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Subject: RE: TWL: Electrical Problem
From: Jim Donohue (jim_donohue@XXX.XXX)
Date: Sun Feb 03 2002 - 00:37:46 EST
Well yes and no. I think 25 years ago I had a study run on touch switches
which found I think 10 or 12 practical methods of doing so. The best are
the parametric amp things which use the change in capacitance when coupled
through a finger rather than through air. One can also base the change by
simply running an oscillator and using the touch pad as part of the
capacitance but it has a number of likely problems like questionable ground
return. A thing that often works well is a hum detector. Simply see how
much 60 cycle you can find and presume a touch if it jumps way up.
I would not use these things on a boat. They do not overwhelm us in
standard usage because they have always proven troublesome. When you get
them all set up and working then you run your VHF or your microwave and they
change state. Get them damp and their operating parameters change. Look at
them sideways and they decide to slide up and down their range without human
intervention. They will most likely choose to do this while you are trying
to pick out a boat bearing down on you. Murphy requires such behaviour.
These generally very high impedance circuits should simply be avoided on
boats. They are difficult on land and treacherous at sea.
Jim
> -----Original Message-----
> From: owner-trawler-world-list@XXX.XXX
> [mailto:owner-trawler-world-list@XXX.XXX]On Behalf Of Arild Jensen
> Sent: Saturday, February 02, 2002 2:09 PM
> To: Rollsdoc@XXX.XXX
> Cc: Trawler-world-list@XXX.XXX
> Subject: Re: TWL: Electrical Problem
>
>
> At 07:42 AM 02/02/2002 EST, Rollsdoc@XXX.XXX wrote:
> >Did you say the invertoer and gen set were not grounded? They should be.
> They
> >both should have green wires bonding them to the boat at all times.
> >Rodger Wrona
>
>
> REPLY
> Not the same thing!
> In the case of the touch switch the body capacitance works against the
> ground plane.
> Most often the trigger circuit is something called a blocking
> oscillator.
> The body capacitance loads the free running oscillator circuit by
> interacting with a part of the cirucit which in effect acts as a
> radiating antenna. The ground plane of the earth is apart of
> this circuit.
> and the person's body afffects this by grounding the osc. into the earth.
> When the oscillator stops running, a semiconductor switch toggles on or
> off to suit.
> National semiconductors ( among others ) makes such a chip for industrial
> and comsumer product applications.
> For a detailed technical description refer to their application
> text books.
>
> You do not have the same relationship to ground in a boat as you
> do ashore
> because the system is floating with a single round point. he popular
> duplex test plugs will often get confused when testing outlets aboard
> a vessel running on a genset or inverter due to the changed
> relationship
> of ground, neutral and hot lines.
> More than once I have seen them give faulty test results with inverters
> or gensets while testing correctly when shorepower is used in the same
> boat and testing the same outlet.
>
> Cheers
>
> Ariild
>
>
>
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