Ring Voltage Technical Bulletin
In the old days (a few
years ago?) we primarily dealt with ringing from real phone lines.
Sometimes we'd have to deal with a channel bank or station port on a
phone system, but ringing wasn't much of a problem. The ring voltage
was pretty similar, and most of us never had to worry about or
In general, ringing coming
from every real phone company Central Office looks pretty much the same
(but true rural phone companies can do whatever they need to). It's around 90VAC
at 20 cycles per second, with a true sine wave, at about 5 REN.
Ringing coming from
"fake" phone lines can look very different from manufacturer
to manufacturer, or even from model to model. Channel banks, VoIP
gizmos, cable company phone gizmos, and analog station ports on phone
systems all ring, but there's no way to know how a particular engineer decided to make it ring.
Because these devices only
give you "fake" dial tone, the manufacturers don't have to
follow any standards or regulations. Today's inexperienced engineers are
told to do it the fastest, easiest and cheapest way possible. Often the
engineers designing this stuff have never even used a real phone line -
they might have a simulator and a couple of books. If every piece of
phone equipment looked the same, that wouldn't be a problem. The reality
is that different makes and models of phone equipment look very
different today. Because
most of the phone equipment sold in the US today is made in China or
Europe, very little looks like the old equipment we used - even three
A lot of
the phone systems and phone equipment used in the US actually is made
for European standards - which although similar, are different
enough to cause big headaches for telephone men trying to hook this
stuff up here in the US. In Europe, 60VAC ring voltage is common, so in many cases US
phone equipment connected to phone system station ports that were
designed in Europe just don't work well (because they're looking for a
minimum 75VAC ringing).
In the old days (in the
US) ringing was 2 seconds on, and 4 seconds off. Then distinctive
ringing from the phone company became commonplace, which offered
shorter rings. PBX station ports have often had shorter rings, with different ring
indicating an inside or outside call. The ring cadence is also
different in Europe. There's a possibility that the station equipment
you're installing won't recognize a distinctive (or short) ring.
Your assignment, should you
decide to get paid, is to figure out why the stuff doesn't work - and fix
The easiest thing to fix
is ringing that's too short for the telephone equipment to recognize.
A common fax switch usually puts out the standard 2
seconds on / 4 seconds off ringing, as long as it recognizes the ring. There's a pretty good chance that sticking a fax switch
between the line with the short ring cadence and the telephone
equipment will fix the problem, but it will likely delay ringing for
one or two rings, and will probably eat the Caller ID signal - which
normally comes between the first and second ring here in the US. It's
an easy thing to try to get you going in the right direction.
AC (Alternating Current),
which is used for ringing, means the voltage goes positive, to zero, and then negative by the
same amount over a given period of time (for a phone line ringing at 20
cycles, there are 20
complete cycles per second).
If it's a sine wave, the voltage
positive to negative, making a waveform that looks rounded at the tops and
bottoms (see the chart below).
If it's a square wave
(which is cheaper to make electronically), the voltage goes straight
up to the maximum positive voltage, stays at that voltage for the duration of
that part of the cycle, and then goes straight down to the maximum
voltage (see chart below). On a square wave, the line going from
positive to negative is usually vertical, as opposed to sloping on a
Some real bells (ringers)
have a problem with square waves, so some manufacturers shape the
waves into trapezoids (with extra electronic components) to
improve how their device works with real bells, and to reduce some of
the noise that can be generated by using a square wave (see the chart
below). Most electronic ringers work fine with a
The reading you get with
a particular meter on ring voltage depends on the waveform type. See
the chart below.
It's unlikely that any two of us will get the same
reading measuring the same ring from the same source, since we all
carry different makes and models of meters. That makes tech support
pretty difficult, and makes it important that you use your meter to
take readings at lots of customers where you're not
having a problem - so you have a good feeling about what's normal
and what's not. It's impossible to use your meter for the first time
where you're having a problem (to make a diagnosis), since you'll
have no basis of comparison.
