Re: REN Booster
 

The distance from the Exchange will have some effect on ringing current and subsequently available REN.

In the days of hard wired telephones the bell coils were 1K impedance and were wired in series, with a recommended maximum of six in total on any circuit (I use the term circuit as this encompasses the anomalies experienced with shared service or certain extension plan working) so based on that recommendation, in theory, you should be able to get away with a REN of six without using anything like a REN booster.
It may be, however, that the (presumably electronic) ringing generators used in modern exchanges don't quite have the current to offer that the older mechanical ones did which is why the REN was reduced to four.

One other thing to beware of is the fact that not all types of ringer will operate correctly together on the same line, this is mentioned in a lot of early BT instruction manuals after the event of the "Plug and Socket" system; An inductive ringer, such as a bell motor, and a Capacitive ringer, such as a tone calling device, may if connected to the same line, act as a notch filter at ringing frequency and effectively sink the ringing current causing poor or no ringing.
I have to admit that I have never experienced the effect in practice but can see that it could be possible.

Andrew

Re: REN Booster
 

With your capacitor values, an assumed ideal voltage of 75V RMS at 25Hz at your premises and using typical type 59A bell-motors in your telephones, the current, per telephone bell, for 1, 2, 3 and 4 instruments would be 12.3mA, 9.8mA, 8.1mA and 7mA respectively. These values are not a million miles from the currents if a 3k3 resistor were to be used in each telephone (1,2,3,4 telephones each drawing 14.8mA, 10.7mA, 7.6mA and 5.7mA respectively).

So yes, it would work, and you might even obtain a more pleasant 'ting', as the resistive element of the circuit is limited to the wiring and the winding resistance (at a.c.) of the bell-bobbins. You could back warm in the knowledge that none of your precious ringing current was being dissipated as heat!

If no telephones were connected, you'd have a test-line impedance consisting of a 4uF cap inseries with the 470k resistor (I have neglected this resistor in all of my calculations), but I don't think that would be a problem. With four telephones connected, each with a 1uF cap in place of the 3k3 resistor in series with the type 59A bell and a 4uF master LJU capacitor, you'd see 2uF at the incoming line. With one, two and three telephones you'd see 0.8uF, 1.33ruF and 1.7uF respectively - I guess all would work.

Re: REN Booster
 

Indeed... I've limited my calculations / experiments to telephones all using a type 59A bell-motor - and there are even differences in values between those! Theory is nice, but one must run tests on one's own line, as lengths, impedances, etc... vary so much.

Re: REN Booster
 

A few years ago, I got a hint about the Panasonic KXT 308 and 616 PABXs would help me with several problems.

I got a KXT 616 from UK (and have long rings for internal calls, and short double rings for external calls)
This PABX solved several problems. I got 16 internal lines (16 integratedmodular sockets of std US type) Each just powering nicely at least 3 old telephones, (unmodified). I may call internal between the 16 different extensions) I may program witch lines to ring daytime, and witch to ring night time. (or not at all) I may restrict some if I want. E.g. The one in my often uloced garage is restricted to only call emergency numbers, C-net numbers, and my mobile.
I do not have to modify my telephones exept those with reverce dial. The PABX accepts tone and rotary, but is programmed to send tone only.

To start up, it is only to hang the box on the wall, and plug in the power.
Programming is solved by using a special phone on the first ext. (21 on mine)

It is several other options, wich I not have tested. Thancs to C*NET I have been able to connect via an ATA both direct acsess to each line, and free connections to other entusiasts all ower the world. The biggest problem is the time difference. The next is my internet connection, who may be some unstable.

dsk

Re: REN Booster
 

It occurs to me that the only way to 'boost' the REN is by substituting locally generated ringing current for the incoming one from the exchange.

I don't think it is particularly difficult and it should be possible to build a suitable unit for considerably less than £150! In fact, if you've got a well filled junk box, it could cost next to nothing!

