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Where to Measure Voltages?

Started by Lewes2, September 16, 2014, 12:30:07 PM

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I read a thread somewhere I believe in this section regarding line voltages from Vonage VOIP routers.  There was discussion about 24 volts on newer routers versus 48 volts on older routers - I believe the discussion was in a ringer volume thread but not sure. 

So, my newbie questions:  Are these voltages AC or DC.  I'm thinking the latter.

Where does one measure the line voltage from the router?  Contacts on the Network or . .  .? 

How does one measure the voltage a ringer is using?  How does one measure REN? 

Is there a diagram (for lay people) where to take various voltage and continuity measurements -



The operating voltage (I.E. Talking, dialling, etc., not Ringing) is DC, and you can measure that just by using a multimeter set to DC and connect the probes somewhere easily measured (E.G. where the L1 and L2 wires connect), the same method can be used to measure ringing voltage, but set your meter to AC for that, and keep you hands clear as the ringing voltage can often be as much as 100 Volts depending on the device or line used... :)

As for REN, I'll let someone answer that, I'm still confused by it!!! ;D


I am sure Vonage routers where discussed in various places here and I have not taken a survey, but here is one of my posts with measurements:

A modern telephone jack, on the wall as well as on a Vonage ATA, uses the RJ11 modular jack. It is a 6P2C female connector, which means it has a total width of six pin positions, while only the center two pins are used. However, most of the time, there are actually four pins installed.   I have a VDV23 which has four pins. On this model of ATA, the two outer pins are unused. Vonage also makes a VDV21 and a VDV22 ATA and these units do use the second pair of pins for the second line that may be activated. On the first jack, the first pair is on the center pins, the second line on on the outer pins, while on the second phone jack the second line is on the center pins, reversed from jack no. 1.

Considering only a single-line jack, the center two pins are the tip and ring leads. Typically they are color-coded as green and red, respectively, or white/blue and blue, respectively.  The standard central office battery voltage from tip to ring is -48 V direct current (DC), and tip is usually close to ground level but may still be slightly negative.

Many voice-over-IP ATAs don't reference the leads to ground at all. The levels are free floating. Some ATAs reverse the polarity of the pins, because they assume the user should plug the ATA into a house wall outlet that is without service from the telephone company, to connect to all telephones in the house on the existing wiring. When doing so with a standard modular cord, the pins get reversed again. However, this really doesn't matter any more for modern telephones.

Some ATAs indeed operate at a lower DC voltage, but most, I believe, use 48V, since that is nowadays a DOCSIS compliance specification.

So, you want to measure with your meter set to DC VOLTAGE (range of at least 50V) between the center pins of your outlet and you should read anywhere from + or -24V to + or -48 V maximum when no phone is connected or all phones are on-hook.

It is also useful to measure directly in the phone on the L1 and L2 terminals, to verify that the telephone cord is good. The voltage should be very close to the same value, as it is unlikely you have very long wiring in the house.

When the phone is taken off-hook, the voltage should drop to about 4 to 5 V as the resistance of the phone permits current to flow which should be anywhere from 20 mA to as high perhaps as 100 mA. Most carbon granule transmitters won't work well anymore if there isn't at least 20 mA of current available.

Ringing voltage can be measured in the same places, but it is alternating current (AC) between 50 and 90V, typically.  ATAs typically don't provide the high voltage anymore that used to be the standard. The frequency of ringing is typically 20 Hz (hertz) and this poses a problem with some meters.  Many AC voltmeters that are intended for general electrical work only work accurately in the 50-60 Hz range, which is what the power company delivers world-wide.

So you want to be sure to use a high quality meter, preferably one that measures true root-mean-square (RMS) values at any frequency.  RMS measurements reflect the voltage that corresponds to the true power dissipation of a device.  The values measured are approximately 71% of the peak value during the AC voltage cycle, which is the value you might get from an oscilloscope.

REN is a measurement unit of the AC load that a telephone represents to the ringing source.  It is a complex and somewhat obscure measure, because it measures the AC resistance and the capacitance that a telephone represents in one simple value. It is a historically derived quantity that was originally set to be approximately equal the load that a 500-type Western Electric telephone represented.  Later, it was more formally defined by the FCC and ANSI in terms of electrical conditions.

The typical CO line can support a total load of 5 REN, meaning that one can run up to five telephones that each have a REN 1.0 ringer equivalence.  Many, perhaps most, VOIP ATAs support less than that, and the value of 3 REN is probably more common.

Measuring REN of a telephone is a little more involved, as one really needs to use a test set that outputs the right frequency and voltage for this test.

In the picture below, I am showing such a device, a 3M Dynatel 965 Subscriber Loop Analyzer.   It is a nicely packaged rugged box that combines several common telephony measurements and can be bought on eBay quite cheaply, cheaper often than a good all-purpose multimeter.  It can measure line voltage and current, ringer impedance, REN, and more.

