I recently went over to digital voice for my landline phone via a FTTC hybrid fibre internet connection. My ISP has supplied a battery backup which has a single 12 volt output that uses a splitter Y cable to supply a router and ONT through DC barrel connectors. In my case the power connection for the ONT is spare.

I would like to use the spare (ONT)12v connection to power the cordless phones, which use approx 200mA at 5.5v when charging, 50mA when charged and 60mA when being used for a call. To do this I have connected a variable input/adjustable output 2 amp DC to DC buck converter between the 12v ONT power connector and the cordless phones). This arrangement does not work with the phone line cord connected between the phone base unit and router; the display on on phone displays one of several error messages dependent on the order in which I connect things up.

If I power the router through the ISP battery backup and the phones through a separate power source (various 12v output AC/DC adapters or 9v battery) still using the buck converter the phones work without a problem. I can even run the phones on a USB battery backup outputting 5.3 volts.

I guess there is something passing/circulating through the phone line cord when the router and phone are powered from the same DC source.

ISP battery backup output - 12v 3.5A.
Phones base unit input - 5.5v DC, 200mA when charging, 42mA when using phone and 50mA in standby.
DC-DC converter - LM2596 DC-DC Buck Converter Adjustable Power Supply Step Down Module , Input voltage range:4-40VDC, Output voltage range:1.25-37DC adjustable, Output current 2amp

My background was in industrial electrics(now retired) with very little electronics knowledge.

Any suggestions on how to overcome this problem would be much appreciated

Many thanks

 

I would suspect the problem might be that the ISP battery and buck converter output are sharing the same common or "ground" connection.
Possibly might work using a buck converter that's output is fully isolated from the input.
What model buck converter are you using?

 

Bought off eBay see https://www.ebay.co.uk/itm/40599430...-a4c6-0a75cf8db5f5|iid:1|vlpname:vlp_homepage

 

That is a non isolated buck converter.
Will the phone work on 5.0 volts?
5.5 volts is not a common voltage and there are fixed output voltage isolated converters that output 5 volts.

 

It works on the following:-

1. Original 240/5.5v 500mA mains adapter that outputs 6/6.2v with the phone connected.
2. 240/5v 3a mains adapter but that outputs 5.75v with the phone connected.
3. 20Ah USB mobile phone backup which outputs 5.3/5.4v on the USB A port with phone connected and on the USB C PD port which outputs 5.2v with the phone connected.
4. 2Ah USB mobile phone backup which outputs 5.3/5.4v on the USB A port with phone connected.
5. 5V from a dying 9v lantern battery barely runs the phones which cut in and out.

The ISP battery back up also has a USB C PD port which the phone also doesn't seem to like presumably due to common ground.
I have several 240/5v AC/DC power adapters but I expect, on such a low load, they will all output 5v+

I wondered if connecting one of these in series with the buck converter would do the job. They Output 12V from a 5-36V Input and appear to be isolated.

 

That was my thinking if the phone doesn't work on 5 volts.
One issue is the efficiency when running through two converters.
When using the lattern battery was the 5 volts steady?

 

The 5 volts was steady ish, it was slowly falling from 5.1v to 4.95v. I'll try using a 12v car battery and the existing converter, as that should give me a stable 5v, and see how low a voltage the phones will run on; I'm just a little concerned about running the phone on too low a voltage and causing damage.

One thing to bear in mind is that both my DVMs are quite old now and haven't been calibrated for years so the voltage/current readings I'm getting may not be accurate.

 

Generally, with electronic stuff, running on too low a voltage may cause incorrect operation, it does not cause damage.The possible exception would be hard drives.

 

I've now connected the phone base unit to 5v from the buck converter which is powered from a fully charged car battery. The phones run just about on 4.75 volts and this includes calling in, calling out, taking and listening to answer phone messages.

sghioto Your suggestion of using a fixed output voltage isolated converter that outputs 5 volts could be the way to go. I'll see what I can find.

