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Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
Oh wait!!!! I got it.
What you do is put the tree as the top capacitor of a low-voltage Tesla coil, say 500 volts pk at a few hundred kilohertz.
So the tree volts bounces up and down at 500v and on each branch you have the bauble. So the current flows in along the branch to each bauble and out again, each cycle.
If the bauble has a capacitance of a pF then we get half a milliamp or so.... So what you do is, you put a ferrite bead transformer in and step down the volts/up the current and recify it and use it to power the LED (2-3 v @ 5 mA).
OK, potential problems:
* safety: it's 500 volts, but the actual current is tiny, below the lethal 10mA cutoff. If you touch the tree when Earthed, you'll discharge the Telsa coil/tree and you might get a static discharge, but it's too quick to kill you. After that, you've got the power supply current going through you, but it's a high impedance source, so it doesn't matter. If you're not Earthed, you might get a slight shock, but you'll pretty much just bounce up and down with the tree.
* resistive losses in the tree: it's about a kiloohm or so, so at 500 volts on the capacitance of the tree, you're looking at milliamps, so well under a milliwatt; you're just feeding the capacitance of the tree through a 1k , not putting 500 volts across the tree.
* radiation: there's hardly any current, so it won't couple well to the far field, resistance is too high for that to be significant.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
I've got a plausible circuit running in LTSpice simulation.
There's problems though. The short circuit current is quite high, over 100mA, as with the circuit I have the tree ends up at 70v if somebody grounds the tree through their finger. I'm not sure if this is a necessary problem though yet, but the tree and the person are similar resistances, and this looks like it is causing issues.
I had to ditch the ferrite rings, they gave too much impedance, so the diodes are directly in the capacitive circuit right now, I may add some transistors later to control the current.
The safety issue may be dealt with in a number of ways; one interesting one is to have the tree detect the change in capacitance as the person comes near, and shut the lights off. That would also improve EM health issues of having a several hundred volt field. However, reliability of such a safety system is a potential issue, if it failed people could die.
It might also be possible to detect the DC connection to ground, and use that to perform an RCD-like function, but unless it's well integrated, failure could be embarassing.
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Wireless christmas tree?