In the attached circuit, the local source is based on a mains transformer, bridge rectifier and smoothing capacitor, plus a surge limiter.

Providing a replacement 50Hz voltage won't provide the correct ring, so I'm suggesting switching a DC source in sympathy with the incoming ringing current.

Instead of a bell, the incoming line feeds a relay via a diode and the usual capacitor.

RLA is closes for half a cycle of incoming ring current, then opens for half a cycle when the diode is reverse biased. The contacts of this relay reverse the polarity of the local DC supply every half cycle to produce a roughly square wave AC output current at the same frequency as the incoming signal.

Another capacitor couples this local AC to the output whilst providing isolation from the DC when there is no incoming ringing current.

A mains transformer with a 25 - 30V secondary will provide an RMS equivalent output from about 63 - 75V. None of the other components are critical provided that they are adequately rated. The output current demand is low, so just about anything available will be suitable for smoothing - and output ripple isn't going to be a problem, anyway. The surge limiter can be anything handy - 100 to 500Ω, say.

Any handy DPDT relay should be ok, with an optional series resistor to mop up excess voltage.

Other things you might consider are replacing the mains derived DC with a suitable battery supply - 3 x PP3 in series, perhaps - to make the unit independent of mains power; snubbers across the relay contacts; an isolating transformer on the output to improve the waveform.

See what you think.

Terry

Re: REN Booster
 

One of these

http://uk.rs-online.com/web/2079519.html


These were made by Gemini Connection Ltd for BT

Re: REN Booster
 

Hi,
The REN booster that Kevin has provided the link to is the type that I am familiar with.

Terrys circuit looks fine apart from the fact that the relay may have to be fed via a bridge rectifier to accommodate for line reversals, the only other problem that I can foresee is that, unlike "off the shelf" REN boosters, it wouldn't be a BT approved device so there is a (small but unlikely) possibility that, in the event of a visit from a BT technician, the connection of the additional home built REN booster would be frowned upon.

Regards

Andrew

Re: REN Booster
 

This is indeed the unit that I have - and at a much more sensible price...

What a shame it is headed:
Discontinued Product

Re: REN Booster
 

No! The whole point is that detects line reversals! AC is AC, whichever you look at it! The diode - which can be either way round - ensures that the relay only operates on alternate half cycles. It is this action that ensures that the output frequency matches that of the incoming AC.
which is the reason that I suggested the alternative of using a battery supply ...

Re: REN Booster
 

In about 1983 I extended the number of phones in the house because my wife was getting hard of hearing. I was advised to use 4 wire cable and put two (or three) bells in series off a single capacitor, thus making each group a single REN (extensions used to be wired that way I was told) I did manage to work out which jumpers to move!!!
Worked like a charm! Would that work now?
Also used a thermistor to stop dial tinkle.

MM

Re: REN Booster
 

I've successfully connected a type 300 and an extension 59A bell in series on a standard BT plug and socket arrangement; no audible difference in ringing, and, of course, four bells connected in series-parallel like this means that no resistors are necessary to dissipate energy that would otherwise be used for the bell-motors, as four in parallel would be (assuming one was aiming for a REN of 4).

Trouble is, such a system becomes unique to one's installation - fine if one isn't swapping out telephones all the time!

Re: REN Booster
 

There seems to be no reason that it shouldn't, subject to the line length limits from the exchange I suppose.
Thermistors were often used to prevent bell tinkle, as were additional gravity switches on some installations.
The "official" method of preventing Bell tinkle on a Plan 1A installation, which would have probably been the most common of standard domestic installations (along with Plan 4 plug and socket as maybe the second most common), was to use an extra "Anti - Tinkle" wire which effectively shorted out the bell circuitry during dialling from any of the telephones.

With series wired bells each telephone in the circuit was designated either Main, Intermediate and Last - for instance a two phone installation would not have any intermediate stations and a four phone installation would have two intermediates; all the intermediate stations were wired identically but the Main and Last were exclusively wired.

