GPO 746 - The Resistor

[wkirkman]

Hello,

I have had a GPO 746 for some time now and had converted it over myself. Recently the phone has started to ring once and then no more.

So, I took a look and checked to see if the resistor was an issue, replacing it with the original strap. Seems to ring normally now.

My house runs BT Digital Voice VoIP and this is the only phone on the line (connected to the ATA at the back of the Smart Hub 2) which needs voltage for a ringer (the only other phone on that same line is a modern DECT phone which I believe therefore isn't afflicted with the need for a bell wire as it creates its own ring digitally).

So, my question here is - Do I need the 3k3 resistor at all?

The ringer coils appear to be 500 ohm each, so it's a 1K ringer which as far as I understand it would lend the need for a resistor — however, if there is nothing else in the house needing the voltage to ring, do I need it, or can I leave terminals 4-5 with its original strap?

If I do need it, then could someone be kind enough to explain why this is so? I am an IT engineer and so pretty much able to follow technical speak, but I am not an electrical or telephone engineer so I'm not entirely sure of the consequences here.

I read that a BT line would be giving about REN 4 and the phone would only be REN 1 - (and this should be the same coming from the back of the router as it simulates the master socket of PSTN).

So, after my ramblings:

Do I need the resistor? And if yes, why?

Many thanks in advance,

Wiliam

[Dave Moll]

You probably don't need the resistor (effectively to reduce the REN of the 746), though you may want to check what REN the DECT 'phone presents to the line (it should appear somewhere on the unit)*. The problem is that with a 1k ringer, the 746 presents a REN of 4, so your total REN will probably exceed the official limit of 4.

*Even though it may generate its own ring, it does contribute toward the load on the line.

[AC/HL]

Wired analogue phone lines could support a REN load of four. Traditionally, an extra phone would have had the bells wired in series by BT. When the new customer wired installations were introduced the high impedance ringers were introduced to allow parallel plug and play working. The resistor was a cheap and cheerful wheeze to allow two low impedance ringers to ring in parallel on longer lines.

[russell_w_b]

Hi William,

You won't need a resistor. The resistor was an expediency originally designed for a Trimphone to shift the REN to 1 from a REN of 4. On a magneto bell telephone, the impedance at 25Hz of a 1000 Ohm bell-movement is typically 1776 +j 2085, or an impedance of 2739 Ohms Z. With the 1u8 capacitor in series it becomes 1776 -j 1451 or an impedance of 2293 Ohms Z. With the resistor in cct you'll run 14mA or so instead of 33mA but over half the ringing voltage will be lost heating the resistor instead of having just over the applied ringing voltage clanging the bell.

A magneto bell will work down to about 12mA satisfactorily on a sine-wave but below that it becomes a bit iffy.

Don't know about your Digital Voice VoIP adaptor by my Grandstream one claims to deliver oomph equating to a REN of 5.

[wkirkman]

Hello,

Thank you for your replies - I’m glad I checked as I didn’t receive any email notifications and so hadn’t thought about it much these past few days.

So the general consensus appears to be that I don’t need a resistor, probably.

So a further question, if I may. Would not having a resistor, (if it was needed) do any damage?

I have tested the existing Dect phone on the extension and it certainly rings and the same time as the 746 so it isn’t appearing to cause problems that I can detect… but most problems that are real problems (causing house fires) are those you don’t detect, so just checking

Many thanks,

William

[russell_w_b]

Originally Posted by wkirkman

So a further question, if I may. Would not having a resistor, (if it was needed) do any damage?

You'll need to look up the spec of your VoIP router and see what sort of REN it will support. As I said, mine will support a REN of 5 (Grandstream HT801), i.e: one 'traditional' REN 4 magneto bell instrument plus one modern REN 1 instrument, but yours may well be different.