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Old 22nd Feb 2017, 3:28 pm   #1
els1967
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Default Question about the 3.3k resistor in vintage phones

I was wondering about what sort of wattage was required for the 3.3k resistor installed on vintage phones to allow them to work on modern telco lines. I did find a thread here...

https://www.vintage-radio.net/forum/...ad.php?t=83570

...that answered my question but it did get me wondering about it. Sorry if I'm missing something blindingly obvious but when I plug in the numbers to find the required wattage for the resistor on the ringing current I get...(using a 75vac ringing current)


75vac x 75vac / 3300 ohm = 1.7 watts

so shouldn't the minimum spec be for a 2 watt resistor?

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Old 22nd Feb 2017, 4:16 pm   #2
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Default Re: Question about the 3.3k resistor in vintage phones

I haven't done the calculations, but you've missed out the impedance of the bell coils and the capacitor in the master jack. Also the resistor isn't passing continuous current.
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Old 22nd Feb 2017, 4:42 pm   #3
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Default Re: Question about the 3.3k resistor in vintage phones

It's easier to start off with the typical ringing current of 20mA AC.

Watts = IsquaredR= 0.02x0.02x 3300, which I reckon is 1.3 watts.

I guess that a conservative approach would suggest a 2W resistor, but as ringing current doesn't last long, the rating is strictly intermittent, and a 1W resistor should do the job.

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Old 22nd Feb 2017, 5:17 pm   #4
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Default Re: Question about the 3.3k resistor in vintage phones

Never had a problem with 1/4 W jobbies.
 
Old 22nd Feb 2017, 6:43 pm   #5
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Default Re: Question about the 3.3k resistor in vintage phones

FWIW I normally fit 2W resistors here. It's not as though there's limited space for them. And the extra cost for the few I've fitted is 'lost in the noise'
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Old 22nd Feb 2017, 7:11 pm   #6
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Default Re: Question about the 3.3k resistor in vintage phones

as I recall the bell coils are resonant at 17.6 Hz How does that affect the mathematics? EI7KA.
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Old 23rd Feb 2017, 9:06 am   #7
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Default Re: Question about the 3.3k resistor in vintage phones

Quote:
Originally Posted by merlinmaxwell View Post
Never had a problem with 1/4 W jobbies.
Nor have I. I wouldn't worry too much about wattage.
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Old 23rd Feb 2017, 10:53 am   #8
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Default Re: Question about the 3.3k resistor in vintage phones

Quote:
Originally Posted by space charge View Post
as I recall the bell coils are resonant at 17.6 Hz How does that affect the mathematics? EI7KA.
That would be very surprising because the bell coils would need to have a very high inductance around 40H to resonate at 17Hz with the 1.8uF series capacitor .

In any case, the typical 20mA ringing current figure would still apply.

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Old 23rd Feb 2017, 2:03 pm   #9
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Default Re: Question about the 3.3k resistor in vintage phones

A telephone bell movement (59A) is typically 2740 Ohms Z at 25Hz, or around 1776 + j2085. When the 1u8 capacitor is in circuit, the impedance becomes 1776 - j1451 (around 2294 Ohms Z) and for, say a 90V ringing supply, a current of about 42mA will flow.

The cap in the master LJU will have about 140V dropped across it.

A 3k3 resistor added will result in a series circuit with bell and capacitor (for one telephone ringer) equating to -j3536 (capacitor) +j 2085 (typical bell-bobbins) + 1776 Ohms (a.c. resistance of bell-bobbins including eddy current and hysteresis loss, being in-phase) + 3300 Ohms. This is an impedance of about 5279 Ohms Z.

Assume a 75V ringing voltage and you'll draw a current of about 14mA. Even with a higher ringing voltage pushing 16mA through the 3k3 resistor, you'll only be dropping 53V across it, and it will dissipate a power of just over 800mW. And that would be continuous use!
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Old 23rd Feb 2017, 2:16 pm   #10
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Default Re: Question about the 3.3k resistor in vintage phones

Quote:
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as I recall the bell coils are resonant at 17.6 Hz
The bell-bobbins approach resonance (but are still a way off it) when two are in series through one capacitor. You then have an impedance of 3552 + j634, so not all inductance has been cancelled out by the 1u8 capacitor.

Earlier bells were wound to be optimised at 16 2/3Hz, being one-third mains frequency but I haven't done any investigation of those. 2uF caps were used back then too.

A 59A bell-motor has an inductance, fully loaded with magnetic ironwork, of about 8.6 Henries at 16 2/3 Hz, diminishing to 8.1 Henries at 25 Hz, and dropping to 2.68 Henries at 1kHz. The impedance, respectively varies from 1100 + j900 at 16 2/3 Hz to 12350 + j 16850 at 1kHz. The impedance, therefore at 1kHz (speech) will be 20891 Ohms Z and will not affect speech circuits.

These figures, incidentally, were acquired over a frequency range driving a constant 42.5uA through the bell motor. At a higher current of 3mA, the inductance ranges from 14.4 Henries at 25Hz to 3.7 Henries at 1kHz. I couldn't do 16 2/3 Hz as the PA 100V line-output amp I was using started to wobble a bit!
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Old 23rd Feb 2017, 2:25 pm   #11
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Default Re: Question about the 3.3k resistor in vintage phones

Maybe worth pointing out that I believe this bell-design to be no accident. Most power will be developed in the bell-motor when the resistance is the same as the inductive reactance. As is seen from my earlier post, the resistance of the bobbins is similar to the reactance.
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