A slight side issue - I noticed that when the handset battery gets near to fully charged the current demand of the base unit starts to rapidly rise and fall (hunt) from about 200mA to 50mA every half second or so. This continues for several minutes until the the current drops to a steady 50mA, which is its standby current. Is this hunting normal; it's the same whether powered by it's original power adapter or other power source? Could my setup of the DVMs, one connected for current monitoring and the other for voltage monitoring, be causing the issue?

 

I wouldn't think so.

 

When the battery reaches full charge, the charger evidently changes modes, (reasonable), and then the applied voltage deops a bit because it does not need to be charging. So, then some part of the system sees a lower voltage and switches back to the higher voltage, which after a bit, the control system senses as fully charged, and so it switches back to the lower voltage. So you have oscillations until the decision part of the system stabilizes.

 

I noticed that when the handset battery gets near to fully charged the current demand of the base unit starts to rapidly rise and fall (hunt) from about 200mA to 50mA every half second or so. This continues for several minutes until the the current drops to a steady 50mA, which is its standby current. Is this hunting normal

Hi duff.
Yes, that is a typical, charge > test > charge >test sequence for that type of charger when the battery is almost fully charged.

E

 

Phone running OK after 20 hours on 5v through original DC/DC converter connected to 12v car battery.

Thanks for all the comments and assistance.

 

Then it should work on that 12 volt backup battery.

 

sghioto
The problem is not with running the phone's base unit on a separate battery it is powering it from the same battery back up that the router runs off, because whenever I connect the phone's line cord to the router the phones fail to operate correctly. As you suggested probably due to a common ground.

I've looked around a lot for an isolated Dc to DC converter and prices seem to range from very cheap to ridiculous high for my project. Many of the cheaper ones turn out to be non-isolated, however, I found this one on eBay.

I messaged the seller, because the description appeared to be confusing and contradictory (least ways to me), asking "Please can you confirm this module does not have a common ground and the input and output are electrically isolated" The seller responded with "The DC-DC Isolated Power Module features isolated and regulated output, ensuring that the input and output are electrically isolated. This design helps solve interference noise issues caused by shared power supply."

The converter seems to be suitable but I'd be grateful if someone could take a look at the item on eBay and advise if they think it will solve my problem.

Many thanks

 

Yes I know what the problem is.
I was speaking about the 12volt supply not the converter.
It says isolated so I would believe it is.

 

BY DEFINITION, an isolated module does not share any connections between input and output.
And now for the previously unasked question: Is there another connection between the cordless phone and the FTTC hybrid fibre internet connection?
It seems to me that there must be for it to cause a problem.
And an isolated DC to DC 12 volt to 5 volt supply is fairly common, in my experience.

 

sghioto
Thanks, puts my mine at ease.

MisterBill2
Not sure what you are asking but the phone base line cord plugs into the phone port on the router either directly via an RJ11connector or via an LJ5113 line adapter.

The phone base(5.5v) and router(12v) plug into the 12v battery back-up with the phone base obviously connecting via the 12v/5v DC/DC converter. The converter is not isolated and perhaps should be replaced with the one linked to in my last post

As far as I'm aware there are no other physical connections within the setup other than the Router WAN and a wired LAN to a PC, see drawings - Setup in Diagram 1 works without problems but setup in Diagram 2 does not work and the phone displays a Phone line error.

 

The obvious difference is the lack of isolation in the 5 volt supply to the cordless phone 5 volt supply. That implies that the phone line connection to the router is connecting two opposing grounds. One option worth trying is to connect the cordless phone to the router using an only two conductor (RED and GREEN ONLY) cable. THAT might be sufficient. And both cheap and easy.
Otherwise you will need an ISOLATED 12 volt to 5 volt converter module.

 

The phone line cord used is two conductor red and green. I noted this on the diagrams but then deleted it coz it seemed over-the-top.

I'll get an isolated converter and see if that resolves the issue.

As a point of interest the problem I've experienced is likely to be a widespread as people with cordless phones change to digital landline (VOIP). The ISP's are addressing the issue of keeping routers powered during a power cut but do not appear to have addressed the issue of keeping the phones powered despite having to provide free-of-charge power resilience measures for vulnerable customers, who need to maintain there phone service in case of emergencies, as per OFCOM requirements.