# max amount of phones???

Discussion in 'General Electronics Chat' started by Mathematics!, Jun 13, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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I am wondering how many phones you can install in a house such that all of them will work. ( assuming all the phones are on the same phone number i.e all use green and red wires )

As you add phones in parrellel you are cuting the current down. While the voltage -45 volts DC and 90 volts 20 Hz AC for the ring stay's the same.

I am wondering what the current normal should be and how many phones in parrellel would make it so the current would have to be amplified before the phone's could be use.

I have used a few splice connectors to install a couple of phones. Just wondering about the max phones that can be installed before problems occur. (i.e like ringer won't ring or sound won't be heard...)

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Each phone has a "Ringer Equivalence" number on it. If you're planning on adding more than a couple of phones, call your phone company and give them the total of all of your ringer equivalence numbers.

3. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I have a phone with Ringer Equivalence 0.0B what does this even mean.

And since all the phones are hooked in parrellel is their a min current that each phone must have to beable to hear the voice?

Or is it just the Ringer Equivalence that matters when installing alot of phones?
Because I just read that exceeding the max of Ringer Equivalence could make it so the phone won't ring.
But if you happen to pick it up would you still beable to talk. I would think the voice would be controlled by a min current?

Last edited: Jun 13, 2009
4. ### Audioguru New Member

Dec 20, 2007
9,411
896
A ringer equivalence of 1.0B needs the same ringing power as one old fashioned mechanical ringer.
A ringer equivalence of 0.0B is zero power to operate the ringer.
My wireless phone has a ringer equivalence of 0.1B.

I think the phone company is supposed to provide enough ringing power for 1.0b so it will ring 10 of my wireless phones.

You don't have six phones off-hook and talking on one phone line all at the same time, do you???? the phone system is designed for only one phone at each end. If you have two phones off-hook then the volume at each one is cut in half. If too many phones are off-hook then none will work.

5. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Is their any ratings of min current for the phones to work?

Like if you had so many phones off the hook what would be the min current to a phone such that all of them would still work.

If the ringer equivalence is 0.0B then in theory you could have as many phones as you want on the hook and they would all ring. Yes/No?

6. ### hgmjr Moderator

Jan 28, 2005
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If you go with cordless phones you can expand to quite a few handsets.

hgmjr

7. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
I realize this but I am interested in the above questions for the corded phones.

8. ### hgmjr Moderator

Jan 28, 2005
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I see. Then your question is posed simply out of curiousity about the standard capacity of the incoming phone lines to support parallel phone connections.

hgmjr

9. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Yes..............................................

and since their all in parrellel the voltage will be the same just the current will be cut down.
So that is why I was asking about if their is a min current before all the phones won't work.
And if all of them where on the hook except one would it even matter.

10. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
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Does anybody know?

11. ### Externet AAC Fanatic!

Nov 29, 2005
780
58
About ringing, the REN has been explained above, and how many telephones on-hook can be paralleled is infinite. They are not 'connected' until you lift the handset.
How many can be off-hook at the same time depends on a few factors:

-How many volts are present on the telephone line pair when more than one is off-hook. It is usually 7 to 10 Volts for a single, varies with the distance to the central office in your neigborhood.

-What current is supplied by the telco. It is usually from 30 to 40 mA

-The circuitry and chips used in the design of the telephone itself. Some brands can work down to 4 Volts and 15 mA. Others want more to operate, the typical values are 6V and 20 mA.

-The designed resistance of the internal circuitry of the telephone, being the usual 200 Ohms. But many can be from ~40 to 400 Ohms. Depending on that, will load more or less the telco pair and allow more or less paralleled off-hook telephones.

-If the telephone is telco line operated or uses a power adapter to make its circuitry run from it.

Miguel

12. ### leftyretro Active Member

Nov 25, 2008
394
2
The old originals phones had a off hook nominal impedance of 600ohm, so you could be able to calculate the effect on send/rec volume as additional extension phones are also taken off hook at the same time. Two is usable in my experience after that nobody can hear what grandma is saying.

Ringer requirements should not be a real problem with modern phones as their ringers take little current.

At least that's my take on the subject.

Lefty

13. ### Audioguru New Member

Dec 20, 2007
9,411
896
The voltage for two or more phones in parallel and off-hook is much lower than only one phone off-hook. Then the current for each phone in parallel and off-hook is also lower than one. Then the signal power supplied to each phone is much less than the signal power fed to one phone.

14. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Question 1
I don't see how the voltage is lower all the phones are hooked in parellel?

Question 2
So if all my phones have ringer equivalence of 0.0B does this mean in theory I could have infinity phones and they would all ring fine?

