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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.

Davide Andrea
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Przemek B
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  • There's about 30% glass fill in a transformer coil former and that separates the coils by up to a few mm. – Andy aka Oct 01 '22 at 19:54
  • 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. – user57037 Oct 01 '22 at 19:58
  • @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? – Przemek B Oct 01 '22 at 20:05
  • 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. – user57037 Oct 01 '22 at 20:15
  • So another example is the mobee mouse charging where the mouse sits on the device to charge but no lead is connected, google that... – Solar Mike Oct 01 '22 at 20:45
  • Can it be a specially made window? Getting electrical leads through glass while maintaining an airtight seal is a solved problem (you have to use a metal called kovar), because it's necessary for the construction of vacuum tubes and some types of light bulbs, so you could theoretically have a window with leads going through it in the corner. – Hearth Oct 01 '22 at 21:04
  • @Hearth this is pretty interesting. Definitely will read about it. Original problem sadly does not allow to modify or open the window itself. – Przemek B Oct 01 '22 at 21:16
  • @PrzemekB Expect it to be very expensive to do, even more so than buying a new window. It's possible, but definitely not the preferred way of getting wires through a wall. – Hearth Oct 01 '22 at 21:20
  • what if i dont need a lot of Watts on output? what if i only need it for some LED lights or something else with low power consumption, and not oven or anything like that. does it change the rules ? – Przemek B Oct 01 '22 at 21:25
  • Yeah for sure. If it is not like an extension cord substitute, but instead just two coils that you slap up on the window plus some electronics, you can totally do it. One side, the indoor side plugs into the wall or could be USB-C powered, and the exterior part delivers 5 or 10 watts at low voltage DC, I think this could totally work, just like a wireless charger. – user57037 Oct 01 '22 at 21:49
  • It should be possible to use microwave power transmission, although it would likely be dangerous as a DIY project. There are many articles on the subject, some of which date back to the 1970s (https://space.nss.org/wp-content/uploads/NASA-CR134886-Microwave-Power-Transmission-Vol2.pdf) and Tesla's proposals over 100 years ago. – PStechPaul Oct 01 '22 at 23:16
  • I found an interesting paper that describes the construction and testing of a small microwave power transmission system that is capable of transmitting enough power to light an LED at a distance of 40 cm. The input power was on the order of 20 dBm (100 mW?) and received power was about 25 mW at about 5 cm distance. The power density was limited to 1 mW/cm^2 per FCC regulations. https://www.csun.edu/~jaf35230/Microwave_Wireless_Power_Transmission_System.pdf – PStechPaul Oct 02 '22 at 00:10
  • Search: Inductive Power Transfer. This has been done commercially for decades. Frequencies in the 100 kHz+ range help reduce size. Resonant coils are esscential for decent separation. || I saw demionstrations of operation at 400 Hz in 1972 !. These were large and clunky due to the low frequency 0 limited by the switching devices of the day. – Russell McMahon Dec 20 '22 at 11:26

4 Answers4

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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.

Transistor
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  • But one can wirelessly charge a phone, which requires lots of Watts. Could you go more into quantitative description? – Przemek B Oct 01 '22 at 19:54
  • Not necessarily high losses. Just poor power coupling, right? – user57037 Oct 01 '22 at 19:59
  • @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](https://en.wikipedia.org/wiki/Inductive_charging). Google is your best friend ;) In case you want to _experiment_ yourself, you can follow [this](https://www.instructables.com/DIY-Wireless-Charger-1/) tutorial. – Daniel Melendrez Oct 01 '22 at 20:40
  • 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. – Rich S Oct 01 '22 at 21:00
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    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. – user1850479 Oct 01 '22 at 21:27
  • @user1850479 I didn't ever think about the efficiency gains from using matched lighting source. That is interesting. It also might work to some extent when the sun is out, even without the LEDs being energized, which is interesting, too. – user57037 Oct 01 '22 at 21:51
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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.

user57037
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  • Aren't all of this small enough to fit into my phone and my little wireless charger? – Przemek B Oct 01 '22 at 20:35
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    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. – user57037 Oct 01 '22 at 20:38
  • 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. – user57037 Oct 01 '22 at 20:39
  • good point with not needing AC in case of charger – Przemek B Oct 01 '22 at 20:59
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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...

Tim Williams
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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.

Russell McMahon
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Maurice
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  • There are also demonstrations of perpetuum mobiles on YouTube. Please provide an explanation. – ocrdu Dec 20 '22 at 09:27
  • 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](https://www.swrtec.com/technology) so most likely that's what the product is using. – Maurice Dec 20 '22 at 10:32
  • 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. – datenheim Feb 18 '23 at 15:25