Author Topic: Resistors in switchboard circuits  (Read 1094 times)

Offline RB

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Resistors in switchboard circuits
« on: March 22, 2017, 03:01:28 PM »
Hi.
I am reading a lot about switchboards...and learning a little.
My question for the day:
Are there any examples of RESISTORS in schematics for Kellogg crank switchboards?
or for any other type using crank phones to communicate.
I refer to the small ones "supposedly" located inside the board. I have not seen this yet.
and used to lower the voltage to the subscriber phones when used in common 24 volt batt installation.
also, used to lower voltage for the supervisor lites etc...
I believe this is accurate, but, well... still learnin a bunch about these things.
Regards.
Rod

unbeldi

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Re: Resistors in switchboard circuits
« Reply #1 on: March 22, 2017, 03:37:12 PM »
A single resistor is used to limit current, not voltage.
But multiple resistors may be used to design a voltage divider, based on the concept that the resistor has a voltage drop across it, when current flows through it.  But when no current flows, the voltage before and after a resistor is identical, there is no voltage drop across the resistor, according to Ohm's Law:  E = I x R.

Old switchboard usually don't have many resistors, but the need does arise for limiting line current and for those purposes they often employ tungsten lamps, they were called resistance lamps.

I think it would help answering your question if you cite your source for "supposedly located...".
I suppose you mean a manual magneto switchboard with "crank switchboard" ?  Typically, resistance lamps are only used in powered common battery switchboards, I believe.
« Last Edit: March 22, 2017, 03:39:48 PM by unbeldi »

Offline RB

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Re: Resistors in switchboard circuits
« Reply #2 on: March 23, 2017, 09:22:45 AM »
Hi Unbeldi, Thanks for the reply.
A little background...
Always a tech. limited only by my tiny brain, always lookin through a glass ohm meter, if you will.
What can I do with that?
Well, I have 3 crank phones. and two grandchildren.
Why not a switchboard? Sooo...
I am building one so my grandchildren can experience a piece of history sadly gone from life.
And so they can talk to grandpa, and each other. "home day care".
I am building my switchboard based on a pic in the CAphonehandbook.pda, page 137
My situation, I don't have nore want simple...it just does not work for me, too many limits.
I need to be able to connect almost any type of phone together...crank type that is...
Thats why I am building a board.
The Type A model 6 switchboard allows both 2 and 3 wire connections. grounded, and metalic. as I understand it any way.
So I am basing my board on that one.
Now the hitch... I don't have resistor lites, and correct relays/drops, from the original systems.
I need a work around.
So, Reading what I can find about these old boards, I hear about resistors, and resistor lites.
I have not read that deep yet, head is still spinnin from all the variances in these things.
My board will use regular lites for the "line busy", and or ring off indicators.
I have the tip and ring lines wired.
I am now working on the sleeve circuit.
I understand touching the tip of a cord to the sleeve of a subscriber, would light a lite if the sub phone was off the hook...???, or on the hook?
That's my goal for this part.
I need the lites to lite when the call is terminated.
I thought from my reading, that resistors were used for part of this circuit. but cannot find any examples.
Maybe I just need to read some more? for simple circuits, these things get complicated.
Sorry about the book...any help is appreciated!

unbeldi

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Re: Resistors in switchboard circuits
« Reply #3 on: March 23, 2017, 10:49:00 AM »
Ah, now I remember your earlier posts about building a switchboard.  I didn't make the connection until now.

I think you have a lot of stuff to learn, don't be offended, please, but good preparation and a clear concept is needed to succeed with such a project.

I thought about you when I saw an eBay sale recently of about a dozen switchboard key panels that likely came out of a cordless switchboard, but the cost was rather high.  IIRC, it was around $200, certainly more than I would pay for a small board.   I got my cordless Kellogg board for only $50.

The first concept you have to understand is the difference between a local battery magneto switchboard and a common battery switchboard.

Local battery (LB) magneto boards don't have indicator lights, they don't have any power supplies to provide the current.  Supervision is solely by using the generator to alert the operator, in which case a mechanical signal, such as a 'drop', is activated on the board, and the user is required to 'ring off' when done with the call, to alert the operator to disconnect the call. The operator alerts the user of an incoming call by also cranking the generator so that the subscribers bell rings.

Indicator lights are activated on common battery circuits when the subscriber goes off-hook, completing the circuit so that current flows to power the lamp.

