DAA specifications

Long ago I wondered why the phones and then modems were made the way
they were, why transformers had the word HIPOT on them, and other
mysterious things. Then I found out about FCC Part 68, and its
requirements. Later, I saw some designs that were seriously
deficient, mainly because they were made cheap to sell cheap. Many
cheap phones don't even have a transformer. As for the DAA using the
solid state relay, they don't get into that in this doc.

I found a document that explains some of the circuits and reasoning
behind them. Unfortunately this document doesn't go into designing a
DAA that minimizes RFI, which so often becomes a real problem around
larger installations. In the circuit shown, I would have made the
relay dual contacts so that both sides of the DAA are disconnected
from the line during on hook. This minimizes the unbalance of the
line. Anyway, now I've learned how to calculate REN, always wondered
how they figured that. All I knew was that with the usual cap and
coil, the impedance was several kohms at the ringing freq. Enjoy.
http://www.web-ee.com/primers/files/AN812_1.pdf

~~So Happy Holidays~~

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The answer to your question lies in the telecom safety issues of each
country.

The basic safety for telephone equipment is there MUST BE an isolation
(insulation) barrier between the
telecom circuits (which have telecom network voltages TNV on them) and those
circuits which
which the user may touch (safe extra low voltage circuits- SELV)

For a simple telephone the isolation is the plastic of the hand peice and
the plastic of the outer casing of the base.
Plastic offers insulation ! (thus the whole internal telephone circuit can
be at TNV potential).

For a modem the serial port connection is something that the user can touch
and the users equipment connects to
(thus most modems have an isolation barrier at the DAA). The transformer and
opto-couplers offer this isolation.

The HI-POT test is a High Potential test to test the breakdown of the
isolation barrier - regaardless if it is a modem transformer
or the plastic insulation of a standard telephone....

Telephone safety standards EN60950 (Europe), UL1950 (USA), CSA950 (Canada)


If you do a search on EN60950 and Transformer ... I am sure you will find
lots of transformers that are compliant to this standrd.

JG

 

I asked a question????

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###Got a Question about ELECTRONICS? Check HERE First:###
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My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 <at> hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
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No, you didn't. But thanks for the link you provided.