# DC power line communication problem!

#### dubu

Joined Mar 14, 2022
9
I wrote a circuit as shown in the figure to use DC Power line communication.
However, if 12V or more of the main power is applied and a signal (High, 1) is applied to PLC_TX, OCP occurs in the power supply. I wonder why?

#### AlbertHall

Joined Jun 4, 2014
12,190
Q1. What is the value of R35?
Q2. What is the current rating of the main power?

If you divide 12V by the answer to Q1, is the result greater than the answer to Q2?

#### dubu

Joined Mar 14, 2022
9
Q1. What is the value of R35?
Q2. What is the current rating of the main power?

If you divide 12V by the answer to Q1, is the result greater than the answer to Q2?
A1. The value of R35 is 3 ohms / 6432 Size.

A2. Current rating of main power is 600mA.
If connect another 36V smps to the main power, the R35 resistor burns out.

yes. Divide 12V by Q1 (3 ohms), which is 4A, which is greater.
I tried changing the R35 to 42 ohms but it was the same.

Joined Feb 20, 2016
4,251
Are you actually switching a 3R resistor onto the bridge output?
I fail to see how your circuit would work well at all.
What is the signal you are wanting to send?
Are you sending smoke signals ?

EDIT: What data rate are you looking for?
And just switching a 4A (or 12A) load onto the rectifier output is not the way to go!

Last edited:

#### dubu

Joined Mar 14, 2022
9
Are you actually switching a 3R resistor onto the bridge output?
I fail to see how your circuit would work well at all.
What is the signal you are wanting to send?
Are you sending smoke signals ?

EDIT: What data rate are you looking for?
And just switching a 6A load onto the rectifier output is not the way to go!
Here's my desired circuit behavior:

Send the desired Low/High (0,1) signal from PLC_TX connected to the MCU, turn on/off Q10, and load the signal (0,1) to the PLC_IN power line through this.

However, at present, if PLC_TX is Low, Q10 does not turn on, so R35 resistor is safe.
If PLC_TX is High, Q10 turns on and R35 resistor burns out.
Therefore, no signal is loaded on the PLC_IN power line.

I assumed it was because the current was high and tried to increase the resistance value to about 42 ohms, but the resistor burns out as well.

The signal you want to make through PLC_TX is the same protocol as UART.

Joined Feb 20, 2016
4,251
Do you need to modulate the power supply for some odd reason, or just have an input for the PLC?
Is "PLC_IN" the PLC +Ve power supply?
Are you trying to power the PLC?

If you are just looking for a signal, change R35 to a 2K2 resistor, than feed the PLC input signal from the Q10 pin3.
Do not use the PLC power as a signal. They should be separate.

Joined Feb 20, 2016
4,251
I cannot figure out what you are trying to do. PLCs already have inputs and outputs so what is all the extra stuff for?
Can you add the actual PLC to the circuit as it does not make a lot of sense at the moment.

Are you trying to use the PLC power lines to both power the PLC and send comms to it?

#### dubu

Joined Mar 14, 2022
9
I'm sorry for the late responses

Your Q. Are you trying to use the PLC power lines to both power the PLC and send comms to it?

A. Yes!

That circuit is a client product. There is another host product with the same plc circuit and it is to communicate with the host and client on dc plc power!

Joined Feb 20, 2016
4,251
A. Yes!

That circuit is a client product. There is another host product with the same plc circuit and it is to communicate with the host and client on dc plc power!
Ok. Now, what comms are you looking for? Do you have the data rate? This will have a great impact on how you need to design your circuit.
Is there any way you could see to use a radio link? That may be the easiest option. Small radio links, based on ESP8266 or ESP32 boards may be worth looking at.

#### dubu

Joined Mar 14, 2022
9
Ok. Now, what comms are you looking for? Do you have the data rate? This will have a great impact on how you need to design your circuit.
Is there any way you could see to use a radio link? That may be the easiest option. Small radio links, based on ESP8266 or ESP32 boards may be worth looking at.
I want to communicate with the PLC power line using the same protocol as the UART signal.

#### dubu

Joined Mar 14, 2022
9
Ok. Now, what comms are you looking for? Do you have the data rate? This will have a great impact on how you need to design your circuit.
Is there any way you could see to use a radio link? That may be the easiest option. Small radio links, based on ESP8266 or ESP32 boards may be worth looking at.
As you said, changing R35 to 2.2k does not burn the resistor.
Also, as shown in the figure below, if you check before R35, a signal is created while swinging 0V-36V, but if you check after R35, it maintains 36V and does not swing. What I want is to make a signal like 30V-36V swing in PLC_IN, even if it does not swing up to 0-36V.
Can I use a Zener diode?

Joined Feb 20, 2016
4,251
You will most probably have quite a problem modulating the power between 30V and 36V at "UART signal" speeds. For one thing, the PLC power filtering will need to be isolated from the signal.
Why can you not use radio? It looks you are adding external circuits to modulate the power anyway. An extra small matchbox sized board at each end would give you a radio link.
Still, adding a series transformer to each end of the power line could work, then impress a tone onto the line for the data signal.
I wonder why just running a twisted pair wire link is not an option?

#### AlbertHall

Joined Jun 4, 2014
12,190
You might consider a system like DCC used for model railways.
It uses a square wave symmetrical around zero so a bridge rectifier will produce a DC supply voltage with the data being passed in the pulse timing.
https://dccwiki.com/DCC_Tutorial_(Power)

#### dubu

Joined Mar 14, 2022
9
You will most probably have quite a problem modulating the power between 30V and 36V at "UART signal" speeds. For one thing, the PLC power filtering will need to be isolated from the signal.
Why can you not use radio? It looks you are adding external circuits to modulate the power anyway. An extra small matchbox sized board at each end would give you a radio link.
Still, adding a series transformer to each end of the power line could work, then impress a tone onto the line for the data signal.
I wonder why just running a twisted pair wire link is not an option?
you means..
-> Still, adding a series transformer to each end of the power line could work, then impress a tone onto the line for the data signal.

Can you draw a brief circuit for this explanation?

#### dubu

Joined Mar 14, 2022
9
You might consider a system like DCC used for model railways.
It uses a square wave symmetrical around zero so a bridge rectifier will produce a DC supply voltage with the data being passed in the pulse timing.
https://dccwiki.com/DCC_Tutorial_(Power)
Actually, the place I want to use with this circuit is the parking guidance system. The main controller is trying to send and receive commands to and from various parking guidance sensors through DC PLC. But the problem is that I don't have enough knowledge.

Joined Feb 20, 2016
4,251

#### dubu

Joined Mar 14, 2022
9
First of all, the circuit I posted at the beginning was written by looking at the actual product and circuit in use elsewhere.

I wrote the same circuit, but I had a problem with the resistor burning. Even if you look at the actual product, you can't see any other parts other than that circuit.

I guess I didn't have enough explanation.

A circuit and system schematic are attached for reference.

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