
  • \$\begingroup\$ It is not really my field. I don't have any formulas handy. But normal line frequency power transformers rely on a high permeability core material. There is no way to achieve a high permeability path through a window. Wireless chargers make up for this by using much higher frequencies. One way to look at power transfer is to look at the energy stored on the magnetic field of the transformer * the frequency. If the energy storage is low, the frequency must be high. The glass gap greatly reduces the potential for storing energy in the core. \$\endgroup\$
    – user57037
    Oct 1, 2022 at 20:15
  • \$\begingroup\$ So another example is the mobee mouse charging where the mouse sits on the device to charge but no lead is connected, google that... \$\endgroup\$
    – Solar Mike
    Oct 1, 2022 at 20:45

4 Answers 4

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The permeability of air and glass is about 1/1000th that of iron. Introducing the airgap would result in high losses in the system.

The technique is used for low power applications, commonly in medical devices. My niece has a cochlear implant hearing aid with a receiver coil and magnet under the skin. The external microphone and amplifier couple the audio energy through the skin to activate the implant. The magnet holds it in place.

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  • \$\begingroup\$ But one can wirelessly charge a phone, which requires lots of Watts. Could you go more into quantitative description? \$\endgroup\$
    – Przemek B
    Oct 1, 2022 at 19:54
  • \$\begingroup\$ Not necessarily high losses. Just poor power coupling, right? \$\endgroup\$
    – user57037
    Oct 1, 2022 at 19:59
  • \$\begingroup\$ @PrzemekB Certainly, the power required to charge a battery is significantly higher than the one required to power on a cochlear implant. You can easily find a whole bunch of online resources on inductive charging. Google is your best friend ;) In case you want to experiment yourself, you can follow this tutorial. \$\endgroup\$ Oct 1, 2022 at 20:40
  • \$\begingroup\$ You might also look at electric tooth brushes (Philips Sonicare), etc. They use a frequency of 80kHz to pass few layers of plastic to recharge the 'AA-size' Li-ion cell. Of course the power delivery is slow, which is OK, since the user only partially drains the cell, and only a few times a day. \$\endgroup\$
    – Rich S
    Oct 1, 2022 at 21:00
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    \$\begingroup\$ You can get 80% efficient LEDs and photovoltaics well over 50% efficient for monochromatic light, so if you want to transmit through thick optical glass I would consider light. Can probably do 50% efficiency at thickness of inches, maybe more, and without the weight of a huge transformer. \$\endgroup\$ Oct 1, 2022 at 21:27
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Wireless chargers use high frequency excitation of air-core coils. Essentially, the wireless chargers are high frequency transformers with weak magnetic coupling. This does not necessarily reduce power efficiency too severely, but it does not work at line frequencies such as 50 Hz or 60 Hz. The coils would need to be enormous.

So for a practical (somewhat) system, on one side of the window you would need to convert line AC to DC then generate high frequency AC to excite a coil on one side of the glass. On the other side, you would have a coil which would be excited by the magnetic field from the first coil. The induced voltage on the second coil would be rectified to DC then converted to line frequency AC on the other side of the glass. I don't think it would be possible to make something simple or small enough to just stick to the glass with suction cups.

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  • \$\begingroup\$ Aren't all of this small enough to fit into my phone and my little wireless charger? \$\endgroup\$
    – Przemek B
    Oct 1, 2022 at 20:35
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    \$\begingroup\$ Well, if you want to charge your phone through a window, that is one thing. If you want to run a toaster oven, then no, it won't fit in your little wireless charger. Does your wireless charger work through a window? Please try it and let us know. It might very well work. \$\endgroup\$
    – user57037
    Oct 1, 2022 at 20:38
  • \$\begingroup\$ The heaviest part of the circuit would be the part that converts DC to 50 Hz or 60 Hz AC. Your wireless charger doesn't have to do that. Inverters are heavy. \$\endgroup\$
    – user57037
    Oct 1, 2022 at 20:39
  • \$\begingroup\$ good point with not needing AC in case of charger \$\endgroup\$
    – Przemek B
    Oct 1, 2022 at 20:59
  • \$\begingroup\$ Depending on what you're doing, you may not need to go back to line-frequency AC. If you're just lighting up LED Christmas lights or something, they probably won't care if it is 60 Hz or 30 kHz, because they're rectifying it right back into DC anyway. Heck, that's also true for incandescent lights, presumably. \$\endgroup\$
    – dgatwood
    Nov 11, 2023 at 21:43
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A couple of problems:

