Unknown Transformer----details required

[vinodquilon]

I am planning to tap a telephone line (to feed DTMF decoder as shown in the attachment) through an isolation transformer 600Ω/600Ω of 1:1 type. In our local market it is not readily avail. After a lot of search I have found one Dax 56K DX-56PU modem contains one Transformer .

Find the attachment T1 AASUPREME.
For using that Transformer I have to know the following details.
How can I know the following 4 specifications of the transformer,
Impedance matching (intended value is 600 ohms to 600 ohms)
Frequency response (intended value is 300-3400Hz for Plain Old Telephone System)
Turns ratio (intended value is 1:1)
Does it pass DC current through windings (otherwise I have to use separate capacitors at primary to block DC)

Complete Transformer specification-
aasupreme by
C US 0514
S022168A

I cannot get the specifications through internet.

 

[FvM]

Simply assume, that any available 1:1 modem isolation transformer perfectly fits your application.

Rather than starting one or more new posts related to your DTMF decoder project every day, why don't you simply add them
to your original thread?

You don't provide any new information except an out of focus photo of a modem card. Those forum members who will possibly
answer your post have plenty of similar devices in their electronics trash box.

 

[betwixt]

In the schematic you show, it clearly does pass DC but I would strongly advise you not to do that. When a normal POTS phone is off-hook (in use) it can draw as much as 100mA and that current is needed to alert the exchange equipment that the line is active. Small signal transformers will easily saturate at such high currents so it is best to keep the DC path separate and only let the AC signal reach them. In most telephones and modems, the signal is capacitively coupled and on the line side of the capacitor there is a constant voltage clamp to provide the current sink.

If you use a blocking capacitor make sure it is rated at 250V or more. Most phone lines are 50V DC when on-hook but have as much as 125V AC superimposed on them when 'ringing' voltage is applied. The peak will therefore be as much as 175V.

Brian.