Question 3
If all the phones operate with different resistance , current ,...etc
Then what is typically the most phones you can hook together in parrellel and still have them all work off the hook.

Thanks for the help.

15. ### flat5 Active Member

Nov 13, 2008
403
17
Put a current meter in series with one phone.
Record the current. Play with some resistors and see what happens.

16. ### franzschluter Active Member

Jun 1, 2009
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0
I had this topic and assignment a few years ago in college... There is a typical "threshold" current to operate a phone. In our assignments it was 20mA but it can be different in reality. To know the answer to your question..
Measure the resistance or current from your phone "off hook". If you can give the amount of current/resistance being drawn by one phone then it is easy to compute for a maximum amount of phone units. There are phones in the textbooks that acts parasitic and phones that are active and are not parasitic when off hook. If they are not parasitic phones then you can add an unlimited number of phones but I am unsure if this is only seen as "ideally" only in textbooks. PBX systems for example have active components to maintain the threshold current when off the hook.

17. ### Externet AAC Fanatic!

Nov 29, 2005
780
58
If the off-hook telco voltage supply is limited to say 10V, three telephones will load the telco line because the wires from the central office to your premises have relay coils resistance and the resistance of wires. Those relays engage when the handset is lifted and stop the ring generation, and connects capacitors for the voice path.

+Bat-----uuuuu----------------\/\/\/\/\---------------Tel+
50V
-Bat------nnnnn----------------\/\/\/\/\----------------Tel-

The 50V telco battery goes into relay coils (uuuuu and nnnnn) of about 200 ohms each resistance.
The 2 wires to reach your home have resistances \/\/\/\/\, can be around 1000 Ohms
The voltage available at your 'Tel' will depend on the internal paralleled resistances of all 'Tels' hooked with handset lifted.

With all telephones on-hook, you have 50V at 'Tel'
Lifting the handset(s) places a resistance across; then current flows and relays at the telco engage.

Calculate voltage and current for 200, 100, 67 Ohms across 'Tel' corresponding to 1, 2, 3 equal 200 Ohm units in parallel and you will understand what happens. It is just mathematics.

In theory, yes. But real world may be different.

Typically, three. With decreased audio level as they have to share a single resource (voltage and current) from the telco pair.

Miguel

Last edited: Jun 14, 2009
18. ### mitch2 New Member

Jan 23, 2009
2
0
Q1 - Externet's answer covers this pretty well

Q2 - First, a phone with truly 0.0B shouldn't ever ring. However, if yours does truly exhibit 0.0B and rings, then it has a very small REN.

http://en.wikipedia.org/wiki/Ringer_equivalence_number

Connecting multiples in parallel should work for ringing.

Q3. The North American rule of thumb for telephone installers is 5.0REN maximum, including answering machines, fax machines, caller ID boxes, etc.

Last edited: Jun 15, 2009
19. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
There is lots of information using google about phone networks. I don't know the answer, but I've seen it, just wasn't worth memorizing at the time. A search through here will point out some of the better websites.

You probably already know this, but most of the phone standards predate tubes. Once a standard is set, they are loath to change it (kinda like Masons in that regard). So when you see something incredibly strange that is usually the reason.

20. ### Externet AAC Fanatic!

Nov 29, 2005
780
58
Hello. You have it reversed
If the telephone apparattus is stamped 0 REN means it does not load the 86V 20Hz telco ringing signal, capable of up to typically 5 REN as you correctly say. It can be the case of a 'no load' mosfet gate circuit to sense the ringing signal.
The telco supplies up to 5 REN, the telephone(s) loads it in 0 to 5 REN

If the telco power is 0 REN, then yes, it won't ring.

Example : telephones stamped with 1.0 REN ---> you can connect up to five in parallel and they will all ring if the telco supplies 5.0 REN ringing power

The previous explanations were based in POTS, but modern telephone companies use "line cards" which have somewhat complex electronics and reduced dimensions instead of electromechanical relays to sense the off-hook condition, interpret DTMF, pass audio, ringen and several more functions

By the way 1 REN = 1 Watt at 86V RMS 20Hz. =~ 8KΩ Z Load in USA. I showed a 10K resistor as approximation to fake REN1 at 110V.

The ring detector circuitry in a telephone is nearly impervious to the frequency, 20 to 60 Hz , 20Hz is just the standard set a century ago.
It is nothing else than a 0.1~1μF/250VAC capacitor in series with a bell; or in series with a bridge rectifier to make the 20 Hz into DC to turn on a buzzer or an optoisolator.

Miguel