Cap Recharge

[Mike Granger]

I know how to fix this for you. You need to put a 1n4004 diode across
the relay coil, with the banded end closest to the positive end of the
supply battery. This will be reverse biased when the relay is energized,
and will absorb the voltage spike that the coil generates upon the
magnetic field collapsing as the coil curent ceases. This is called
reverse EMF, and can be several hundred volts for a very brief time
during the collapse.

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
>
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