Telephone relay selection help

[rjrl]

Hello all,

I'm looking for help in choosing a suitable relay for a telephone switching project. I have an internet router with VoIP capability and so have my telephones plugged into that, not directly into the BT line. While the power is on, it works fine and with dialling rules or by prefixing the number I dial, I can make either a VoIP or PSTN call and the router switches it appropriately. However, during a power cut, all phones plugged into the router stop working. Unlike my old Draytek router, the new one does not fail-over to the PSTN when power is lost.

I would like to remedy this by making a switch box for the phones, so that when power does fail, all phones are automatically switched back to the BT line. There is no requirement to maintain a call in progress when the power fails.

My plan is to power a DPDT relay off the router's own wall wart (12V DC, 1.5A output), in series with the router, but what relay would be suitable? I've found a category of telecoms relays on the Digikey website, in which the following product appears:

Digi-Key - PB387-ND (Manufacturer - V23105A5303A201)

Would a relay like this be OK, or does the 'telecom' category refer to some other use? Any problems with powering the relay in the way I've described?

I believe the specs for a UK telehpone line are -48V line voltage, 40 mA current, 90V ring voltage.

many thanks,
R

 

[Reloadron]

Telecom Relays:

The relays meet Telcordia specifications and are suitable for use in telecom equipment such as central office switches and broadband routers, which must typically provide many channels of switching within a confined area.

Not to deter from your project goal but have you considered or given any thought to placing the router on a UPS (Uninterrupted Power Supply)? That is pretty much how I have things configured and it seems to work fine. Since the current drain of a router is so minimal even a small UPS can last for hours.

However, if you want to build your own switch box if you could provide a simple drawing of what you have in mind I am sure something could be developed.

Ron

 

[mneary]

For the application as you describe, the relay is perfect.

 

[rjrl]

Thanks for the quick replies, good to know I was on the right track! I'll have a look at UPS units and see how the cost of one compares to the components of the switch box once I've worked out exactly what I'll need for it.

**broken link removed**

Here's a rough sketch of what I was intending to make. I hope it's clear enough - I've not quite got to grips with Xcircuit yet. The box marked "BT" is where the telephone line comes into the house.

Can I also ask about the "Control On Voltage" (Max/Min) parameters for the relay. What exactly are these? I'm probably being stupid but it's not clear to me.

thanks again, R

 

[mneary]

Can I also ask about the "Control On Voltage" (Max/Min) parameters for the relay. What exactly are these? I'm probably being stupid but it's not clear to me.

thanks again, R

A 12V relay won't wait till it gets 12.00V to energize. In this case (12V 200mW coil) it's guaranteed to operate at 70% of 12V when it's 25c, and at 80% of 12V when it's about 60c.

Likewise, it won't drop out when the voltage falls below 12.00V. Due to mechanical and magnetic factors, relays tend to release at a much lower voltage. For these relays, the voltage needs to be <5% (???) before you can depend on them to release.

In practical terms, when you lose power, these relays will take a little while to drop out (transfer to the BT lines) because your 12V might need to decay down to less than a volt.

 

[ljcox]

As a retired communications engineer with many years experience with complex relay circuit design, I can assure you that it is essential to connect a diode across the relay coil.

The coil is inductive so when the circuit is opened it will produce a back EMF which could be several thousand Volt. It is therefore likely to damage other components.

I'll draw a circuit and post it for you later.

 

[ljcox]

Here is the circuit.
I have assumed that the positive side of the relay operating voltage comes from the router.

If not then you will have to reverse the diode.

The cathode of the diode must be connected to the + side.

 

[Diver300]

You should put the relay in parallel with the router. The diode is a good idea.

If the router runs from 12V, you don't need a UPS. You just need a battery, a 12V sealed lead acid battery is the simplest. You then use a battery charger to keep the battery full instead of the wall wart. You should use a charger that is regulated so that you don't overcharge the battery.

That is how most domestic burglar alarms work.

 

[mneary]

Circuit diagram is wrong.. As said by Diver300, the relay must be in parallel with the router.

A diode is not necessary in this circuit as the current collapse in the coil is extremely gradual, taking place over the period of one to several seconds. The collapse of the magnetic field will not generate a noticeable voltage.

A diode would be used if there was a switch of any kind directly controlling the relay.