PS: If anyone is going out now to look for one of these 965 units, make sure to get the accessories, at least the four-conductor probe cable. The cord and the tool are available separately, but often at a much higher price than the whole instrument. The unit is self-calibrating in such a way that it assumes it has the proper original test leads connected. The terminating contact tool is used in this procedure. I remember looking at quite a few eBay offerings before I found one that had the tool.


Wow!  A good lesson in basic telephone electronics.  It's perfect for me.  Thank you!  And you too, twocvbloke.  All this is really helpful.

I had to laugh, Unbeldi, because the thread to which I was referring but couldn't remember is the same one you mentioned. 

The Volt/ohm meter I am using was a hand-me-down from my dad.  It's a WW2 unit he picked up at Sandia base back in 49 or 50.  He was a Ham and collected trunks of excess equipment the AF was disposing. (I can remember ten or twelve, 36"x36" aluminum shipping trunks packed with radio and electronic gear.  Just before he passed away, he donated all that equipment to a Ham radio club in southern California.)  Long story to say I need to test the meter to see how it does. 

I think I'm going to end up with more than five ringing phones on our system.  As you mentioned, I have one output from the Vonage router feeding our prewired telephone network.  I believe I read (may have been in the same thread) that the ringers in the 302 and 202s have higher RENs.  I assume the outcome will be reduced volumes when the combined RENS exceeds the router's ability.  Is damaged caused to the router or the phones when this happens?

Again, thanks



Here is a guide for REN values.
I just picked some examples from my measurements.

WE 554C subset (8AA/2x500 ringer, 21AP cond) with 1051C desk stand: 2.9
WE 534A subset with a 2x700 ohm ringer and 21BW condenser:  2.5 to 2.8
WE 634A subset with 2x500 ohm ringer and 149A condenser: 2.4
Dutch Heemaf 1955: 2.2
WE 302 (1953) with B1A ringer: 1.3
WE 302 (1952) with B1AL ringer:  1.2
WE 500:  ~0.9 to 1.0

The Panasonic KX-T61610 PBX manual claims 0.4B

The WE 302s seem to vary between 1.2 and 1.3, this is probably because of small variations in capacitor and ringer coil windings, and age producing perhaps some leakage in the condensers.

The "true" REN value of a device is actually dependent on the conditions of the loop.  The standard test procedure according to ANSI rules calls for a total of four different types or methods of measurement, and the value that should be reported is the highest one obtained from the four.  In this measurement process, type A and type B ringers are identified and the method of measurement is relative to that type. The ringer type is suffixed to the value, e.g. 1.0B
Of the four measurements involved, the largest value is usually obtained from an on-hook AC impedance measurement at 20 Hz. The REN value is calculated in this test by dividing that impedance into 8000 ohms. So, a 1.0 REN ringer has a nominal impedance of 8000 ohms.

A good way to increase the ringing capacity of a home system is to install a cheap Panasonic EasaPhone PBX (KX-T30810, or KX-T61610) and many collectors here have done this. The REN value of such a system is only 0.4 to the line, which I listed for this reason above.  Each port on the system can support up to REN 3 loads and it recognizes rotary dials too. But some of the Vonage ATAs already recognize DP.



How much REN can you have on one phone line?

What are the consequences of having too much?

How about internet connection?


The rule of thumb is you can have up to 4 REN on a standard phoneline (may vary country to country), so if you have phones that are 2 REN each, then you can have 4, if you have phones that are 4 REN each (like an unconverted GPO phone with a 1K Ohm ringer), then you can only have one... :)

Consequences are usually that phones won't ring or won't ring properly on the line if it's overloaded, they'll still work, but you won't hear more than an occasional ding at best... :)

And the internet connection is a different kettle of fish as it doesn't require a ringing signal, so shouldn't in theory use any REN at all (though in reality a DSL type modem is probably about 0.1 REN), and dial-up modems are usually classed as a telephony device so usually carry up to about 1 REN... :)


Unbeldi said:

"A good way to increase the ringing capacity of a home system is to install a cheap Panasonic EasaPhone PBX (KX-T30810, or KX-T61610) and many collectors here have done this. The REN value of such a system is only 0.4 to the line, which I listed for this reason above.  Each port on the system can support up to REN 3 loads and it recognizes rotary dials too. But some of the Vonage ATAs already recognize DP."

Two questions here:  I read back through the posts but "DP" is going right over my head??

And, I did look on Ebay and saw several of the Panasonic models you cite,  one or two for less than $100.  What is involved with installing them?  Do they require modification or are they essentially a plug and play?  (this is probably a very naive question)  :D

But . . . . factoring in TwoCVBloke's comment regarding operating on a Vonage router, does the router obviate the needs for the Pano unit?  Does anyone know if the Vonage router has an upper REN limit?  Or turning it around, will the Pano unit even work on a Vonage VOIP system? 

I appreciate everyone's patience but I'm just trying to get my brain wrapped around options and needs. 




