How can a VDC pulse data signal share the same conductor with AC Voltage?

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jchavez

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For some heat pump mini-split systems, the same conductor is used for the proprietary data communication signal (i.e. 12~24VDC) with a power voltage of 208~230 from the outdoor unit (heat pump) to the indoor unit (fan coil). See Mitsubishi Electric's M and P Series systems, aka, Mr. Slim.


I would like to read an explanation of how both DC and AC voltage can share the same wire :?
 

GoldDigger

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For some heat pump mini-split systems, the same conductor is used for the proprietary data communication signal (i.e. 12~24VDC) with a power voltage of 208~230 from the outdoor unit (heat pump) to the indoor unit (fan coil). See Mitsubishi Electric's M and P Series systems, aka, Mr. Slim.


I would like to read an explanation of how both DC and AC voltage can share the same wire :?

If the data communications is modulated high frequency rather than direct current pulses, it is pretty easy to do, and relies on the fact that a high frequency signal can be coupled to the wire through a capacitor which presents a high impedance to 60 Hz. And the 60 Hz load provides a high enough shunt reactance to avoid totally grounding the HF signal.

Using actual pulsed DC is going to be more difficult, but if the pulses are short enough it is really just a high frequency signal.
 

xformer

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For some heat pump mini-split systems, the same conductor is used for the proprietary data communication signal (i.e. 12~24VDC) with a power voltage of 208~230 from the outdoor unit (heat pump) to the indoor unit (fan coil). See Mitsubishi Electric's M and P Series systems, aka, Mr. Slim.


I would like to read an explanation of how both DC and AC voltage can share the same wire :?

The AC rides on top of the DC. Short explanation.. :)
 

GoldDigger

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The AC rides on top of the DC. Short explanation.. :)

But this requires either an isolated AC supply or an isolated DC supply to work, and will result in DC in the motor windings or other AC load unless a differential (phantom) connection is used.

I also would not say one rides on the other. They share equally under the wiring rights act of 1963.
 

jchavez

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Mini-Split - Outdoor unit and Indoor Unit. Mitsubishi Electric M and P Series Systems

Mini-Split - Outdoor unit and Indoor Unit. Mitsubishi Electric M and P Series Systems

Mini-Split - Outdoor unit and Indoor Unit. Mitsubishi Electric M and P Series Systems


The outdoor unit (heat pump) has an inverter driven compressor for speed and capacity control, also known as, variable speed. The system operation is driven by proprietary software. The outdoor unit has an input voltage rating of 208/230VAC on L1 and L2. Then the voltage goes through a PCB and lands on terminal block with three terminals (S1, S2, S3). The outdoor units powers the indoor unit aka single source power. S1 and S2 are 208/230. S2/S3 have 12~24VDC with a pulse signal communication (the software data stream). S2 has both 208/230 and 12~24VDC.


The indoor unit blower motor is a DC motor (brushless digitally commutated motor) which draws .76 F.L.A. So the AC voltage and motor is not a strong inductive load. The blower motor fan speed is changed by a voltage range of 175~350VDC. The outdoor unit PCB with the software provides commands and receives feedback (communication) to and from the indoor unit to maintain comfort as well as system operational integrity.


The question has come up, how can VAC and VDC share the same conductor?


Perhaps it is frequency. The information provided by Mitsubishi Electric is not forthcoming with the information to understand how this can occur.


Here is a link to a sample schematic.


http://usa.mylinkdrive.com/uploads/documents/1019/document/wiring.pdf
 

GoldDigger

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Mini-Split - Outdoor unit and Indoor Unit. Mitsubishi Electric M and P Series Systems


The outdoor unit (heat pump) has an inverter driven compressor for speed and capacity control, also known as, variable speed. The system operation is driven by proprietary software. The outdoor unit has an input voltage rating of 208/230VAC on L1 and L2. Then the voltage goes through a PCB and lands on terminal block with three terminals (S1, S2, S3). The outdoor units powers the indoor unit aka single source power. S1 and S2 are 208/230. S2/S3 have 12~24VDC with a pulse signal communication (the software data stream). S2 has both 208/230 and 12~24VDC.
The question has come up, how can VAC and VDC share the same conductor?

