Dwarak Technology Solutions: The Dual Power Phone Adapter… Finally ;)

Everything has a story behind it. Do you agree ? Once again, I’m going to stick to the same old style of why and how the Dual Power Phone Adapter came into being.

Okay. Down South in Chennai is where I live. We have the worst power shortages here. Nobody knows when the power may go and come back. Real Worse. So, one problem that usually irritates me is when I talk over my cordless phone and power goes down all of a sudden. Line gets lost. Well, after all it’s just 9 volt DC supply that’s powering the base station from the adapter. Why not, use it with a battery cum AC voltage ? So, you know, we could have it set it up like, whenever the power goes, we can use the battery for alternate supply. Cool, right ? I soon decided to make one.

Okay, that was the story behind and now we go on to the schematics of the circuit.

Circuit

Essentially, you’ll need

A 230v, 5A AC supply –> 9v DC supply transformer
4 nos. of 1N4001 Diodes or something similar
Electromagnetic Relay – Something like RSM822 (Data sheet here : http://www.ges.cz/sheets/r/rsm822.pdf )
A 9v DC Battery
Connecting Wires
Phone Adapter Pins

In India, the standard power supply is 230v, 5A. Choose the transformer appropriate for you to step down the input voltage to 9v or required power supply that your phone needs. Check with the product manual.

First step, its obvious. You’ll need to step down, your input power to 9v DC supply.
So, connect, your input to transformer and output secondary winding to a bridge rectifier. So okay. Bridge rectifier is one with which you make your input AC into a DC voltage. Essentially DC voltage is a single polarity voltage. It doesn’t have a positive and negative voltage concept at all. So, we’re going to clip the negative side of the AC voltage to get a single polarity. But, that alone is just not enough. It is still a pulse. –> pulsed voltage. To make it tend to much more DC characteristic, we may use a capacitor. Although, we didn’t use capacitor in the circuit given, you may use it for best results as in a full wave rectifier.
To switch back and forth between the AC and DC supplies, we’re going to use a Relay coil. It is basically a 2-input, 1-output switch, controlled by electromagnetic coil. Whenever the coil gets powered, output from 1 terminal and when the power goes down, output comes from another terminal. We’re going to power the coil in switch with the rectified power from Bridge rectifier and use it send AC input to phone. Whenever, there isn’t AC supply, DC shall take over.
Find more about Relays here : http://www.kpsec.freeuk.com/components/relay.htm
Connect the negatives of both the AC and DC together and use the Relay switch to control the adapter.
Use appropriate adapter pins to connect the supply to your phone.
This might not be the ‘perfect’ solution to the problem. But, suffices, one may say.