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Mike & speaker funnybusiness..

R

Robert Baer

Jan 1, 1970
0
Well, my trusty AT&T phone died and a got a new one.
So,i thought to take apart the handset..err..BASH apart,as it was
*NOT* made to be taken apart in any nice way.
Surprise!
The electret mike had a _diode_ (signal) in parallel, reversed polarity.
Say what? Waffer??
And the dynamic speaker had _two_ diodes (1N4000 series),
back-to-back in parallel across it.
Say what? Waffer?
Yes,i know that diodes are cheap, but since there is no useful
function for them, what gives?
 
T

The Great Attractor

Jan 1, 1970
0
And lightning.

Lightning IS an electrostatic discharge, and no, one cannot generally
protect against a true lightning intrusion. They start at like 6MV.

You would have a hard enough time stopping the EMP jolt they cause.
 
M

miso

Jan 1, 1970
0
Possibly the diode across the speaker is for back-EMF. Drop the phone
and speaker generates a pulse.
 
J

John Devereux

Jan 1, 1970
0
That is what he is talking about here. And ESD in an electronics context
is considered to be something else. As in, say, human body model as
opposed to struck-down-by-the-almighty model.
A lot can be done to harden electronics against nearby lightning hits.
Telephones without such protection won't last long.

Careful, you'll attract the real lightning nutter.

Oh why not, lets try anyway: Lightning rod, mains whole house protector,
lightning surge protector.

See what turns up.
 
J

John S

Jan 1, 1970
0
Underground utilities help. As near as I can measure, the max jolt
from a lightning strike at the substation 2 miles away from me is
around 800V. And I haven't lost anything since I added a mains
protector ~15 years ago.

...Jim Thompson

Actually, I had underground telephone service in the country. It seems
that lightning will follow the tree it hit down the trunk, to the roots
and into the nearby telephone cable. I have several pieces of equipment
which testify to that fact.
 
Actually, I had underground telephone service in the country. It seems
that lightning will follow the tree it hit down the trunk, to the roots
and into the nearby telephone cable. I have several pieces of equipment
which testify to that fact.

This causes a common mode into the armored cable. The current flows
along the cable shield in both direction, eventually spreading into
the surrounding ground. The size of the common mode voltage depends on
the total resistance from the point the cable was hit to neutral
ground and the current (U=IR), typically hundreds or a few thousand
volts.

For a simple telephone set, the common mode is not much of an issue,
since it is floating and relies of the differential signal between the
conductors in the pair and hence not much need for diodes across
microphone and headphone. However, there should be a large gap
between the headphone and the human head, so that the human does not
get hit by this common mode voltage in the head by a small spark (that
is the U=IR losses, not the megavolt range lightning).

However, when the "telephone" is also grounded locally or at a power
substation, such as in mains powered phones, answering machines,
modems and ADSLs, there is a big risk that the common mode voltage in
the telephone pair will flashover to ground, through audio/mains
transformers and optoisolators or simply flashover between too closely
mounted PCB tracks, causing a lot of damage.
 
J

Jasen Betts

Jan 1, 1970
0
Actually, I had underground telephone service in the country. It seems
that lightning will follow the tree it hit down the trunk, to the roots
and into the nearby telephone cable. I have several pieces of equipment
which testify to that fact.

Yeah, underground phone service didn't protect me eiither, I had a few
MC1488s and MC1489s in serial ports die before I learned to also
unplug the modem
 
T

Tauno Voipio

Jan 1, 1970
0
Yeah, underground phone service didn't protect me eiither, I had a few
MC1488s and MC1489s in serial ports die before I learned to also
unplug the modem

The 1488 and 1489 were called 14-leg fuses, for a reason.
In the 70's, we had a standing order to set up good quality
sockets for them when changed for the first time, to save
the PCB's.
 
And the dynamic speaker had _two_ diodes (1N4000 series),
back-to-back in parallel across it.

Diodes are for transients created during normal phone operation. As onlyone other noted, parallel 1N4000 series diodes limit voltage to the speaker and protect your ear from excessively loud noise transients. Diodes conduct only when speaker voltage is excessive.

Other speculated anomalies are made irrelevant by solutions required at the subscriber interface; where phone wires enter each building.
 
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