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Re: Wireless ON/OFF for Tesla Coils- anyone done it?


From: 	Geoff Schecht[SMTP:geoffs-at-onr-dot-com]
Sent: 	Tuesday, November 04, 1997 11:50 AM
To: 	Tesla List
Subject: 	Re: Wireless ON/OFF for Tesla Coils- anyone done it?



>The other day, I was thinking back to a Richard Quick post, where Richard
>described the 60Hz house ground "The last line of defense" or something
>like that.  He was refering to situations where the discharge from the
>secondary could strike the primary, and the operator console, potentially
>subjecting the operator to primary circuit currents.
>
>What I am wondering is anyone on the list has tried switching a coil on
>and off by remote, i.e. no wires or conductors connecting the user to any
>part of the system.  I was thinking of imploying a simple infra red
>system, whereby presence a specific infrared signal (from a tv-like
>remote control) would be used to switch power on through a suitably heavy
>relay in series with the normal SPST manual key switch.  Pressing the
>remote button and _holding_ would fire the coil, and releasing the button
>would cause immediate cutting of power.
>
>Of course the receiver and relay circuit would have to be nearly fault
>proof, to avoid having a coil stuck in the on position for any reason,
>but this can certainly be safe guarded against by a good design.
>
I was thinking about that myself a while ago. You can buy IR transceiver
modules from companies like Philips, Sony, HP and several others but it
would probably be easiest to find a dead TV set that has a working remote
with it. If you can't find the remote, several companies make universal
remotes (Fox brand seems to work well). IR receivers are obtainable as
self-contained units, with both the IR receiver and the decoder built into a
single subsystem.

Obviously, something that lives on a TC is going to require a lot of EMI
shielding! I'd recommend putting the receiver in its own metal box (Pomona
cases can be had that have good EMI gasketing) with a bonded copper-screen
window for the IR port. The receiver and decoder (you'll need a
microcontroller, explained below) need to run off of their own battery
supply. I'd recommend coupling the actual TC control signals out of the
receiver enclosure through a fiber optic cable.

For safety, a "dead man" technique of control is required. Should the
receiver fail (or the battery die), a keep-alive signal that is sent from a
healthy receiver to the power controller goes away and a watchdog timer
forces the TC to shut down. For that, you'll need some sort of simple
controller. I'd suggest looking into a PIC chip or, better yet, something
like a Basic Stamp.

So, you have a fail-safe receiver feeding a power switch of some kind. The
power switch controller should probably also be battery operated for
complete isolation and only the wire going to the power switch itself should
emerge from the shield (after passing through a good common-mode choke).
"Dead man" controls are also a good idea here. If you're switching a big
load, mercury contactors work very nicely and can be driven from a fairly
small control switch. Since they're encased in glass, mercury contactors
have very good load/controller isolation.

You could use a long fiber cable to control the TC if an IR link is too
involved. That would be somewhat simpler. There are a bunch of ways to
tackle a problem like this; you just have to decide what you want to
accomplish and how exotic it needs to be.

Geoff Schecht, Austin, TX