The N diagram for Plan 1A can be seen here the Anti-tinkle wire is strapped from T6 (via BT1) to each telephone.

Regards
Andrew

Re: REN Booster
 

Terry,

Very interested to find your REN 'booster' circuit. Please could you clarify something?

As far as I can see it will only supply bells wired to terminal 3, and will not help with running additional modern phones that have a 2-wire supply and derive their ring tone internally.

Am I right, can you suggest a way round this?

Re: REN Booster
 

You are right.
I would like to see that relay following 20-25 hz.

I have without asking, modified Terrys diagram.
If you have some kind of 25 hz power (15-30 hz) it will be the best:

This could be an old ringing machine (frequency dividing unit) or a frequency converter and a transformer.

If not, 50 hz will actually ring many telephone ringers, I have an Ericsson PABX ringing at 50Hz 45V, it sounds a little bit odd.

The rectifier bridge will protect the CO equipment from high ren loads, and strange feedback from the unit.

The 3rd relay contact set solves the need of ringing on 2 wire phones.

Relay contacts 1 and to may be moved to primary side of the transformer, and if you only have a rely with 2 set of contacts, you may skip one of these.

What is the drawback putting in this 3rd contact set? When picking up the handset during ringing, the ring current will continue until the ring is finished, and the voice circuit will be given more current than expected. The 1.8 microfarad capacitor will limit the current.

dsk

Re: REN Booster
 

Hi
I would definitely go for the PBX - I have worked my way through several BT REN boosters and I agree that they do work up to a point. However, I found 2 problems
1. They would die after about 2 years of use - you could smell the component failure!
2. They have nothing like the "oomph" of a PBX
So I now use a Revelation PBX. Cheap via ebay, works like a dream and all my numerous old phones ring with enthusiasm all over the house!
Regards
Trev

Re: REN Booster
 

Wow! 18 months on, I'd forgotten all about this, so I've had to re-read the thread!

The first thing, which was enshrined in the first post in this thread, was that the requirement was for a REN Booster to operate a number of vintage phones.

These will have independent access to the bell circuits and would, therefore, connect to my circuit on a dedicated bell wire.

However, if you ensure that a relay with a 4000Ω coil is used, or a relay + series resistor equalling 4k in total, the circuit will have a REN of 1, so two wire phones can still be added up to the maximum line limit in addition to older phones with a 3/4 wire configuration.

Re: REN Booster
 

If that means that you doubt that the relay will do so, bear in mind that a special type of relay, called a vibrator, was successfully used for many years to generate HT supplies for valved equipment, such as radios, driven from car batteries! Moreover, they ran at 50Hz and tended to be used continuously for long periods, unlike the present application.

Not a problem ...! :)

The whole point behind my idea was to generate exactly the same frequency as the incoming ringing current. If ringing current at a suitable frequency is already available, I'm sure that there are better ways of using it!

Re: REN Booster
 

Those vibrators are tuned to a specific frequency, and they need a little time to start up. And yes they works well. I have a PAX probably from 1948 doing that.
The vibrator starts when I go off-hook so it is ready when I have dialed.
Quite stable at 20H, in 2012!

dsk

Re: REN Booster
 

Thanks Terry - I guess you are confirming my conclusion. I actually want to run a couple of bells - so your idea helps, but also have rather too many phones in the house (generally the phones work, but the (small) bells struggle, despite some extra capacitors!
Maybe a PABX is the only real answer, assuming there are no other 'simple' solutions.

Re: REN Booster
 

I have thought about this for some time.
They used to have boxes that worked like a party line except both users could use it at the same time.
They had a Ni-cad battery in them that was trickle charged at a rate well below the level that takes the line off hook.
With this you could add a small audio amp without bias. This could just have a couple of zena diodes in series in opposite directions to stop the speech ringing the bells.
I would run two transistors into a 12-0-12 transformer from a 4-6 volt battery and the primaries connected to the bell capacitor line.