On Mon, 28 Nov 2005, Anand Dhuru wrote:

> In most of the carrier current based systems, as in X10 transmitters, I have 
> seen a signal transformer at the mains end. I beleive this serves 2 purposes; 
> one of the windings generates the high frequency carrier signal (usually 
> around 120 KHz), and the other(s) couple the actual data, modulated by the 
> carrier, into the mains.
>
> Am I right till this point?

Yes and no. The transformer serves to match the generator impedance to 
the line at (and only at) the working frequency. It strongly rejects 
mains ac and glitches by being selective.

> The transformer does not provide any input-output isolation, as the input and 
> output sides often share the ground and so plays no role in the safety of the 
> circuit anyway.

Yes and no. Most carrier current systems that have something earthed or 
touchable on the user side will have galvanic separation.

> My question is, can the data, be modulated by a high frequency oscillator 
> (say using a 555) and fed into the mains thru' a capacitor, thereby not 
> having to use the transformer?

The capecitor will have to have a certain value to pass the signal. The 
same capacitor will couple conducted EMP from the line back into the 
circuit and destroy it asap when that happens.

You want to use a relatively narrow band signal. Think radio or 
ultrasound.

> I am trying to design my own X10 transmitter (I do know about the PL513 / 
> 523, but would still like to design my own), and was wondering about the 
> above aspect.

Take a ferrite antenna from a cheap radio (for LW) and use that. Provide 
your own 2nd secondary using insulated wire and it should work fine.

Peter
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