Page updated 22/03/2022 to reflect further tests
The router currently being provided by BT for its broadband service is the Smart Hub 2.
Power Supply
Input barrel connector measured dimensions: outer diameter 5.5m inner diameter 3.0mm. No source in the UK has yet been found for this connector. A connector with these dimensions is marketed in the USA by Philmore (their type number 206).
Power supply output rated at 12V 1.5A DC
Power demand of the Smart Hub 2 as stated in BT instruction leaflet: Idle 0.036W Maximum 14.04W (∴ presumed intended maximum current = 1.17A)
Curent Demand From 12-Volt Supply (As Measured)
Values in red were obtained in a later series of tests.
No phone connected
Power off = 47μA (power supply connected; Hub power switch off)
Power on: at startup rises gradually from about 250mA to 840mA then varies around 730mA
During video download: about 730mA
During download from Speedtest.net: 960mA 996mA
Ringing
voltage A-wire to B-wire = 50V AC RMS 25Hz 53V RMS
Ringing wire to B wire = 66.6V AC RMS 25Hz 53V RMS
Two DECT cordless base units plus two corded phones with electronic sounders connected
Maximum during ringing = 780mA
700-series telephone with high impedance bell
Maximum during ringing = 803mA 834mA
Maximum during ringing while
downloading from Speedtest.net = 1.144A 1.050A 1.084A 1.094A
Ringing
voltage A-wire to B-wire = 50V AC RMS 25Hz 53V RMS
Ringing wire to B wire = 66.6V AC RMS 25Hz 66V RMS
700-series telephone with high
impedance bell, plus three corded phones with electronic ringers, plus two DECT base units (listed below)
Maximum during ringing = 873mA 907mA 915mA
Maximum during ringing while
downloading from Speedtest.net = 1.042A 1.168A 1.145A 1.131A
Ringing voltage A-wire to B-wire = 54V AC RMS 25Hz
700-series telephone with low impedance bell
Maximum
during ringing = 1.004A 1.020A
Maximum
during ringing while downloading from
Speedtest.net = 1.286A 1.259A
Ringing voltage A-wire to B-wire = 65V AC RMS 25Hz
Ringing wire to B wire = 64.6V AC RMS 25Hz
The
current from the power supply fluctuates, even when the Hub appears
inactive, so readings can be inconsistent. Also, an increase in current drawn from the power supply during ringing may partly be caused by the higher current demand imposed by the Hub itself in processing data transfer while the incoming call is handled.
A multimeter with capacitance range shows a reading of 1.7μF between Terminal 2 (B-Wire) and Terminal 3 (ringer wire) of the Hub's phone socket. This is similar to the value of the 1.8μF capacitor used in the bell circuit of BT phones and later in the BT master socket. In effect, the Hub's phone socket appears to replicate the output parameters of a master socket, except that the DC voltage on Terminal 2 (B-Wire) is +47.9V with respect to Terminal 5 (A-Wire). This is slightly lower and in reverse polarity compared with the 50V encountered on a PSTN line.
(Note the reference to terminal numbering shown on BT socket outlets. The pin numbering on the BT plug is in reverse order, although it is uncommon to refer to the pin numbers)
Typical Ringing Current Values
Measured using Post Office hand generator with voltage output about 60V AC RMS
700-series phone with high-impedance bell = 14mA
700-series phone with low-impedance bell = 20mA
BT Viscount corded phone = 14mA
South Western Bell Freedom Phone corded phone = 12mA
BT Vanguard corded phone = 12mA
Panasonic KX-TG7302EB cordless base unit = 150μA
BT Freestyle 750 cordless base unit = 400μA
These values are for current at 60 volts. The calculated minimum loads imposed on the 12 volt supply would be: 70mA for the high-impedance bell; 100mA for the low-impedance bell. That assumes no losses in the Hub's internal inverter supplying the ringing current. Clearly, these figures allow a more generous interpretation than those observed on testing.
Conclusions on Ringing Current
All the tests with one high-impedance bell and three electronic ringers connected while certain data transfers were occurring showed current
drawn from the 12-volt power supply just below the intended
maximum calculated from BT's specified power rating (worst test 1.168A; BT rating 1.17A). Although there is no significant drop in ringing voltage with the low-impedance bell, with one low-impedance bell the maximum intended current from the power supply is exceeded.
Ringing current is generated in bursts, so it is possible that this
has been factored in to the design and that the Hub output is
intended to support the same ringing current maximum demand as a
conventional PSTN line (ie four high-impedance bells or two of low-impedance). However, in the absence of any published
specifications, it may be prudent to restrict the use of corded telephones to four instruments with a high impedance
bells or equivalent sounders, which could work along with several cordless bases.
Using Older Phones
Under the old system of hard-wired telephones, bells were connected in series, up to a maximum of four, fed from a capacitor in the first phone in the chain. Bells were low-impedance types (with two 500Ω coils in series). When the phone socket was introduced, a maximum of four high-impedance (two 2000Ω coils in series) bells could be connected in parallel, fed by the third "ringer" wire from a capacitor in the master socket. The concept of the Ringer Equivalence Number (REN) was introduced: a telephone with a high-impedance bell was given a REN of 1. The REN value is a whole number, so a REN of 1 encompasses any ringing current below that of the high-impedance bell.
Tests show that some older corded telephones with electronic sounders draw a similar ringing current to the high-impedance bell. It is possible that some electronic sounders may require a lower current. A cordless phone base unit requires only a very small ringing current to trigger its sounder circuit, which derives power from the base unit's own power supply. However, such instruments are still allocated the lowest REN of 1.
The Smart Hub phone socket could support several phones, the maximum depending on the total ringing current, different models of phone having different demands. If a bell is disconnected in a phone, then that phone can be discounted for the purposes of ringing current. The parameter which determines the maximum number of phones is ringing current. The speech circuit is powered by the 50-volt (nominal) DC supply on the A- and B-Wires. This voltage applied across the phone drops to a very low value when the handset is lifted, and in practice could almost be considered a short-circuit. In any case, it is unlikely that all phones would be used simultaneously, so any number of instruments without bells or sounders could be connected.
Older electronic phones may need ringing current fed via the third "ringer" wire. Newer instruments may not require this provision, but they will need ringing current to be applied between the A-wire and B-wire. The Smart Hub caters for both options. There is a slight difference in the ringing voltage measured between A- and B-wires, and the voltage between the "ringer" wire and A-wire.
A rotary dial phone can not make outgoing calls on Digital Voice, but it could be used for incoming calls. If necessary to reduce the ringing current demand, a bell can easily be disconnected as these old phones have screw terminals and jumper strips which can be configured as required. If the bell is to remain in use, the internal capacitor can be disconnected and the third wire used to feed the bell.
Useful Links
Sam
Hallas' website contains numerous
technical documents, including the following:
BT Phone Socket wiring instructions (includes information about cabling , ringing devices and internal
configuration of 700-series and similar telephones; written before the NTE5 outlet with customer-removable faceplate was introduced)
Post Office N Diagrams (numerous wiring and circuit diagrams, for example the Telephone No 746)
https://www.britishtelephones.com/btdigitalvoice.htm
Openreach Suppliers' Information Note 351 (characteristics of the existing analogue PSTN line)
Two informative articles by Draytek with some technical information: Part 1 Part 2 (Part 2 mentions the bell isolating capacitor having a value of 300μF. The correct value is 1.8
μF)
