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Old 8th Aug 2017, 4:25 pm   #1
ThePillenwerfer
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Default REN, Resistors &c.

Following on from this thread to save dragging it Off Topic: http://www.vintage-radio.net/forum/s...273#post966273


There's a Wikipedia article here: https://en.wikipedia.org/wiki/Ringer_equivalence_number which says that it is a thing that's used internationally, though with differing definitions.

I know that the GPO/BT maximum of 4 is conservative and, unless you are on a very long line, it'll really be more. When I was young and daft (guess which one of those still applies ) I connected a load of 700s to a BT line and they all rang. Assuming the ones with 500Ω coils and no resistors have a REN of 4 I must have been pulling about twenty.

The problem I've had with trying to work it out is not knowing the inductance, and therefore the Inductive Reactance of bell coils.

The only problem I've had was quite recently. I converted an ATM T3903 to a 232 configuration. I remarked at the time that it's bell coils were 1,000Ω and didn't expect this to matter. With a 2.2kΩ resistor that bell made a pathetic tinkle but was fine without. I then found that for all that that one worked it was pinching all the current so changed those coils for 2,000Ω ones. I did notice that the originals seemed very skinny so am inclined to think that it was their Inductive Reactance, or lack thereof, that was the problem. I also wonder if this is why things like 250s and 704s have two 250Ω coils which are short and fat so, maybe, more inductive than the usual type.

I've never fully understood the need for resistors as if I put several bells in parallel the total Inductive Reactance will increase and reduce the current drawn anyway. If this is the case though I can't see why the GPO would have bothered to change over to 2,000Ω.
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Old 8th Aug 2017, 6:46 pm   #2
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Default Re: REN, Resistors &c.

Quote:
Originally Posted by ThePillenwerfer View Post
I've never fully understood the need for resistors as if I put several bells in parallel the total Inductive Reactance will increase and reduce the current drawn anyway. If this is the case though I can't see why the GPO would have bothered to change over to 2,000Ω.
Come again? If you put inductors in parallel then unless there is some magnetic coupling between them (which in the case of telephone bells would seem unlikely) they behave like resistors in parallel. The reactance will decrease.
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Old 8th Aug 2017, 6:59 pm   #3
ThePillenwerfer
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Default Re: REN, Resistors &c.

AC Theory always gave me nose-bleeds. I was obviously mixing them up with capacitors. Thanks for the clarification.

The point about different shapes possible having different reactions stands though.
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Old 8th Aug 2017, 8:01 pm   #4
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Default Re: REN, Resistors &c.

Hi,
REN has always been a bit of a grey area as far as I'm concerned and I agree that in most cases line length will be a contributing factor to how much, or how little, REN is available on any line.

I recall seeing a comment, in many telephone instruction manuals in the early/mid 1980's after the introduction of the "new" plug and socket system that went something along the lines of "mixing different types of calling devices on an exchange line may result in not all the (your) phones ringing"

I always understood the "different types of calling device" comment to mean tone callers and bells and figured that the reason all the phones may not ring, if an inductive bell and a capacitive tone caller were connected on the same lane, would be due to the ringing current being shorted out by a combination of the inductance and capacitance of the bell motor and tone caller being resonant at a particular frequency which looked like a short circuit to the ringing current.

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Old 8th Aug 2017, 8:06 pm   #5
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Default Re: REN, Resistors &c.

Quote:
Originally Posted by ThePillenwerfer View Post

The problem I've had with trying to work it out is not knowing the inductance, and therefore the Inductive Reactance of bell coils.
I got my figure for a type 59A bell movement by driving it with a 25Hz 90V supply via a resistor and a true RMS ammeter, and measuring the voltage across the bell-movement (and hence its impedance), the supply, and the resistor.

From this I could draw a triangle with each side representing the aforementioned voltages, and using the Cosine rule, extrapolated the in-phase and quadrature components for the bell-movement: typically 1776 + j2085, or a Z of 2739 at an angle of +50 degrees.

This differs from that of a bridged measurement because of the current and the 'in-phase' losses (eddy current and hysteresis losses) and gives an inductance of about 13H.
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Old 8th Aug 2017, 9:19 pm   #6
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Default Re: REN, Resistors &c.