Even worse is that many
modern meters, even expensive True RMS meters, won't read AC voltage
on a DC telephone line. I guess the inexperienced engineers who designed
this stuff just never had to work with phone lines, or measure AC hum on a DC power supply?
When you try to
measure AC on a phone line using one of these screwed up meters,
the display will hunt all over - like from 30 to 100 to 60 to
200 to 100 to 80, etc. If your meter hunts when you put
it on the AC scale on a phone line, you need to get a different meter. Since you
can't test a meter before you buy it, that's sometimes hard to do!
phone man has to be able to measure AC on phone lines these
days! Besides ring voltage,
a phone man has to be able to measure the induced AC on a phone line. When
the AC, measured from tip to ground or ring to ground is over
.5VAC, strange things start to happen on phone systems and devices
that have a reference to ground (AC powered). Battery operated devices
with no reference to ground are usually not effected by AC on the
If your meter starts hunting as soon as you put
it on a phone line on the AC scale, you won't be able to check for
induced AC or ring voltage (CLICK
HERE for an explanation of problems that can occur with
induced AC, in our Longitudinal Imbalance Tech Bulletin).
Meter Comparison Chart
for Various Waveforms
Model 179 (True RMS)
Sidekick T&N (Analog)
60 Cycle Sinewave
20 Cycle Sincewave
CO Line Ringing
20 Cycle Squarewave
20 Cycle Trapezoid
20 Cycle Squarewave
20 Cycle Trapezoid
20 Cycle Squarewave
1. All Ring Voltage Readings Taken with a 1.4 REN
2. Scope readings are calculated RMS
3. Model 179 is a True RMS meter
4. All other meters read Average
AC (the Sidekick is
Copyright © 2009 •
Mike Sandman Enterprises, Inc.
Looking at the above chart
convinced me that taking AC ring voltage readings can be a real problem.
Some phone equipment won't respond to a 60VAC ring. Some won't respond
to a 40VAC ring. Since any two models of meter could read a 10VAC or
more difference, that makes troubleshooting pretty hard!
If you already
have a meter that works, your best bet is to use it to take readings on
many different CO lines, VoIP lines, cable lines, and analog station ports, as you come
across them. Make yourself a little chart with all the readings.
When you come across a ringing problem, you'll be able to compare the
readings on other systems to the one you're having the problem with, as
a sanity check.
If you use the meter for the first time at the premise
where you have a problem, you have no basis for comparison and you really can't
trust the readings your getting (and the readings aren't particularly useful to
a tech support guy, unless he's familiar with that particular model of meter on
that particular type of phone system).
A very good technical
explanation for variances in AC voltage measurements was put out by
(formerly HP), who makes their own meters ranging from
handhelds to lab equipment. I was amazed to read that meter
manufacturers calculate True RMS by reading the heat produced by the
AC voltage (4 page PDF file):
Application Note on Errors in Digital Multimeters (PDF)
Voltage and waveform is
only half the equation in dealing with ringing problems. The amount of
current that the phone equipment uses, and the amount supplied by the
line (or equipment supplying the ringing) can also cause problems.
Since most people would have a hard time measuring AC ringing current
with a meter, the phone company came up with the REN (Ringer Equivalence
1 REN is the amount of
AC current that's used by an old Bell type 2500
set with mechanical bells. The phone company has traditionally
supplied about 5 REN from the CO - enough to ring 5 of the old
fashioned phones. Since AC ringing current is limited at the CO, if
you put 6 REN worth of phones on the line the ringing will either stop
on all of the phones, one or more phones will sound very weak, or some
phones will ring and some won't.
If you have a meter that
will read AC current on a phone line (AC ma), you can use an old 2500
set, which uses 1 REN of current, to measure how much current it takes
to ring the bell on that set on your meter. Once you know how many ma
of current it takes for 1 REN, you'll be set to figure out how many
REN a particular phone takes, or how many REN a phone line provides
(before it stops ringing).
