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Wireless christmas tree?

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BigBad

Mon Dec 06 2010, 02:42AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

I was toying with the idea of driving my entire christmas tree with AC current (at about a megahertz), and then hanging on several individual lights, I probably would be using a christmas bauble as a capacitor in series with an inductor so it rings, and then using it to drive an LED light or two. I worked out that the capacitance would be a few tens of pf, so I would probably need fairly high voltage (well over 100v) on the capacitor, but the capacitance is tiny, so it should be safe(?) Probably low volts on the tree itself though.

Not sure if it would work though; has anyone tried doing this kind of thing with the electrical characteristics of a live tree? It's kinda the thing that people must have tried before. I want mainly near field (obviously) but in practice does it radiate unacceptably?

Is there any massive gotcha that anyone knows about before I try it?

Conundrum

Mon Dec 06 2010, 10:38AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
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Posts: 4061

the field needed to light up an LED wouldn't be that high, but you would run into problems with RF interference to nearby devices.

seems a very inefficient way to light LEDs, due to inverse square losses...
-A

BigBad

Mon Dec 06 2010, 03:30PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Inverse square law shouldn't be a problem, because I would use the tree as one conductor (and capacitively couple to Earth as the other).

Pinky's Brain

Mon Dec 06 2010, 06:15PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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A LC circuit isn't going to help you AFAICS.

AFAICS the purely resistive part of the impedance from the bark (where you're power supply is attached) to the leaves (where your lights are attached) is going to be much larger than the resistance directly to ground through the tree. For every mW of light you are going to be putting a hundred into the tree most likely.

BigBad

Mon Dec 06 2010, 06:31PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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I was planning on putting a probe directly into the tree to contact the sap (or maybe experimenting with driving it via the roots, putting a probe in the soil). The point is that sap is conductive. (Theoretically I could put a ferrite around the tree and drive it wirelessly by that).

To couple to the light I could do the same thing, or use close range capacitive coupling.

But I'll think about it, and maybe try it and see.

The idea of the resonance is to avoid having a return wire.

Bjørn

Mon Dec 06 2010, 08:34PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
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It is an interesting idea, but a combustible conductor is a really bad idea.

BigBad

Mon Dec 06 2010, 08:59PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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The biggest problem I've found so far is that the resistance of the tree is probably higher than I'd hoped (~1k ohm). That might scupper it, since I'd have to make the voltage on the tree itself quite high, which might make it unsafe.

Also the inductors are looking annoyingly heavy/bulky.

Ash Small

Mon Dec 06 2010, 09:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

This is a very interesting concept. I hope you can find a solution, however, if you decide it can't be done, maybe you should consider replacing the star on the top of the tree with a tesla coil (or similar device).

This should light up all the lights, and more!!

(just an idea)

Mattski

Tue Dec 07 2010, 12:44AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

It's kind of a silly idea, but this would be pretty cool to see. Have you actually measured this 1kOhm resistance? That isn't so bad, if each LED in parallel has a 1kOhm resistance to your source at the base of the trunk that only drops 20V from the source to the LED. Obviously you can't pack the LEDs in too densely though. How have you estimated the capacitance of a metalized spherical ornament? Just using a random estimate of 1pF you would need ~25mH inductance, which is pretty big. You probably would want to put a chicken wire ground plane beneath the tree, or even around it though it won't do much for the aesthetics.

Obviously if you do this experiment indoors you should keep a very close eye on it as well as a ready fire extinguisher or two. Christmas tree fires can be very dangerous.

BigBad

Tue Dec 07 2010, 01:36AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

I'm not worried about the bursting-into-flames angle, the currents would be very low, so the I^R is negligible.

I'm much more interested in shock hazards. Still, it's currents that kill, not voltage, and we can presumably arrange to have high voltage/very low currents (think static discharge).

And yeah, the inductances are looking really awkward.

Incidentally, capacitance of a sphere in free space is just 4Pi E0 R, Where E0 is 8.85e-12 and R is the radius in metres (but the proximity of the tree will probably reduce that by a factor of 2 or 4 or something.)

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