Sounds like you just described the triboelectric(sp?)
effect and teflon insulation has one of the highest
"electron work functions" you will find.
Mechanical motion causes a separation of the insulation
>from and along the conductor. That mechanical separation
causes a GUARRANTEED charge separation. Generates a
voltage.
This effect is the bane of cable makers, that is why the
insulation is so tightly wrapped and stuck to the
conductor. If not, 1000 feet of cable and the slightest
slippage and you have voltage.
This principle is used in the security industry to make
buried cables that detect any motion for the cable. Cables
in walls detect drilling and digging through them. Cables
buried in the ground detect footsteps of people walking
around above ground. Yes, it is that sensitive.
An example of such a cable is a coax made with teflon
insulation, very loosely wrapped around the center
conductor. So loose that if you grab the center conductor
with pliers and pull, you can pull the whole 20 feet of
center conductor right out of the coax.
I thought we were talking millivolts when I first heard of
such a cable. So to check signal level, used a scope on
the cable and with a light tap produced over 8 volts! The
scope is a high impedance load and the cable is a high
impedance source, but still very energetic.
It is my understanding that this is one of forms of
deterioration for aging cables.
Alas, the solution is to not let a cable move.
- Robert -
Robert A. Macy, PE .. m...@california.com
408 286 3985 . . . .. . . fx 408 297 9121
AJM International Electronics Consultants
101 E San Fernando, Suite 402
San Jose, CA 95112
On Sun, 31 Aug 2003 15:49:43 EDT
lfresea...@aol.com wrote:
> Hi all,
>
> well, I have found myself involved with an interesing
> problem. I'm testing an
> aircraft system, that only has a few wires... 8 or so,
> that interface with
> the outside world. 28 volt dc power, an programming loop,
> a manually operated
> switch and an indicator.
>
> Well, evey time the push button is pressed, I see a 10 to
> 50 nS pulse, very
> consistant. The problem first was noticed during RE
> testing, and I found it
> easier to hook my scope to the antenna rather than use
> the SA. I have eliminated
> the EUT operating, since I have disabled the trigger by
> removing the component
> that allows it to trigger. I did leave in place the 2
> resistors that
> terminate the push button wire. I have a predictable set
> up.
>
> I remove either of the two reminating components, and the
> signal drops, it
> almost goes away.
>
> Anyway, after I'd removed the circuit board to make a
> change, I pushed the
> wire harness and saw a very similar transient! So I
> jiggled the harness and saw
> many... Cursing a loose connection, I checked, they were
> all sound. Not sure
> what was going on, I disconnected power, I could still
> got them. So I stripped
> down to my cotton shorts.. still got them.
>
> One section of the harness is sleeved with heatshrink
> sleeving, it holds the
> wires tighly. Jiggling that produced no events.
>
> So, I can only conclude, that the wire is generating a
> charge during the
> movement/jiggling, that when it discharges any one of a
> number of places causes my
> event. The parasitics of the harness set the ringing
> frequency. When I press
> my push botton, I believe I also kick the wiring with the
> impulse, again it
> rings very close to the frequency when I jiggle the
> cables.
>
> My questions are, am I off base with the charging
> supposition? The cable
> appears to be a teflon type insulation.
>
> If so, how can I get around it. I can't really blame the
> EUT for it.
>
> Opinions very welcome
>
> Thanks,
>
> Derek Walton
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