For running just magneto ("crank") telephones you should build a local battery magneto board.  There were two principle varieties of those.  The switching can either be accomplished by plugging cords into the jacks connected to the telephone station lines (corded switchboard, or cord board), or by mechanical keys that use switch contacts to make the connections. These are called cordless switchboards and don't have any jacks or switchboard cords on the front panel.  While easier and perhaps less error-prone to operate, cordless boards were more expensive and were built for only small installations for typically up to twenty stations.  The most typical boards had perhaps seven to ten stations and two or three trunk lines for outgoing calls.

I would recommend reading one of the classic handbooks on manual switching, written by the contemporary experts of the field.
Here is just a small selection, some of which should not be hard to find online.

*Miller, Kempster (1905) 4th edition: American Telephone Practice
*Miller, Kempster (1933) Telephone Theory and Practice Volume 2; Manual Switching and Substation Equipment.
*McMean S.G., Miller K. (1912) Telephony
*Cyclopedia of Telephony and Telegraphy (1919) Vol 1 of 4
*Principles of Electricity applied to Telephone and Telegraph Work (various editions 1928 to 1961), AT&T Long Lines Department

As a sample, I attached the chapter of McMean & Miller about simple magneto switchboards as a PDF file.  Perhaps this helps.  With some search you will find many other texts that discuss the principles.

Coming back to resistors. All switchboards and telephone circuits require certain resistance levels for proper operation.  In the early switchboards most of these requirements in the switchboard were achieved by proper design of the relays involved for switching and signaling.  A resistor basically wastes energy, so it is better to let the current do useful work, such as dropping a signal flap, while being wasted.

Getting an old switchboard working again, in conjunction with some telephones, is fun and a rewarding experience.  Building one from scratch is challenging, despite the apparently simple principles involved. I commend you on your ambitions!

« Last Edit: March 23, 2017, 11:26:44 AM by unbeldi »

Offline dsk

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Re: Resistors in switchboard circuits
« Reply #4 on: March 23, 2017, 11:32:57 AM »
I'm not sure I will recommend such project for only 3 phones, you may need one of them for the exchange service, and then...
An the other hand, the simplest working circuit I know is to this x-German exchange: http://tinyurl.com/lqv8e74  (more documentation linked)

I would probably go for ring signals e.g. 1, 2 or 3 rings, and all 3 phones on the same pair.

dsk
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Offline RB

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Re: Resistors in switchboard circuits
« Reply #5 on: March 23, 2017, 11:50:14 AM »
Thanks for the replies. I started out to build a simple crank board. but I don't have any drop parts, so no rattle.
I have to use lites. so, it got more complicated.
Thanks, Unbeldi, for the doc.
there is more info there, than I have found so far. so more reading.
I have a pretty good handle on crank/two wire connectivity.
However, finding the correct parts to make it work...well...
So, I decided a common batt board would be easier to work with...I say easier...
it is a bunch more complicated! but, I will get there.
can you tell me how, if the sub phone runs locally on about 3-5 volts, how did they kick that down on a common batt board?
or was the 24 volts common to all sub phones as well as the board?

Offline dsk

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Re: Resistors in switchboard circuits
« Reply #6 on: March 23, 2017, 12:39:03 PM »
If you are going for a common battery board, the LB telephones will not be suitable.

If you build a board like this , using regular 2 pole on/off switches as TT and stereo plugs/jacks, you will almost there, a battery lamp (or led) circuit with an thyristor as relay, you could get it working.  the thyristor holds the current after receiving a signal, until shorted, or power is removed. You have to make use of diodes, and resistors to tune in the sensitivity.  It should not trig far to easy, let say it should have at least 25V to trig, and still dont break down at mains voltage.

To your last question, the voltage at the subscribers end is not important, but to feed the transmitter with a suitable amount of energy the current should be 18-50 milli amps, where 25 should be ideal. This will often cause a voltage at the phone of something like 5 Volts.  The same current will be used in a relay to indicate off hook.  Coils or relays in both lines offers high resistance (impedance) for the AC/sound part of the signals, so the signals will not be weakened to much.  Suitable battery voltage for such circuits will usually be from 24-60V

dsk
« Last Edit: November 13, 2017, 10:31:32 AM by TelePlay »
Please notify me of dead links in my postings.