  1. Just open the window -- the obvious answer is worth the slight inconvenience (of having to lay the cord, or open and leave the window cracked, or etc.), relative to the much greater cost of a device specifically to do this.
  2. Even a fairly naive solution is possible: say, a relatively large iron-core transformer, which will mostly hold itself together by attractive force, and while it will draw a fair amount of reactive current, it'll still do alright as a transformer. But:
  3. A naive solution is bulky, very heavy, likely crushes double-glazed windows (or doesn't work at all through them, due to the increased thickness), and only works when both sides are lined up correctly, and if they're misaligned, probably blows a fuse instead. Or if operated without the two halves placed together. An example would be something like, two microwave oven transformer cores facing together, which would be good for a few hundred watts -- much less than the nameplate rating of the microwave for several reasons, most importantly the effect of the separation distance.
  4. A technological solution exists, and indeed works down to fairly low power levels (some watts, as in phone chargers), but doesn't scale well, and especially isn't going to be affordable. A rough rule of thumb for power electronics is to figure $1/W of capacity; an extension cord might be rated for 1500 W so expect an isolator like this to cost over a thousand bucks. That's only a ballpark figure of course, and could range from tens of thousands (for a one-off, significant engineering effort required), to a few hundred (if produced in the millions). Is anyone really going to buy such a thing, when an extension cord will do almost all the time?

To be clear, there are niche applications for such technology -- even back in the 2000s, wireless (or at least, contactless) EV charging was a thing. Granted, EVs themselves were largely speculative in those days, but Magne Charge was in use. There are numerous modern proposals of course, ranging from in-road coils (laughable, at least for quite a while) to chargers that basically connect physically (like the Magne) but with looser tolerances so a vehicle can simply be parked up against it. (One in particular I've seen, uses resonant power transfer mechanisms (which allows more distance and misalignment), targeting industrial application -- recharging fork lifts for example.)

So, in summary -- entirely possible, unfortunately just not economically feasible. Or, if you can convince enough people they need it -- but that's an entirely different matter...

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  • \$\begingroup\$ can you provide some rudimentary math for output current and voltage given input current, voltage, some coil properties and window thickness? \$\endgroup\$
    – Przemek B
    Oct 1, 2022 at 21:12
  • \$\begingroup\$ @PrzemekB For what kind? I'm not going to detail each design approach. \$\endgroup\$ Oct 1, 2022 at 23:47
  • \$\begingroup\$ the one you think is most appropriate for this problem or the most DIY one \$\endgroup\$
    – Przemek B
    Oct 2, 2022 at 8:45
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enter image description here This demonstration on youtube shows that it is possible.

There are numerous internet pages with examples of wireless power transfer. These are NOT at mains frequencies. While mains frequency transfer is possible the use of frequencies of 100 kHz+ greatly reduce size.

This google image search provides a web page linked to each image. Many are directly relevant.

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    \$\begingroup\$ There are also demonstrations of perpetuum mobiles on YouTube. Please provide an explanation. \$\endgroup\$
    – ocrdu
    Dec 20, 2022 at 9:27
  • \$\begingroup\$ They don't provide much of an explanation on their website. They do refer to Magnetic-resonant wireless power transfer (MRWPT) on their technology page so most likely that's what the product is using. \$\endgroup\$
    – Maurice
    Dec 20, 2022 at 10:32
  • \$\begingroup\$ Rofl, this video does only pretend that it works. Nothing is demonstrated really, how could that convince anybody? The receiving box could have batteries, the "window" and transmitters never shown closely. Not better as all those "free energy" harvesters. Anyway. \$\endgroup\$
    – datenheim
    Feb 18, 2023 at 15:25

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