Simple solution to the low bell situation is to fit a BT socket as used on systems . Don't know the type/part no, and don't have one to hand, but it had circuitry inside, which when Driven by a 50v from the system gave extension bells some oomph. Then again, there's the ringer converters as fitted to system phone systems( where there was no provision for an extension bell ) to have an outside bell ring on external calls.
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Old 8th Aug 2017, 9:45 pm   #7
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Default Re: REN, Resistors &c.

Quote:
Originally Posted by AndiiT View Post
if an inductive bell and a capacitive tone caller were connected on the same lane, would be due to the ringing current being shorted out by a combination of the inductance and capacitance of the bell motor and tone caller being resonant at a particular frequency which looked like a short circuit to the ringing current.
But would they not be in parallel, and so draw minimum current from the line ringing supply at resonance?
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Old 8th Aug 2017, 9:52 pm   #8
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Default Re: REN, Resistors &c.

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Simple solution to the low bell situation is to fit a BT socket as used on systems . Don't know the type/part no...
A REN extender? I have a BT one of those: RS used to sell them years ago. They take a 75V 25Hz signal and regenerate it, rather than supply a 25Hz ringing current from d.c. but yes...

An external mains supply is needed, of course.

http://uk.rs-online.com/web/p/teleph...ories/2079519/
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Old 8th Aug 2017, 11:06 pm   #9
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Default Re: REN, Resistors &c.

Evidently the European Telecommunication Standard REN is defined (REN 1) as a simulated ringer detector circuit consisting of a 55 H inductor in sries with a 7 kilohm resistor, and is based on the characteristics of the dynamic impedance of a type 59D bell movement.

From my extrapolation method described earlier I reckon the 59D (the one with the 4000 Ohm @ d.c. bell-movement) is 7186 + j8275, or a Z of 10960 Ohms at 25Hz, which near as dammit fits the spec.
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Old 9th Aug 2017, 4:16 pm   #10
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Default Re: REN, Resistors &c.

I wanted to put a 312 and a modern phone/answerphone on the same extn but when I tried this with the unconverted 312 the answerphone wouldn't answer the call until I "converted" the 312. I presume the 312 was hogging so much current the modern phone couldn't detect the ring. Or something.
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Old 9th Aug 2017, 5:55 pm   #11
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Default Re: REN, Resistors &c.

The insets below may shed some light ono issues of ringer loads, dating
back to the step-by-step days,

As an aside, the wire chief desk test gear included a very sensitive
galvanometer. In days where additional (unpaid) phones were hunted down,
the kick-up of that instrument could detect if extra ringers lurked on a line.

Needless to say, being raised in a telco family, sometimes served well
in days of youth.
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Old 9th Aug 2017, 6:43 pm   #12
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Default Re: REN, Resistors &c.

The above presumably relates to ringers in series, given that this was the norm in the days before plug-in 'phones, where the greater proportion of the ringing voltage would be across a high-impedance ringer. A slightly different situation arises with the present arrangement of parallel ringers where the greater proportion of the available current will pass through the low-impedance ringer, which is significant if the load causes the total voltage to drop. The situation encountered by els1967 may be that the 312 was more tolerant of a reduced voltage than was the answerphone.
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Old 9th Aug 2017, 8:16 pm   #13
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Default Re: REN, Resistors &c.

If the answering machine has a ringing detector which relies on a reverse-biased Zener diode going conductive, then it's conceivable that a high-REN phone in parallel with it could pull the voltage down just too low for the Zener diode to start conducting. It probably will impose some additional loading of its own, just to guard against false positives from noise on the line.
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Old 9th Aug 2017, 8:51 pm   #14
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Default Re: REN, Resistors &c.

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conceivable that a high-REN phone in parallel with it could pull the voltage down just too low for the Zener diode to start conducting. .
The voltage dropped across an unmodified 312 bell (or any 59A bell movement phone) is greater than the voltage dropped across a modified (bell plus 3k3 resistor) telephone: for a 75V 25Hz supply, say, 89V and 78V respectively. But this is without the extra loading of the high-REN answerphone, which I wouldn't have thought to be that severe a load anyway.

I was going to suggest the ringing voltage might be too high, but the spec says 100V down to 40V, so any BABT equipment should be OK.
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