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I have even got a regular New York number :-) 646 570 1796

unbeldi

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Re: Resistors in switchboard circuits
« Reply #7 on: March 23, 2017, 07:02:36 PM »
Thanks for the replies. I started out to build a simple crank board. but I don't have any drop parts, so no rattle.
I have to use lites. so, it got more complicated.
Thanks, Unbeldi, for the doc.
there is more info there, than I have found so far. so more reading.
I have a pretty good handle on crank/two wire connectivity.
However, finding the correct parts to make it work...well...
So, I decided a common batt board would be easier to work with...I say easier...
it is a bunch more complicated! but, I will get there.
can you tell me how, if the sub phone runs locally on about 3-5 volts, how did they kick that down on a common batt board?
or was the 24 volts common to all sub phones as well as the board?

Local battery telephones are fundamentally different in design than common battery telephones.  You do need to learn the difference first.
A local battery telephone does not work on  common battery switchboard, and vice versa.
If you want to use your existing magneto telephones, you need an LB magneto switchboard.

The lack of historical parts is always difficult, but they do show up.  But if you don't mind modern parts and technology, a traditional electromagnetic drop can be replaced and simulated with an LED circuit.

This forum post contains a book (Understanding Telephone Electronics by Bigelow) that you might find useful for the principles of the telephony.

Victor Laszlo

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Re: Resistors in switchboard circuits
« Reply #8 on: March 23, 2017, 07:15:39 PM »
Originally, and historically, LB telephones did not require or use a switchboard. The phones were installed in parallel on a pair of wires (originally one wire) and were used to intercommunicate among the members of the party line, generally farmers in a very rural environment.  It was only after several lines were run in a radiating footprint from a central location (a town center of commerce, as an example) that the parties of one line, needing to communicate with the parties of the other lines, joined forces and installed a switchboard and hired an operator to act as the "central."

To build, from scratch, a switchboard, either LB or CB, is going to be a daunting task. You would be better served to allow the grandchildren to use and understand the phones as they were originally intended, that is, using coded ringing to call, and one long ring to "ring off" at the conclusion of a conversation.

The alternative is to find a military switchboard that was designed for magneto phones. They are available occasionally on Ebay and at telephone collector meets.  Look for, as an example, the Signal Corps model SB-22.

http://www.prc68.com/I/SB22.shtml#Compatible


unbeldi

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Re: Resistors in switchboard circuits
« Reply #9 on: March 23, 2017, 07:39:38 PM »
Originally, and historically, LB telephones did not require or use a switchboard. The phones were installed in parallel on a pair of wires (originally one wire) and were used to intercommunicate among the members of the party line, generally farmers in a very rural environment.
Yes, in a previous thread, we already went through that.  It would be a good way to start some experience with the technology, of course, if a switchboard is actually needed.

Offline RB

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Re: Resistors in switchboard circuits
« Reply #10 on: March 24, 2017, 11:49:53 AM »
Thanks guys for your insight.
very helpfull!
is is a long journey, but fun as can be at the same time.

Offline dsk

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Re: Resistors in switchboard circuits
« Reply #11 on: March 24, 2017, 01:06:43 PM »
You have to hurry up before your grandchildren grows up  ;D This page has some ideas about thyristors, this may help replacing the indicator http://www.electronics-tutorials.ws/power/thyristor-circuit.html, and the extreme simplified circuit her does really work so you may use it. It is still magnet, and you will need one of your phones as operators set. 

Please hook up the 3 phones on a common line until you have your exchange running, maybe they will get and keep interest for old technology.  ;)

dsk
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Offline RB

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Re: Resistors in switchboard circuits
« Reply #12 on: April 07, 2017, 11:50:19 AM »
Hi All...
Still at this switchboard project. Thanks for all your help thus far, the project progresses.
This is the next part of it. A 20-A repeating coil, wired through a 4 pole 2 throw rotary switch.
As I understand it, this pic shows how to wire a Repeating coil for CB or LB application.
I will be installing this in the rear of the board.
Please take a look at it, and tell me if I am on the right track.
also, I see caps used for various reasons, too, anywhere from 1/2 mf to 2 mf.
Could you explain to me the reason you would use a 1/2, 1, or a 2mf cap in this application?
I assume, it has to do with the complete circuit, board and sub set ... but not sure.

unbeldi

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Re: Resistors in switchboard circuits
« Reply #13 on: April 07, 2017, 12:14:28 PM »
Hi All...
Still at this switchboard project. Thanks for all your help thus far, the project progresses.
I would think it will take quite a bit longer yet...   :o
No reason to apologize.

