Telephone Line Powered SX

william chan

Hi,

Any of you geniuses have any ideas on the feasibility of running a 50Mhz SX28 from the telephone line power?
The circuit will have to charge a rechargable battery when the phone line is in use, without disrupting the normal operation of the telephone.
Of course the SX will have to sleep most of the time or be able to change it's own oscillation frequency.

Thanks.

KenM

My guess is that the phone line will not supply enough current running the SX at 50 MHZ.

For example, I remember the phone line measuring about 48 volts dc with nothing connected, then droped to less than 10 with the phone connected.

From the size of phone line wire it looks like it will not carry much current.

Can you go into more detail about the project, do you have to run the sx at 50 MHZ?



Ken

william chan

Actually the wire can carry more than 500mA. But a typical phone line may only provide 25 - 60 mA when in use.
If we want the SX to do DTMF Detection and DTMF Generation, it probably need to run at 50Mhz or somewhere near there.
The SX uses about 80mA when running 50Mhz at 5V, so definitely we need rechargable batteries to help out
Do you know of any simple circuits to steal currrent from the phone line?
Do u know of any method for the SX to change it's oscillating freq during run-time?

Thanks.

William

KenM

If the phone line can "provide" only 60mA, I don't know of any way to get more out of the line than it can possibly deliver.

Regarding changing the osc freq during runtime.....I don't know if that can be done, but some of the SX gurus will likely chime in and let you know about that.

One suggestion is to visit www.sxlist.com and see what they have there about DTMF.

Ken

william chan said...
Actually the wire can carry more than 500mA. But a typical phone line may only provide 25 - 60 mA when in use.
If we want the SX to do DTMF Detection and DTMF Generation, it probably need to run at 50Mhz or somewhere near there.
The SX uses about 80mA when running 50Mhz at 5V, so definitely we need rechargable batteries to help out
Do you know of any simple circuits to steal currrent from the phone line?
Do u know of any method for the SX to change it's oscillating freq during run-time?

Thanks.

William

DntGvaSht

Qouting http://www.ee.washington.edu/circuit_archive/circuits/F_ASCII_Schem_Tel.html#ASCIISCHEMTEL_012

"...They aren't in the business of supplying power, and they ARE in the business of finding faults in their lines. Any substantial power drain from their lines WILL be detected. If it's large, the phone switch will conclude that you've dropped the phone in the bathtub or something like that, and will disconnect your line (and will check periodically to see if the drain has gone away and you can be reconnected). If it's small, the switch will report it to the service people as a possible line problem, to be investigated before it causes a complete failure... and if they investigate and find that you're to blame, they will probably send you a bill for time and trouble. The current you can draw without eventually having it noticed is very small."

qouting http://www.sandman.com/telco.html

"...supplying from 23 to 80ma of power on every phone line... for you to use..."

that last one is funny,·all of their products don't actually "run" on the phone line, they just·trickle-charge batteries.
This link will probably be most helpful: ·http://www.epanorama.net/links/telephone.html

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"OIOOIOOO OIIOOIOI OIIOIIOO OIIOIIOO OIIOIIII OOIOOOOO OIIIOIII OIIOIIII OIIIOOIO OIIOIIOO OIIOOIOO OOIOOOOI"
http://68.11.58.106:69/ircchat2/jicra-1.2.2/index-js.html

william chan

Dear DntGvaSht,

Thank you for your most informative response.
We know that you can't take more than 0.5 mA from the phone line without causing the problems you mentioned.
But we can take 20mA or more DC when the phone is offhook ( in use ).
So, our plan is to charge the batteries whenever the line is in use, and stop charging when the phone is not in use.
The link you provided doesn't seem to have such a circuit.

Thanks.
William

mpuman

Although not exactly what you are looking for; attached is a design that we did several years back. It should give you a beginning to design your telco interface circuit. This design was meant to be installed between the phone line and the phone(s). It worked quite well for its application. Have fun designing!

All the best,

- Mike

william chan

Dear Mike,

Thanks very much for the circuit.
Seems like the bridge rectifier serves to charge the 1000uF Capacitor when the phone is offhook.
Very smart.
But, since the negative output of the bridge is grounded, this circuit will be a problem if more than 1 phone line is connected to multiples of this circuit.
B'cos they share the same ground, short-circuits and cross-talks between phone lines will be a problem.
I also noticed that there is no gyrator or coil to reduce attenuation of the AC (voice) signals when charging is taking place.
Is this normal?

William

mpuman

William,

The 'ground' is not earth, just a local buss. If you want to use multiple phone lines with the same micro you will have problems with this circuit and will need to design one that isolates each line - not an easy task and still be line powered.

The voice signals in such a series circuit are varying current, so the cap 'filters' and smoothes the modulation and a coil or gyrator was not need in the application.



- Mike