I only have limited knowledge of VOIP adaptors, bit I think they usually accept up to 2 REN (not sure if that's per VOIP Adaptor or per port on the adaptor), I only have a Linksys PAP2T to play with (and that's only been set up as an intercom/ringdown for now), so VOIP isn't my strongpoint... :)


DP=Dial Pulse=Rotary Phones
The Panasonic 61610 is plug-'n'-play: In the default programming, you can call from one extension to another, dial 9 for an outside line, and all 16 phones will ring on an incoming call.
You would connect dial tone from the Vonage router, cable modem, cellphone gateway, and/or central office to up to six outside line ports.
"C'est pas une restauration, c'est une rénovation."--François Martin.


In the post that I linked to I gave an example of what I have had connected to my Vonage ATA at the time; it seems to be adding up to about 5 REN, I haven't measured the Cisco router that was connected, but it probably is quite below 1 like the Panasonics. These don't have to power an actual ringer, and only use a ringing detector which consumes much less power.

Standard telephone company local loops should provide 5 REN, because the FCC requires that no device to be connected to the PSTN can exceed the value of 5 REN.
Most ATA specification I have seen, quote 3 REN, such as the Grandstream HandyTone HT502.

When looking for a Panasonic 616 or 308, with patience you can find them for less than $50, even much less.  Look for local sources too, telephone installers, Craigslist.  There are many of them still in use and they are often thrown out.

Sorry for the abbreviation or jargon.


I know this is an old thread but wanted to add some information for anyone seeking resources

Most ATAs like the current and past crop of Sipura/Linksys/Cisco have adjustments for waveform type. THIS IS IMPORTANT!. A telco will do 20hz ringing with a nice smooth sine wave. An ata on the other hand generates the ringing signal from 12 volts or even 5 volts . It is easier for the ATA to generate a trapezoid waveform to ring the phones however some phones may not like that. In Linksys(Cisco/Sipura products this is adjustable. If your power supply is 12V 1 Amp that places a limit on how many phones you can ring. It is worse if it is an ATA withs at 5V 1A

As I always also preach on topics like this the UTStarcom IAN-02EX works great with rotary phones. I discovered this years ago when I had Lingo service. the UtStarcom is the best I have found yet. I can still get them new here in Mexico last I checked. Of course if you use a service like Vonage you are stuck with what you have. here is another interesting note though , I had an IAN-02EX with 2 corless phones (Low REN) and I plugged in the Indetel with mechanical bell (unknown REN but standard Telmex issue in about 1983) . A day later the ATA was dead.  I had never seen this before but I do not believe I ever connected three phones simultaneously to a single port. It may have also just died of age.

I also have here a Vonage VDV21 that I hacked to use with asterisk but i do not have the info to adjust audio gains, etc for my needs . As when connected to Uniden codless phones it gives me an echo , but not on old phones. I think this is because the uniden corless uses compression so overall voice output is higher.

And more convenient than a panasnic PBX is a simple asterisk server and using the ATAs mentioned above you can use rotary phones. I get The UTStarcoms for about 30 USD each with shipping and can connect 2 phones with no risk to each one as each has 2 phone ports. With the FreePBX and ELastix distros this is very easy to do. Also I add a custom context that adds * to a dialed 2 digit string so I can use the echo test etc. It is also a huge benefit if you need to tweak DTMF pads as it is very easy to generate an accurate tone in  asterisk. The more I look at old phone stuff the more I realize things yuy can do with asterisk. I even use it from  my doorbell to an old bakelite Tesla no dial phone to find out who is at the door.


Phat Phantom's phreaking phone phettish


8 yrs since this thread was started, and still I have to admit that its not easy to measure and conclude by one or two readings.
The REN load measurement is within 10% by following this: 
and this:

On the other hand, REN is usually of less interest due to what your system are able to cope with, and that may depend on lots of other factors... :-\

The line voltage in On-hook position may be of interest to se what system you have, e.g. German exchanges might have 60V Norwegian had 48V  PBX's 24 or 48.   Most phones works well when the voltage drop in off hook position gives you the remaining 5V across the line, but it may wary.  Putting an Amp meter in series you should have 25mA  (18-35 or even up to 50) The transmitter capsules does not work equal and an old German W28 needs a different transmitter than the later W48 that works well as most of AE transmitters, but The WE 302 has all the DC current going trough the transmitter and that should be different from the AE transmitter...

All this measurements just indicates that you are probably on the right way, or extremely wrong. :o :o :o

OK do not give up you will find hints about your specific phone somewhere here in the forum, or we may help you. :) :) :)


I think REN is a Purely N. American standard (per FCC?) and therefore does not apply to foreign ringers.  I am pretty certain that the standard itself defines things like peak to  peak or RMS voltage for ringing, and as we know already, 20 Hz ringing.

It is not to say that a foreign phone connected to a N American line will not have an REN but the REN standard itself is based on N American ringing standards and may not translate well when you start adjusting waveform or frequency of the ringing signal.
Phat Phantom's phreaking phone phettish