The schematic makes it very clear.
The two line conductors L1 and L2 are directly connected to S1 and S2. Neither of them is a grounded conductor.
The signal, etc. voltage which is applied to S3 is also not referenced to ground, but rather is generated by an isolated DC supply which is referenced to S2. The result is the common conductor, S2 is carrying the return current for two separate circuits, on AC and one DC.
You could just as easily ask how current from three different phases can all flow through a single neutral in a wye circuit. The answer is that current flowing in S2 is the sum of the current from the S1 and S3 terminals. Not a problem. (See Kirchoff's laws and the Principle of Superposition for more details.) :)

If S2 had been a grounded conductor, would it seem more reasonable to you that both circuits could use it? That does not make any difference in this case.
 

gar

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130315-2932 EDT

My quick look at a portion of the diagram indicates 208 AC is floating relative to ground or neutral. The 24 V DC probably uses the ground wire as one side of the 24 V supply.

.
 

GoldDigger

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130315-2932 EDT

My quick look at a portion of the diagram indicates 208 AC is floating relative to ground or neutral. The 24 V DC probably uses the ground wire as one side of the 24 V supply.

.

it is not floating. L1 and L2 both are solidly referenced to ground in any common household wiring system. There is a big difference between a circuit neither of whose connections are grounded and one which is truly floating. Since the 24 volt DC is shown as being referenced to L2/S2, it cannot be referenced to ground. There is a ground on the PC board, but you do not know where the signal voltage is coming from. I suspect it is from a different board.
 

gar

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130315-2059 EDT

GoldDigger:

Looking closer I see where the DC, whatever it is, is referenced to S2 by virtue of the bracket. Since the wording is "from" the implication is something is coming from the indoor unit. This DC may be both power for the PC board and include data superimposed on S3. Could be bi-directional data also.

Just for clarity, when I said that the 208 wires were floating relative to ground I meant neither was grounded. Whether explicitly referenced to ground or not they will have a potential relative to ground.

Relative to the original post the AC and DC power supplies share a common wire, and with proper design there is no conflict. This has been pointed out in one of the posts.

.
 

jchavez

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130315-2059 EDT

GoldDigger:

Relative to the original post the AC and DC power supplies share a common wire, and with proper design there is no conflict.
.

Does this sound right?

This is what I have heard Mitsubishi instructor's explain how AC and DC can share the same conductor.

Paraphrasing: 'S2 and S3 terminals on both the indoor and outdoor units are a bi-directional communication signals. The terminal S2 is the path of return for the communication signal from the indoor unit to the outdoor. The communication signal is a pulse signal which is very weak at 12~24VDC. The DC voltage rides on the circumference of the conductor because it has less electrons and the AC voltage stays in the core of the conductor because it has more electrons and is more dense. The two different voltages never mix. Your cheaper meters might not pick up the pulse, but a meter that has RMS will pick up the pulse.
 

ggunn

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Does this sound right?

This is what I have heard Mitsubishi instructor's explain how AC and DC can share the same conductor.

Paraphrasing: 'S2 and S3 terminals on both the indoor and outdoor units are a bi-directional communication signals. The terminal S2 is the path of return for the communication signal from the indoor unit to the outdoor. The communication signal is a pulse signal which is very weak at 12~24VDC. The DC voltage rides on the circumference of the conductor because it has less electrons and the AC voltage stays in the core of the conductor because it has more electrons and is more dense. The two different voltages never mix. Your cheaper meters might not pick up the pulse, but a meter that has RMS will pick up the pulse.
I don't think so. DC doesn't "ride on the circumference of the conductor because it has less electrons"; if anything it rides more in the core than does the AC. Skin effect is an AC phenomenon. It's a matter of difference in frequency, much like how several channels of video can travel on the same wire without interfering with one another. "The voltages never mix" is just not correct. If you look at one period of the AC, the data signal looks like a little fuzz if you can see it at all. If you speed up your scope so that you can see the data waveform, the 60Hz AC looks like DC.
 

GoldDigger

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I don't think so. DC doesn't "ride on the circumference of the conductor because it has less electrons"; if anything it rides more in the core than does the AC. Skin effect is an AC phenomenon. It's a matter of difference in frequency, much like how several channels of video can travel on the same wire without interfering with one another. "The voltages never mix" is just not correct. If you look at one period of the AC, the data signal looks like a little fuzz if you can see it at all. If you speed up your scope so that you can see the data waveform, the 60Hz AC looks like DC.