Using an analog port on a
PBX in our office, we used the Network Meter™ to measure the
current it took to ring an old AT&T 2500 set. It took 8.75ma AC.
Then we measured the current it took to ring a Chinese phone we sell.
It measured 8.26ma AC, but the phone said 1.4 REN on the label (if it
was really 1.4 REN, it should have read 12.25ma AC). I guess I'll
believe the meter, and figure that each of these phones is really a
little less than 1 REN.
We then kept bridging more
Chinese phones to the PBX analog station port. It stopped ringing when
we plugged in the fourth phone, which means that particular station
port provides around 3 REN of ringing. When you put 4 REN on the port,
the electronics in the PBX shuts down the ringing. The same thing
would happen on a POTS line from the Phone Company, at about 5 REN.
Now, picture switching
from the real phone company to a VoIP or cable phone company, where
the ringing comes out of a box connected to the Internet or TV cable.
Some of those boxes supply 5 REN, but some of them supply only 1 or 2
REN. That means that although the phones all rang well from the phone
company before, they'll stop ringing when the ringing is coming from
the VoIP device if it supplies less than 5 REN (unless you unplug some
You'll find specific
instructions on measuring REN yourself a little farther down the
The REN number is printed
on the bottom of most phones, but newer
products might show the REN number inside the FCC
registration number. In the registration number US:AAAAAnnBXXXXX, nn is
the REN times 10. Just think of it as n.n REN.
If the registration number
is US:1X23T07A12345, the REN is 0.7. In the new style FCC registration
number, there are 5 characters after US, then the 2 digit REN without
the decimal point. The letter following the REN is almost always A, B
The REN on telephone
equipment can be as low as 0.0 REN (it uses almost no ringing
sometimes as high as 2 or 3 REN (usually older equipment).
phones don't ring properly, add up the REN numbers on the bottom of
everything plugged into the line, and make sure it's less than 5. The
problem is that if it's not a real phone line, you probably have no
way of knowing how many REN the device puts out, unless you measure it
yourself. Some VoIP and cable
devices put out 5 REN of ringing, but most put out less. Unplug the
phones, one at a time, until the ringing starts to work. Few
(if any?) analog station ports on phone systems put out 5 REN.
So how many REN will a
particular device that provides ringing provide? If it doesn't say in the manual or on the box, you'll have to
call tech support (which could be very frustrating if it's a VoIP or
cable company!), or calculate it yourself.
One thing to be aware of
is that the more REN (ringers) are on the line, the lower the voltage
is going to be when you read it with your meter.
The readings on the above
chart were taken with phones totaling 1.4b REN on the line. The
voltages would be higher in all cases with no telephone equipment
connected to the line - just the meter. With 5 REN worth of phones
connected, the voltages would be quite a bit lower. If you're going to
take readings, it would be best to compare apples to apples, and
disconnect the inside wiring when you measure ring voltage.
If you're close to a CO
or SLC, the AC ringing current can be higher than normal, just like
the DC loop current. The phone company wants to make sure the pairs
will work as far away from the CO or SLC as possible, so they leave
everything cranked up. They usually won't adjust it down if you ask
them, since there's no procedure to adjust it back up when that
subscriber cancels and the line card is used for a different
subscriber (who may be farther away from the CO).
Like high loop current,
high ringing current can damage components on a CO line card
(usually through heat). If a premise is near the CO or SLC, it's
likely to have both high DC loop current and high AC ringing
current. Our Loop Current Regulator™ reduces both the DC loop
current and AC ringing current, so it's definitely something that
needs to be installed if the loop current is high (some NT CO Line
cards have a problem with trunks burning out and going low volume if
there is high ringing current on the line).
You could reduce the AC
ringing current getting to a phone system by putting maybe four 2500
sets bridged onto the line with the phone system. The bells in the
phones will use some of the current that would otherwise go into the
CO line card and damage the components. That still leaves you with
high DC loop current, which will also damage the CO line card, so a
Loop Current Regulator™ makes more sense than screwing around
bridging phones to the line.
