In products such as KNX there are only two wires used to communicate between a potentially large number of devices, and also provide power at the same time. For example in KNX the normal voltage on the bus is 30V, and to transmit a bit the voltage changes +/- 5V, see page 33-36 of http://knx.com.ua/attachments/article/132/KNX-basic_course_full.pdf
What I would like to understand is how such a circuit can be built. What I can see in the linked pdf is after the power supply there is a "choke" which has close to 0 ohm resistance for DC and very high resistance to AC, I understand this prevents the power-supply from overriding the data. But how is such a choke constructed?
Furthermore, I would like to understand how to construct a device that can raise/lower the voltage on the bus by 5-6V without using external power.
I do not have a background in electrical engineering, I studies computer science, and have a basic understanding of resistors, capacitors, inductors, and I would like to be able to model such a circuit, i.e., in ltspice to understand how it works (I am not searching for a working model, my goal is to understand it).
Best Answer
Remember that a capacitor acts as a open circuit for DC, and as a short circuit for (high-enough frequency) AC. Complementarily, an inductor acts as a short circuit for DC and an open circuit for (high enough frequency) AC.
So you can construct a bias tee like this:
simulate this circuit – Schematic created using CircuitLab
Now the load sees a DC current coming from
VDCthrough the inductor, and added to that an AC current coming fromVACthrough the capacitor.The circuit you described in your question works essentially the same way, but possibly a bit more elaborate (but since you haven't shared the circuit I can't say exactly how).