Cap Recharge

[René Schmitz]

Seemingly your relay coil generates this higher voltage.
An inductor generates a voltage across it, when the current 
thru it is changing, as when the cup is lifted. 
(Scientifically: U=L dI/dt) 

Maybe even possible that there is some oscillation due to the 
capacitive nature of your switch.

One possible solution is to put a diode "backwards" over 
the relay coil. The cathode should point towards the + pole 
of your battery.

Or you could try to decouple your sensing loop from the switching 
by interposing a transistor which turns on that relay. Anyway that 
diode is mandatory here as well.

Bye,
 René


At 15:38 28.04.00 US/Eastern, Plinio  Barraza wrote:
>Just yesterday I ran into an interesting
>problem.  It might seem like something obvious
>to some of you, but it really surprised me.
>
>I was helping out a friend who wanted to make a
>circuit such that a video signal entering a tv
>would switch from one vcr to another when
>someone lifted a cup from a table (sounds
>wierd?).  
>
>Anyway, I said, "easy".  I made a circuit with
>a dual relay that would switch the video and
>audio source everytime you closed a circuit
>consisting of the relay controll input, and a
>9v battery.  We tested the circuit and it
>worked great.  THe next step was to make a
>table whose top was two isolated metal plates
>that would close the connection when a metalic
>cup was placed in the center of the table (ie
>gapping the two plates. 
>
>Well, the circuit worked great, but someone
>noticed that one would get a shock when
>touching both plates and lifting or replacing
>the cup at the same time.  I thought the
>circuit would be safe, (who can get shocked by
>a 9v battery), but I was wrong.  
>
>I guessed that the shock was somehow the result
>of some form of capacitance between the plates,
>or charging of these.  The shock was actually
>quite hard.
>
>I then tried some intuitive trics to limit the
>current, like putting a small resistor or an
>inductor in series in the controll circuit:
>-battery, inductor, resistor, table, relay...
>but that didn't  work. 
>
>Obviously I have no idea what is going on, nor
>how to fix it.  I thought of a couple
>alternatives.  Isolating most of the table
>except the place where the cup goes, or using a
>nonconductive tabletop with thin copper wire
>strands accross it  -every other strand
>connected to one end of the open circuit in
>order to ensure circuit closure wherever the
>cup was placed.... But my friend is so happy
>with the table top he made, that he would like
>to know if there is another solution.  
>
>What do you guys recomend I do ?
>
>Can anyone explain this effect.  THe plates are
>side by side, laying flat, with a thin gap
>separating them...
>
> Thanks in advance ..
>
>Plinio
>
>---------------------------------------------
>think globally, search locally - Orientation Colombia Email.
>http://co.orientation.com/eg
>
>
>

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