REN of all the phones on the line, or the available REN on the line, yourself...
You measure how
many REN worth of phones are on a particular phone line with
a digital meter that reads AC current
(ma). Just find an old real ITT or AT&T
2500 set with a double gong ringer. The oldest
phones won't have a REN number on them, but the old double gong
ringers are the gold standard that created the REN measurement.
One double gong ringer is 1 REN.
2500 set from 1973
ITT 2500 Set
(Usually no REN value listed)
Put the old phone on
a real phone line (don't use a station port or VoIP ATA since it
might not put out enough current to ring one phone).
Put your meter on
the AC ma scale (not DC). Your meter may have a different
banana jack for the red lead to measure milliamps, or maybe
just for AC milliamps. Be sure to look at the designations on the
Put the meter leads
in-series with either the tip or ring going
to the phone. It must be in-series,
not in parallel (which will short the line) to measure the
current traveling to the phone. If you put the meter in-series
with the tip, the current will come out the tip side of the
phone line, in and out through the meter leads to the tip
connected to the phone, through the phone and back to the ring
side of the phone line.
Call the phone line,
and you'll see the current it takes to ring the old 1 REN phone
(attached in-series after the meter) displayed on your meter.
The reading you see
will be the amount of current your meter reads for 1 REN. Record
that number, because that's going to be the current for 1 REN
that you're going to use to calculate the REN of phones attached
to a phone line, or how many REN the phone line is capable of
To measure the REN
of all the phones on a phone line, connect your meter
in-series with all the phones, ring into
the line and observe the ma reading on your meter. The more
phones that are attached after the meter, the more
ma (current) you'll see.
When you get the
reading for all the phones, divide it by the reference number
you recorded for 1 REN. That's how many REN the phones are using
on that phone line (like 20ma divided by 8.2ma = 2.43 REN)
meter may show 8.75ma AC with an old 2500 set. If you put four 1 REN
phones on the line and they all still rang, your meter would show
When you add the fifth
phone, the ringing may stop on all the phones because ring current
is regulated at about 5 REN from the phone company. When you draw
too much ringing current, you'll see the current drop back down
("fold back") to maybe 10ma - in which case none of the phones
will ring because there's not enough current.
Adding a load
(like one phone at a time) and measuring the amount of current the
phones are drawing after you add each phone is the only way to
measure the available
ringing current on a phone line or station port. You can't measure the available REN on a
phone line or station port by simply putting the meter's leads on
the tip and ring while the line is ringing, since the meter will
short the line and answer it as soon as you put it on the line.
The only way to measure
available REN is to know what your meter reads for 1 REN (above),
then continue to add phones until the ring current folds back - and
the phones stop ringing.
If you have our Network Meter™ (below), it should read about 8.75ma
for 1 REN on many phone systems and phone lines, but the REN number
is going to be different on different phone lines with different
phones. That's because the REN number really takes into account the
impedance of the phone line or phone system providing the ring
voltage, which you can't measure in the field. This will certainly
get you in the ball park.
old 2500 set ringer to ring on lower voltages...
The old 2500 sets with
double gong ringers, and 2554 sets (wall phones) with single gong
ringers, have a small bias spring to adjust the clapper on
the bell. The bias spring was normally shipped in the high
position so the bell wouldn't tap when a rotary phone was dialed or
another phone went on and off-hook - which can put out a spike
that's enough to move the clapper a little.
If you have less than
the standard 90V AC ringing and would like to make an old style
phone ring, you could try setting the bias spring to the low
position to see if it rings.
Bias Spring in Low Position
Spring in High Position
Modern phones with
real bell ringers probably don't have a bias spring. The Chinese
do a good job copying stuff, but I'm pretty sure they had no
idea what that wire was for... so they never designed it into
the cheap phones they make. Too bad, since most of the stuff
they make has pretty crummy ring voltage.
If a phone
doesn't ring with a fake phone line, try this...
