Sending data over DC power lines

[m2pc]

Hello everyone,

I'm working on an automotive project where I need to send both data and power between two points. The only problem is there's only 2 wires (+12V and GND) to work with. These wires are molded in fiberglass, and there's no way to extend/expand them to include data lines.

My original though was to build a square wave generator where I could encode binary "0", "1", and "No data" by varying the width of the pulses, and I could add a rectifier/capacitor on the receiving end to recover a useable DC signal to power the existing electronics, and use an opto-isolator before that to recover the signal for processing with a microcontroller. The receiving end needs about 2-3 amps of current, so I was thinking to switch the 12VDC at the source using a MOSFET fed from a microcontroller via another opto-isolator or buffer IC.

Can anyone else chime in? I can't decide if I should try this using a variation of the above method, or if I should just use a short range RF transmitter/receiver pair to send the data.

 

[Externet]

If the DC+ conductor ends in a capacitor and an inductor, data and DC can be fed and separated respectively on each branch.

 

[crutschow]

Your method sounds ok. Don't see that the complexity of an RF link is needed.

I take it your required data rate is slow.

 

[MrAl]

Hi there,

As Miguel was saying, if you put a relatively low value inductor in series
with the dc line at the receiver end and a capacitor right after that,
and do the same thing at the transmitter end, you effectively raise the
high frequency impedance of the dc line but more or less leave it alone
for dc. This allows you to inject a signal that is maybe 10 to 100kHz
with your encoded data, and all you need at the receiver end is a
capacitor and amplifier and detector to demodulate the signal.
This technique has actually been done only instead of over a dc line
it was done over 120vac 60Hz voltage lines, which is a little harder
but basically the same thing.
What this means is you dont have to deal with pulsing power, which
can get a little tricky and still requires filters. Instead, it's a small
signal that is transmitted down the line that doesnt bother the dc
signal (especially if your encoder uses a zero average value signal)
but still allows the transmission of data.

 

[Diver300]

The aerials on some cars are also the heated rear window elements. It is just a matter of separating out the high frequency from the low frequency.

The cross-over circuit in audio speakers does this.

How much power is going down the DC lines? What data do you need to send?