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Wireless Power and Heating

We covered wireless power before, and one of the points of differentiation was that of inadvertent heating of nearby items. With systems using the lower 200-kHz frequency range, nearby largish metal items like coins and keys can heat up. The systems themselves are designed to detect this and shut the charger down, which addresses the safety issue. It’s just a bother if you think your phone is being charged when in fact it isn’t due to something else around there.

But then it was pointed out that heating can theoretically be an issue for any frequency; it’s just a matter of the thickness of the material and the frequency used. Higher frequencies would create heating in thinner objects; lower frequencies would heat thicker objects. Which means that the 6.78-MHz range of charging can also cause heating for some thinner range of metallic items.

So in the MHz range, keys and coins are fine; is there anything else that might accidentally come in range? Turns out there is one thing: CDs, which have a very thin foil in them (standard kitchen-grade aluminum foil is too thick). And, confirming with WiTricity, yes, they can actually heat up. (And system designers can detect the issue and shut down, just as the lower-frequency systems do. Which means the phone-didn’t-charge bother could happen there too). It could probably be argued that it’s less likely for a CD to be in the way (and, one might ask, who still uses CDs, anyway?) But, they were eager to point out, in cases where there was heating, they had never seen an instance of a CD actually losing any data.

That’s all well and good, at least until I started extrapolating the Cota technology (which we covered today), which uses RF at 2.4 GHz. If the MHz range affects CDs, it’s pretty much impossible to imagine something so thin that the GHz system would affect it. Just following that line of thought, I then realized that integrated circuits have extremely thin films of metals in them. Could this be a problem?

I asked this of Ossia, and they reminded me that the signal power being delivered to charge a phone is no higher than what the phone itself transmits. So if the phone isn’t heating its own metal, then the charging shouldn’t either.

Bear in mind that neither of us did the calculation to see if those IC thin films fall into a range that would even theoretically be affected; the power argument makes it an academic calculation. It also occurs to me on further hindsight that this isn’t resonant charging; it is, as PowerByProxi pointed out, more like harvesting RF for energy. So that might change the entire scenario.

So, in summary:

  • 200-kHZ resonant systems can heat objects like keys and coins; systems can detect and shut down for safety
  • 6.78-MHz resonant systems can heat CDs; systems can detect and shut down for safety
  • 2.4 GHz RF systems should have no heating issues.

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