Some phones say 0.0 REN on the bottom of them, which essentially
means they don't take any current from the phone line when
ringing. If your phone says 0.0 or maybe 0.2 REN, try bridging a
second phone that says 1.0 or so REN onto the pair.
Some fake phone
lines won't put out any ringing if they don't see any current
being drawn. This includes VoIP ATAs (Analog Telephone
Converters) and analog ports on phone systems.
Ring Voltage and Ringing Current if it's low...
If you aren't getting a
high enough voltage or enough REN from the device supplying the
ringing, we sell Ring Voltage Boosters™:
Our Ring Voltage
Boosters™ plug into an AC outlet, and go in-series
with the device that's providing the ring voltage and the telephone
equipment. The Ring Voltage Booster II™ and 25 Line Ring Voltage Booster™ put out 90VAC RMS at 7.5 REN
with a true SINE WAVE.
The Ring Voltage
Booster II™ and the 25 Line Ring Voltage Booster™ can boost ring voltage
as low as 30VAC RMS.
If you need to supply 8 REN, you would split the
stations so that half would be connected directly to the phone line
(with the input to the Ring Voltage Booster™), and half would
be attached to the Ring Voltage Booster™ itself.
You can see lots of
information on testing and fixing telephone line problems on our Testing
& Fixing Telephone Lines page.
If you don't currently own a
meter that reads AC on a phone line, we've done a lot of searching to
find one that would work. It's the Network Meter™ in the list
above, and it's a great inexpensive meter (it's not True
Network Meter™... It's a DVOM and CAT5 Cable Tester!
each, or 3+ at $79.95 each
The Network Meter™ (above)
at least as good at reading AC ring voltage as the $800 to $1,000
Model 5 or Sidekick T&N Meters, and it's accurate on all of
the other ranges too - including DC ma (for loop current). It's got a
nice bright backlit display.
The Network Meter™ comes with a holster with a
police style belt clip, leads with bed-of-nails clips, regular meter
leads, and a case.
It took me over a month of
research to find a reasonably priced meter that reads AC on top of DC
correctly (I bought a lot of meters!). This one is particularly useful
for phone men since it has the Cable Testing feature and bed-of-nails
Butt-set type clips for the meter, basically at no extra cost.
We also sell the Economy
Network Meter™, which is the same meter without cases or Butt-set
type clips for $44.95, or 3+ at $39.95.
If you look inside most
modern meters, there's almost nothing in them. Besides the fact that
all of the components are surface mount (which are very small), the
bulk of the features are controlled by an ASIC (Application
Specific Integrated Circuit),
which is a microprocessor with everything pre-programmed, and then stuck on the
board... they don't buy an off-the-shelf microprocessor from a company
and burn their program into it. An ASIC usually looks like a blob of
black glue stuck to the board, which is protecting the actual
microprocessor and memory (you see a similar thing in one of those
greeting cards that plays music).
Why 90V RIng Voltage?
A lot of guys wonder why the phone
company uses 90VAC at 20 cycles, and the power company uses 120VAC at 60 cycles? Why doesn't the phone company use 120V at 60
cycles? Well, phones became popular before the power
companies used AC voltage. Edison's first
power plants were DC, not AC. Eventually Edison's competition forced him
to use AC, which turned out to be much easier, cheaper and safer to
distribute in towns than DC.
The original phones used a
crank on the side which ran a small magneto to send the ringing signal to the
other end of the line (normally the phone company's switchboard). 90 Volts
AC at 20
cycles were reasonable numbers considering that a human had to spin the
crank by hand, and the bell was a big clapper that had to swing up and
back to hit two gongs (if it went too fast, you'd get a clapping or
buzzing rather than a ringing sound).
Switchboards often used
"drops" in addition to a bell or buzzer, which were little
spring loaded doors that were released by the ring voltage sent from
the magneto on the calling phone. Later switchboards had lights next
to the jack for each subscriber, instead of the mechanical drop.