AVR-Chat

At 01:09 PM 10/21/04 -0500, you wrote:
> > ....
> > Transorbs on power and "local" wiring. Optical isolation on
> > signal inputs with long wires.
>
>While searching the net for info on Transorbs I found this on Lightning
>protection that may be of interest to others:
>http://www.telebyteusa.com/catalog/refinfo/appnote1.htm
>
>I searched Digikey (and International Rectifier) for "Transorbs" but 
>didn't come
>up with anything.  Having seen the above web page I searched for "avalanche
>diode" and came up with 69 items on Digikey ...
>
>Am I searching for the wrong thing?

Transorb (Tranzorb?) is essentially a brandname for a particular type of 
TVS (Transient Voltage Suppressor), rather like Hexfet vs Mosfet.  Digikey 
has many.

>Would I be looking for Schottky or "Standard Recovery" avalanche 
>diodes.  Also,
>I saw none with voltage ratings below 100V.  For the analog data lines I 
>assume
>I'd be looking for something in the 10v range.

Maybe higher.  Series protection is not uncommon, TVS devices don't usually 
have a very sharp breakover so having something like a TVS to catch high 
spikes followed by a zener in parallel with the sense input / resistor in 
series for dropping the voltage to provide a more precise cutout point is 
sometimes used.  Another thing to watch out for is TVS devices are meant to 
be used in a transient mode only. That's how they get such high power 
ratings in such a small package size.


>Since I'm protecting an analog signal line here I assume optos are 
>inappropriate
>(at least I've only seen ones suitable for digital, as opposed to analog,
>links).  I assume that the Transorb does not effect the signal line until the
>voltage exceeds it's rating?  I also assume they conduct in the forward
>direction like a normal diode if the surge takes the data line in the reverse
>direction (i.e.: forward polarity with respect to the Transorb)?

There are analog opto-isolation devices but they are more expensive.  TVS 
devices are often bidirectional.  The other thing to be aware of is that 
they do provide additional capacitance on the line they protect.  I suspect 
that is not an issue for you but if you need high speed it's something to 
watch out for.

Another way to approach isolation is to put the A/D convertor on the 
isolated side and use a serial protocol to get the info back.  Probably too 
complex and pricey for your setup though.


> > ....
> > SMTP is amazingly simple, but the underlying TCP/IP stack is
> > quite a lot of baggage. If you have a dialer, then why not
> > simply call a pre-determined number, have that answer and log
> > the status? Thsi is how many "monitored" alarm systems
> > operate.
>
>If I had a predetermined phone number it would have to be dedicated to this
>function with a system to answer it.  Seems like overkill when the Internet is
>everywhere.

I seem to remember seeing (in the back of one of those product magazines) a 
device that provided a modem with a built-in tcp/ip stack.

Robert


" 'Freedom' has no meaning of itself.  There are always restrictions,
be they legal, genetic, or physical.  If you don't believe me, try to
chew a radio signal. "

                         Kelvin Throop, III