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Comment by simcop2387 | Hacker News Reader

Because they're usually inductively coupled rather than RF coupled they shouldn't cause a problem I would think. They also have to detect a resonant coil near the primary before they'll output significant current because otherwise they'll heat up and burn out the primary coil. So while I wouldn't put the pace maker on top of one, I would think it wouldn't cause problems directly. The standard disclaimer here applies: I am not a doctor, I am not a lawyer, I am not an electrical engineer, This advice should not be taken by those with neck injuries, pregnant women, small children, the elderly, the living challenged, or immortal beings.

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"Inductively coupled rather than RF coupled"? What's the difference?

He means coupled using a magnetic field (like a transformer) rather than using an electric field (like a transmitter).

They're related, to make things more complicated, see Maxwell.

http://galileoandeinstein.physics.virginia.edu/more_stuff/Ma...

That's exactly it. I couldn't think of the correct way to put it earlier and kept thinking of it like an open core transformer rather than an antenna.

Wouldn't the RF coupled version use a magnetic field as well, because the electric field is osciollating?

Seems like the difference is in how spatially localized the two are, and % magnetic power vs electric.

Presumably though there is probably some way to drive (or receive) the wireless charger in a dangerous way so you can microwave things from a distance :p

In "magnetic coupling", the bulk of the power transfer occurs due to strong mutual coupling between the antennas (eg. a transformer).

For electromagnetic, the waves are propagating (oscillating electric and magnetic moving through space) and the coupling between receiver and transmitter is weak (ie. loading the receiver has no effect on the transmitter).

Both are governed by Maxwell's Equations. Sometimes you'll hear these referred to as "near field" and "far field" -- though those definitions are nebulous in terms of modeling the coupling.

There is still strong mutual coupling in the near field if the coils are mutually resonant. The magnetic systems uses a ferrite core to concentrate the field, and these need to be well aligned to achieve high efficiency. These are ferrite based:

http://www.digikey.com/product-search/en/inductors-coils-cho...

The near field systems rely on both coils being within about one radius of one another, and the coils are typically self resonant, but the smaller you make the coil, the higher it's self resonance, thus it's efficiency is lower. That's the issue with resonant near field coupling, the small devices like phones limit the coil size, and efficiency. Near field demo:

https://www.youtube.com/watch?v=4GransrkXd4

It won't be catastrophic but it could affect them. Most pacemakers have a mode that if you put a magnet on them they revert to pacing at 60 beats per minute (VVI mode) like a metronome rather than more advanced adaptive modes (like DDD mode). That's how if a doctor suspects it's malfunctioning due to bad sensing they can get it into a basic mode. I've done it in the ER when a pacer was freaking out and going way too fast due to bad sensing.

pacemakers act up near high voltage power lines and transformers.

Most modern PM's are actually much more resistant to RF induction than one from strong magnetic fields mainly due to the saturation of the RF spectrum over the past 2 or so decades coupled with a decline in high voltage power in populated areas (they are underground as well as farther from residential areas these days).

That said I have no idea if the field those things generate is strong enough to create any issue with PM's or similar devices (like the "jump start" internal defibrillator some people are getting these days).

I would think that these devices are quite safe, especially considering that you see wireless charging station in courtesy area's in lobbies and other public places without any warning signs.

Actually, the health risks of wireless charging is still unknown without longitudinal studies. Are you sure you want this kind of devices at home where your kids can access it easily?

I wasn't talking about long term health risks like cellphones give you brain cancer and WiFi makes you blind.

But there are guidelines for certifying consumer electronics compatibility with medical devices is on that list.

...or those with pacemakers? ;)

I'd hope that they would be current limited at a minimum, I can't see how IKEA would market something with such an easily remedied fatal flaw in it.

They usually are, but if you've got a receiving coil then you can put in a lot more energy before you begin heating up the primary coil. This is because with the other coil there it'll take in energy that would otherwise be turned into heat in the primary coil if you weren't draining it off. This means that if you can detect the presence of the device (usually by a change in the resonant frequency of your own coil I think is how they do it) then you know that it's safer to dump more energy into since it won't all be turned into heat.

No, you simply measure the current drain into the coil and when it hits a value that you associate with a magnetic short-circuit you limit the current to something very low until the short-circuit goes away. That way nothing gets hot.

http://pics.camarades.com/v/jacques/renewables/windmill/dscf...

See those laminates at the edges? They're 'open' for that same reason, that way the magnetic field goes through the coils rather than that it heats up the steel and generates waste heat.

(that's from a windmill I built, diameter is ~ 12").

Could you discriminate between receivers too, based on picking a particular resonant frequency for the whole system? Or is that how you have to do it to do it safely.

You end up with a resonant frequency simply because physics ends up dictating that there is one, and staying at it gives you the highest efficiency. That said it does mean that you can use it as a discrimination between devices/standards.

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