transfering data via DC power line

[ikalogic]

I really hate that too and it seems that Google Domain filter won't remove them from the list either. Someone is being paid.

The reason these work well on the 50/60Hz AC line is that the impedance of the power line is relatively high at higher frequencies so it is fairly easy to superimpose a carrier on the AC line.
Compare this to the 12V DC bus of a car where most of the electronics have a capacitor across the DC input which presents a low impedance path to ground for any carrier you try to superimpose.
As I alluded to, and dknguyen stated, the best way is something like this:

Thanks a lot for taking the time...

can you tell me what are those triangles in your schematic? are those buffers...? i don't quite understand the part where 12VDC is mixed with data.. Also shoudnt we use a High pass filter instead of the LPF?

thx again.

 

[crutschow]

Compare this to the 12V DC bus of a car where most of the electronics have a capacitor across the DC input which presents a low impedance path to ground for any carrier you try to superimpose.
As I alluded to, and dknguyen stated, the best way is something like this:

The filters on the electronics inputs certainly could be a problem. You may need to add an inductive choke in series with each electronic input to maintain the signal.

But I don't understand how the circuit you propose will get around this problem since it's outputing a signal at some frequency, which will also be suppressed by the caps.

 

[kchriste]

can you tell me what are those triangles in your schematic? are those buffers...? i don't quite understand the part where 12VDC is mixed with data.. Also shoudnt we use a High pass filter instead of the LPF?

The first triangle is just a powerful buffer. When idle, it outputs 12V. When sending data, the output swings between 12V and ground. The diode and cap on the input to the regulator peak detects this signal and supplies a relative steady 12V to the regulator. The data rate will be fairly slow, hence the lowpass filter. The receiving buffer is just a simple schmitt type.
If you wanted to make this data path bidirectional, the circuitry would be somewhat different.

The filters on the electronics inputs certainly could be a problem. You may need to add an inductive choke in series with each electronic input to maintain the signal.
But I don't understand how the circuit you propose will get around this problem since it's outputing a signal at some frequency, which will also be suppressed by the caps.

What I'm proposing is running one wire between the master device and the sensor/slave device. No regular devices (Car radio, ECU, lights, etc) are on this data/power wire. I really don't see the point in trying to run power/data on the existing stock car wiring as it seldom goes where you need it and seldom has the extra capacity or state (Batt, Ign or Accy switched).

 

[ikalogic]

The first triangle is just a powerful buffer. When idle, it outputs 12V. When sending data, the output swings between 12V and ground. The diode and cap on the input to the regulator peak detects this signal and supplies a relative steady 12V to the regulator. The data rate will be fairly slow, hence the lowpass filter. The receiving buffer is just a simple schmitt type.
If you wanted to make this data path bidirectional, the circuitry would be somewhat different.

What I'm proposing is running one wire between the master device and the sensor/slave device. No regular devices (Car radio, ECU, lights, etc) are on this data/power wire. I really don't see the point in trying to run power/data on the existing stock car wiring as it seldom goes where you need it and seldom has the extra capacity or state (Batt, Ign or Accy switched).

Thanks a lot for your insight... i think i'll give it a try. at least to learn!

Can i ask you one more favor: Where does the battery's 12V wire goes in your circuit? Can you pin point the node that is to be connected to the 12V line?

thanks a lot.

 

[ikalogic]

Hello again..

I want/need to make some tests, and see with the scope how a car electrical system responds. I just dont have a clue on "how to inject data into DC" i understand the principle, but i can't figure out how to make that work with the components that i know : coil, cap, BJT, resistor, etc...?

can anyone give me hand on that?

 

[blueroomelectronics]

If there was a simple / cheap effective solution to send data through a vehicles 12V DC wiring then why would cars still use CAN bus?

 

[kchriste]

Where does the battery's 12V wire goes in your circuit? Can you pin point the node that is to be connected to the 12V line?

It is the power pin for the first buffer. In the upper left hand corner where it says "Power In 12V". Basically all power flows though the "buffer" placed on the left hand side of the diagram.

I just dont have a clue on "how to inject data into DC"

Well, you could always modulate the field coil of the alternator!
Basically, you would use chokes/inductors to separate the signal from the rest of the system and capacitors to couple into this "isolated" piece of line. The trick is to use chokes that can handle the DC current without saturating:

 

[The Mad Professor]

Seek out the datasheet for the Maxim IC 455, the application notes pdf has a complete circuit diagram. The chip is just a video amplifier and switching, but the application notes show how to send and recieve both DC power and control signals over a "single" wire.

 

[shokjok]

Are you trying to re-invent the ancient BSR X-10 modules?
Transmitting AC signals onto a noisy automotive DC power line is futile. The separate wire is the best approach. Wireless could also be tried, it is unaffected by high frequencies under the hood.

 

[The Mad Professor]

If the wheel had not been re-invented then they would still be made of wood!

As for the 'futility' of getting data over a noisy DC line the problems are not beyond a solution, indeed if anything many real world examples of 'fly-by-wire' can be found not only in aviation but automotive applications.

Closer to home some manufactures of traffic control systems, both temporary systems used during road works and permanent installations communicate both sensor and control data over the DC power line. If you think an automotive DC power line is a bit 'Rough' the you should see what the output of a beat up generator looks like

Sticking with in-car applications a quick search of the web reveals a nifty idea to use the DC power line to inter-connect the various elements of 'ICE' (In Car Entertainment) no rolls of wire for the speakers or getting under the dashboard to run the control head cable back to the CD changer in the trunk. Just put the hardware where you want it and find a live wire.

 

[ikalogic]

If the wheel had not been re-invented then they would still be made of wood!

As for the 'futility' of getting data over a noisy DC line the problems are not beyond a solution, indeed if anything many real world examples of 'fly-by-wire' can be found not only in aviation but automotive applications.

Closer to home some manufactures of traffic control systems, both temporary systems used during road works and permanent installations communicate both sensor and control data over the DC power line. If you think an automotive DC power line is a bit 'Rough' the you should see what the output of a beat up generator looks like

Sticking with in-car applications a quick search of the web reveals a nifty idea to use the DC power line to inter-connect the various elements of 'ICE' (In Car Entertainment) no rolls of wire for the speakers or getting under the dashboard to run the control head cable back to the CD changer in the trunk. Just put the hardware where you want it and find a live wire.

I totally and fully agree with you.

In need i am about to find a solution that solves this problem. in involves switching a high load at different frequencies to cause some battery voltage perturbations (no more than 0.1 volt) then with DFT algorythms that we already made it work, we detect those perturbations back.. still some work to do nut we are on the right track.