Newsgroups: comp.dcom.cabling
Subject: Re: daisychain  POTS phone wiring and DSL filters?
References:  <[email protected]>    
Robert Redelmeier  writes:

> DecaturTxCowboy  wrote in part:
> > The FCC specs out one ringer load not to exceed 12 mA of current
> > and the phone company's central office is supposed to support up
> > to five ringers (5 REN). So that's only 60mA at 2 seconds on and
> > 4 seconds off with five phones ringing. If you add more phones
> > and draw too much current, the C.O. will see it as an off-hook
> > condition and the D.C. line current is limited to 20-40 mA.
> 
> Thanks for the specs!
> 
> But crunch it through:  60 mA @ 90VAC is 5.4W.  Maybe 4.0
> if it's 90 Vp-p.  Still plenty to blow a 1/8W resistor,
> depending on exactly how much drop it takes.  

Anyway the resistance values on the filter are quite lod, 
so only a fraction of this power gets dissipated by them. 
The resistance val�ues I have seen have been around 20 ohms 
per resistor or smaller. 
 
> The resistor had better be below 35 ohms if it's going to
> handle a full house downstream iff the CO drives 60 mA.

Here are details of one ADSL filter I took apart to find information 
what is inside it:
http://www.epanorama.net/documents/telecom/adsl_filter.html

In the end of this document you can find links to information 
on some other filter products. 
 
> I have a single DSL dongle filter in my hot little hand.
> It was graciously supplied by SBC when I got DSL.  My DVOM
> rings it out as 46 ohm resistance on each leg through.
> 
> It might well blow on a full-house load if it only has
> 1/8W resistors inside it's sealed little body.  

The typical ADSL filter there is not just one resistor. 
There is typically some coils (with considerable resistance 
on them) and then the resistor. So not all the power 
gets disspated by this resistor. 

> If the CO
> drivers are constant-current, it'll fry.  If it's 90VAC
> constant-voltage, the extra 92ohms will only drop current
> from 60 to 56 mA and it'll still fry.  Of course, it might
> not fry if my phones draw less ring current (probable).

If this rign current happens to be that 60 mA maximum 
(current limited on the central office), then in the circuit shown 
at http://www.epanorama.net/documents/telecom/adsl_filter_circuit.gif 
the power loss on each of those 20 resistors is 
60 mA * 60 mA * 20 ohms = 0.072W 


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/