Mike & speaker funnybusiness..

Diodes in parallel to earpiece are not so unusual; they are to protect from lawsuits about loss of hearing (no kidding). The diode across the mike is probably for ESD.

Vladimir Vassilevsky DSP and Mixed Signal Designs

And lightning.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
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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?

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.

Possibly the diode across the speaker is for back-EMF. Drop the phone and speaker generates a pulse.

Robert Baer schrieb:

[...]

Anti-parallel diodes connected in parallel to the handset speaker are very common for hearing protection purposes:

HTH

Reinhard

A lot can be done to harden electronics against nearby lightning hits. Telephones without such protection won't last long.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators

polarity.

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.

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.

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John Devereux

polarity.

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

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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I love to cook with wine.     Sometimes I even put it in the food.

polarity.

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.

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

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You burned up two of my chips :-( ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

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.

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Tauno Voipio

oooh, that's gotta hurt Jim.... :)

--

John Devereux

The app-note calls for a series R. If you drive a line directly pulses above or below rail do their "duty" ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

Armored? Where is old crap that used, except where animals chew underground cable? I haven't seen any new like that in 45 years.

--

Politicians should only get paid if the budget is balanced, and there is 
enough left over to pay them. 

   Sometimes Friday is just the fifth Monday of the week. :(

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

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

It just struck me, that, due to Jimbo's secret sauce, the MC1489 is rated to at least +/-25V input, so the board design must have had little or none in the way of ESD protection.

The MC1488, on the other hand, has NPN outputs, so a back pulse exceeding rail has the same damaging (forward bias) effect as it would to CMOS. That's why the AppNote calls for a series resistor. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.