Quote
This is the next part of it. A 20-A repeating coil, wired through a 4 pole 2 throw rotary switch.
As I understand it, this pic shows how to wire a Repeating coil for CB or LB application.
I will be installing this in the rear of the board.
Please take a look at it, and tell me if I am on the right track.
also, I see caps used for various reasons, too, anywhere from 1/2 mf to 2 mf.
Could you explain to me the reason you would use a 1/2, 1, or a 2mf cap in this application?
I assume, it has to do with the complete circuit, board and sub set ... but not sure.

You are going about the project the wrong way; this way it will take many years to get a complete board working, perhaps never.
You cannot start with individual components.  You need to start with a high-level design with features and functionality in mind.
You need to first define what your system can do, how you are going to operate it based on your needs or ideas.

It is ok to experiment with parts, but it seems you need to learn the basics of electronics first, the functions of resistors, capacitors, and inductors, and transformers, Ohm's law at least, and the basics of impedance, reactance, inductance.

We scanned and posted as a PDF file a  Bell System training manual on the forum some time ago: Electricity for the Telephone Man
It has the basics, and provides pictorial examples of concepts and measurement setups.

A little more advanced is this book, cheaply found on EBay:  Principles of Electricity applied to Telephone and Telegraph Work

Spending a few weeks or months learning and planning, will save you years in failed experiments.

Victor Laszlo

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Re: Resistors in switchboard circuits
« Reply #14 on: April 07, 2017, 06:02:02 PM »
Along with the books mentioned, which are certainly good reading to acquaint yourself with the basics, you might consider "jumping ahead" to BSP's for WE magneto boards, or TM's for military signal corps magneto equipment.

Looking at your diagram, I'm not clear what it is you're trying to recreate.  And "recreate" should be your goal, not "create."   In other words, "reinventing the wheel" which is always a time-waster.  For starters, there is a very exact, and very proprietary, method of drawing telephone circuits. Most of the major manufacturers in the US use the same conventions.   For starters, the Tip and Ring of any circuit need to be labeled. There are conventional symbols for capacitors, resistors, lamps, keys and coils. Contacts are generally shown in the "detached contact" method, with an X indicating a make contact, and a - indicating a break contact.  Take a look at any BSP for a telephone or key telephone unit and you will get the picture. We can help better if we all speak the same language.

You mention sleeve leads, but there are no sleeve leads necessary in basic magneto service.  Magneto service, using drops for indicators, was invented before the availability of reliable miniature lamps. Lamps use electricity from a supply. Supplies were not available and reliable when magneto service was introduced.

You asked about touching a cord tip to a station jack sleeve. That is a busy test used by operators at a CB cord board that is not equipped with station lamps, such as a 552 or 556 used in association with a dial PBX.   A station sleeve is open when a station is idle. When a station is busy, its sleeve is grounded through a relay to battery. Since the cord tip is at ground, and there is a small potential difference between it and the grounded station sleeve, the operator will hear a click in her headset when she touches the tip of a cord (with its associated talk key operated) and the station jack sleeve. That indicates that the station is busy and she should  not plug in, which would interrupt the call.

You mention "resistor lites".  Do you mean, perhaps, "resistance lamps?"  They are used in series with machine- or hand-ringing supplies to absorb generator at the moment that a called station goes off hook and before the supply can be released. There is one per switchboard. A standard Edison base lamp can be used as a substitute. Resistance lamps are generally available from collectors or on auction sites.

If you are puzzled by the capacitors you mentioned, that puzzlement indicates to us that you need to educate yourself in basic telephony.  Look at a diagram for a typical LB phone, then look at two of them, wired together. Trace a call through them, both signalling and talking. Then take a big jump and look at a diagram for a LB switchboard and trace the progress of a call through all three items.

You are to be commended for your eagerness to learn telephony, but as you can see, I hope, there are, above, 300 words, written by just one guy, about a tiny sliver of the world and history of telephony. For you to learn, and then apply, everything that you need to know about just one little aspect of the whole realm of this art, would take a lifetime of experience.
« Last Edit: April 07, 2017, 07:19:33 PM by Victor Laszlo »