How does the power get to the phone?

[Dr-of-style]

Hi all.
pondering on power...
I know how the stations get power from a cord board,
but how does a phone get power from an automated co?
Anybody got a diagram of co power distribution?
Regards

[Dave Moll]

My understanding of the situation is that generally, telephones are powered from a DC voltage superimposed on the signals on the line - 48-50V DC when on-hook, dropping to around 10-12V when a loop is made. Traditionally, this power was added at the exchange, though it (along with ringing current of around 75V AC) can be provided in local cabinets these days.

[Radio Wrangler]

It has been usual for exchanges in most countries to have large batteries providing a -48v supply to power just about everything. There would be float chargers from the mains topping up the big battery, and a backup generator for use should the mains fail.

48v is provided down the line to the phone in your house via a high resistance relay coil. On-hook, your phone is mostly disconnected, there is just the bell connected through a capacitor to block DC. Ringing is by a big AC voltage... about 60v

When you lift your handset, the phone connects earpiece and microphone to the line. It takes DC current, pulling in the relay back in the exchange, the relay contacts trigger the rest of the exchange to find which line got picked-up and connect to gear to handle dialling and call handling.

So your phone mustn't take too much current on-hook. Some phones take a controlled amount to charge internal capacitors or batteries without upsetting the on/off hook relay.

It was all planned in the electromechanical era and was remarkably clever. Nowadays it's all electronic and under software control.

David

[emeritus]

In the early 1990's I was involved with a patent infringement problem GEC were having with a SLIC (subscriber line interface chip: one required for every line) that GEC were manufacturing under licence. The GEC engineer who knew about these things at the system level said that the Post Office (BT's predecessor) had changed from constant voltage line driving to constant current, so the SLIC in question was a constant current type. Apparently constant current driving offers superior performance to constant voltage, but I don't now recall the reasons. The engineer mentioned that, being a monoply, the Post Office had been able to implement a country-wide change, whereas the fragmented US phone networks had had to stick with constant voltage for interoperability.

That's about the extent of my knowledge: GEC hadn't been provided with the detailed description of how the circuit worked that was necessary to challenge the threat of infringement litigation, and as I had previous practical experience in designing and laying out integrated circuits, was helping to reverse engineer details of how it worked from the chip layout drawings that had been provided to enable manufacture, in conjunction with an outline description of its operation in an electronics journal. The designers had been less than forthcoming with technical details.The dangers of manufacturing under a licence that doesn't have patent indemnity provisions!

[dagskarlsen]

Constant current will provide more power to long (high resistance) lines, and less to the shorter lines. Traditional telephones performs best at about 25 milli-amps (18-30). Higher current would fry the old fashioned carbon transmitters. Short lines could get more than 50 mA. Long lines could get to little to power the transmitter (less than 18 mA) On Extremely long lines some telco's used local battery speak, central battery signalling.

If you want to power a line for intercom use, just use a relay or something with high impedance. in series with a battery across the line. Ringing is much more complicated to get.

[Oldcodger]

Originally Posted by Dr-of-style

Hi all.
pondering on power...
I know how the stations get power from a cord board,
but how does a phone get power from an automated co?
Anybody got a diagram of co power distribution?
Regards

Try looking for Transmission bridge. This was the early form of powering CB phones for old manual exchanges and formed the way of signalling to a board that a "subscriber" ,i.e customer wanted to make a call. Later auto exchanges followed on his system, but as I'm a transmission bloke ,clunk click is foreign to my way of thinking.

[Skywave]

Originally Posted by Radio Wrangler

So your phone mustn't take too much current on-hook.

Being the total ignoramus that I am about telephones, does that above remark refer to the 'REN' number of a 'phone?

Al.

[Radio Wrangler]

No.

The ringer equivalent number relates to the AC impedance of the bell because the bell (or warbler or whatever) is only AC coupled to the line via a blocking capacitor. So it takes no DC current and when off-hook, the bell is disconnected so it doesn't load the line or tinkle during loop-disconnect dialling.

The DC current your phone takes is monitored. Once it passes a threshold the exchange decides that you are off hook. Some phones used a little bit of current under this value to power things, to maintain data or recharge small batteries. Once your phone is recognised as off-hook, the exchange looks for large pulses of DC current in the form of loop disconnect dialling. Once a call is established, the exchange looks for modulation of the current taken by a carbon microphone or a simulation of one.

If the exchange doesn't see loop disconnect dialling, then it looks for AC tones as DTMF dialling.

REN is important because it is used to calculate the load imposed by however many phones are on one line. If too much load, there may not be enough voltage for some of the phones to ring.

David

[Dave Moll]

Originally Posted by Radio Wrangler

when off-hook, the bell is disconnected so it doesn't load the line or tinkle during loop-disconnect dialling.

That would be true for, say, a 746 if the strap T5-T6 were removed. In, say, a 332 the ringer is permanently connected across the line.

[russell_w_b]

In normal use the bell, and its blocking capacitor, are always connected across the line whether on or off hook. It's just a case of where the cap happens to be (master LJU or telephone) but electrically it's the same. The bell (300 series) or dialling pulses (700 series, CR snubber cct) are shunted rather than disconnected to stop bell-tinkle.

The exceptions are telephones connected as 'shared service' or some continental instruments where the bell is actually switched. The nomenclature usually gives a bell impedance at audio frequency.

[russell_w_b]

Originally Posted by Radio Wrangler

REN is important because it is used to calculate the load imposed by however many phones are on one line. If too much load, there may not be enough voltage for some of the phones to ring.

According to the European Telecomms Standard of 1997 (ETSI) A REN of 1 is based upon the ringer arrangement of an 8746 telephone and equates to 7000 +j8640, or a resistor of 7 kilohms in series with an inductance of 55H.

The ringing signal is expected to see an impedance no less than 3.5 kilohms.