# QI charging over a large area

I am soon building a desk and I thought an interesting feature to have would be for the entire desk to essentially be a giant QI charger for my phone. Obviously, however, this has the problem that maximum charging range for QI is about 1.5cm. I wondered if, using large amounts of copper wire, I could make a large coil - as in the size of the desk - to charge a phone or other QI compatible device placed anywhere on it. I am fairly new to the actual way wireless charging works, so my question is would this work, and if not is there anything I could do aside from covering the desk in coils?

-Matthew

• Physics says this would be an expensive solution, at best. Mar 29, 2016 at 15:01
• Please do research before posting a question, this process is part of the forum rules you didn't read. When you do some research come back and do some question on why you think it would\wouldn't work. Thanks. Mar 29, 2016 at 15:36
• laptop2d: Sorry, I did attempt to do some research but forgot to mention in my question - the only thing I was able to find on the subject was the official explanation of the standard (which I didn't really understand). I DID read the rules, just forgot to mention this. Sorry Mar 29, 2016 at 17:19

Covering the desk in coils will leave dead spots because the current flow in one direction on one coil edge will be opposite to the current flow in an adjacent coil edge. Net ampere-turns is zero = no magnetic field.

It's all down to the value of inductance if you build a large coil. As the coil area gets bigger so does the inductance so this means reducing the number of turns to keep the inductance low. Keeping the inductance low allows you to push significantly more current into the coil for a given drive voltage.

Why do you need more amps for a bigger coil you might ask. Maximum magnetic field density is theoretically closest to the coil - for a big coil and a target device placed centrally in the plane of the coil, the mag field is much weaker hence you need more amps.

However, getting more amps is not that easy - you'll have to probably modify the QI drive circuit to insert a power amp and this might be a bit tricky because the transmit coil is sometimes used as a receiver for signalling from the target device and with a power amp in the way this signalling won't work. So you need to do some research here to see if you can get away without up-link signalling.

• OK, so any solution here would be at least incredibly difficult (and probably costly). Any other ways I could implement this kind of thing? Mar 29, 2016 at 15:15
• If I were intent on doing this I'd build a big coil and try it out but you'll need to do some research on whether it's feasible given what I've mentioned. If you are looking for an alternative method may I suggest the good, old fashioned, and fairly reliable charger with a lead! Mar 29, 2016 at 15:17
• Wouldn't you wire adjacent coils with opposite sense or polarity, to avoid the cancellation you talk about in your first paragraph? Mar 29, 2016 at 15:36
• @DaveTweed I believe that would make the inductance quite high or require some plan of how to series and parallel clumps of coils. Could get tricky but do-able. Mar 29, 2016 at 17:17

A large coil would not work as the coil needs to have a certain value so that it oscillates (together with the driver electronics) at a certain frequency. A large coil would be unable to oscillate at the right frequency for QI operation (around 150 kHz if my memory is correct).

Many smaller coils is possible but impractical as you would need many, I'm thinking more than a hundred ! The placement of the phone on the coil is quite critical, a few cm off and it doesn't work anymore. And you need the same amount of driver circuits. It would be an expensive solution. And power inefficient as well.

Also I would not want to sit close to all these QI chargers probing for a device every second. Because of the magnetic fields they create. It is probably completely harmless but I would not risk it anyway.

Have a look at this article and see how close the coils need to be together.

• Forgot how quick answers come here... Thanks :) Forgive me if this is a stupid question, but is it possible to link multiple coils to 1 circuit to minimize heat? Mar 29, 2016 at 15:08
• You can very easily tune a several-turn 2m x 0.8m coil to resonate at 150kHz though. I have, in fact, done so more than once. The main problem in this approach is that you need shitloads of maths and if you want it 1cm thick or less you'll need 100's of watts to retain 5W to charge in the middle. Mar 29, 2016 at 15:17
• OK, so basically it's possible but tricky and would require a thick coil. I might wait until something like MagMIMO releases and put in, like, three of those.... Mar 29, 2016 at 15:24
• but is it possible to link multiple coils to 1 circuit to minimize heat It probably is, a crude solution would be relays or MOSFETs to switch between the coils. Is that a practical solution ? I don't think so. Will it save power ? Maybe but probably not Mar 29, 2016 at 17:28