If you are looking at a separate voltage/current superimposed on the same wire pair that is providing 60 Hz AC, then the scope will show you that, and you are relying on the impedance of the motor load being high enough at the pulse/data frequency that it does not reduce the amplitude to the point where it cannot be reliably detected.

But when you are talking about a signal circuit and a power circuit that share a common wire, you are better off thinking of it as similar to a MWBC where the neutral potentially carries current from both single-line loads. Putting DC and AC through the same neutral conductor is no more difficult to understand than having current from two single-line loads traveling in opposite directions in the neutral "at the same time". What actually flows is the sum of the two currents. That simple.
 

jchavez

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Location
United States
The schematic makes it very clear.
The two line conductors L1 and L2 are directly connected to S1 and S2. Neither of them is a grounded conductor.
The signal, etc. voltage which is applied to S3 is also not referenced to ground, but rather is generated by an isolated DC supply which is referenced to S2. The result is the common conductor, S2 is carrying the return current for two separate circuits, on AC and one DC.
You could just as easily ask how current from three different phases can all flow through a single neutral in a wye circuit. The answer is that current flowing in S2 is the sum of the current from the S1 and S3 terminals. Not a problem. (See Kirchoff's laws and the Principle of Superposition for more details.) :)

If S2 had been a grounded conductor, would it seem more reasonable to you that both circuits could use it? That does not make any difference in this case.

GoldDigger: Thank you for such a understandable response.:thumbsup:
 

jchavez

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Location
United States
Explanation revealed

Explanation revealed

Everyones contribution has help solve this question. Thank to all.


S2 carries 115VAC potential to neutral, 120 times a second, at these points in time the voltage is actually 0VAC when referenced to the sine wave. During the 0VAC periods, bursts of dat ranging in voltage 12~24VDC are transmitted from the outdoor unit (OC) to the indoor unit (IC). The 24VDC is isolated from the IC and OC microprocessors.
 

GoldDigger

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Everyones contribution has help solve this question. Thank to all.


S2 carries 115VAC potential to neutral, 120 times a second, at these points in time the voltage is actually 0VAC when referenced to the sine wave. During the 0VAC periods, bursts of dat ranging in voltage 12~24VDC are transmitted from the outdoor unit (OC) to the indoor unit (IC). The 24VDC is isolated from the IC and OC microprocessors.
If a separate wire is used for one side of the control circuit, there is no technical reason that the signals have to be sent only at (near) the zero crossing.
You could also multiplex high speed signals on the exact same wire pair as power if you used proper isolators for the two frequencies.
 

mivey

Senior Member
Does this sound right?

This is what I have heard Mitsubishi instructor's explain how AC and DC can share the same conductor.
Let's all hope not.

Paraphrasing: 'S2 and S3 terminals on both the indoor and outdoor units are a bi-directional communication signals. The terminal S2 is the path of return for the communication signal from the indoor unit to the outdoor. The communication signal is a pulse signal which is very weak at 12~24VDC. The DC voltage rides on the circumference of the conductor because it has less electrons and the AC voltage stays in the core of the conductor because it has more electrons and is more dense.
For the most part, the "voltage" is in the space between two conductors in a circuit, not down inside a single conductor. The relatively small voltage drop along the conductor represents a net electric field between two points along the length metal conductor. Any voltage drop across the radius of the conductor would be small indeed.

An imbalance of charges between the two conductors creates an electric field between them. The movement of charges in a conductor creates a magnetic field around the conductor.

The energy for the transmitted signal rides outside the conductor where the electric and magnetic fields cross. There is a very small part of the created signal energy that terminates inside the conductor as losses.

I don't think so. DC doesn't "ride on the circumference of the conductor because it has less electrons"; if anything it rides more in the core than does the AC. Skin effect is an AC phenomenon. It's a matter of difference in frequency, much like how several channels of video can travel on the same wire without interfering with one another.
You speak about current of course. For DC, the electrons in the center move like the ones near the surface because we reach uniform velocity across the radius. With AC, we don't reach uniformity before the direction change because there is a delay in getting the electrons in the center moving. This delay shows up as the skin effect in bigger wires and at higher frequencies and is a function of frequency, dielectric constant, conductivity, and permeability.

When the electric field penetrates the wire, it causes a current. This changing current creates a changing magnetic field which, in turn, creates a counter EMF inside the conductor. This counter EMF limits the current. Naturally, a steady DC signal will not have a changing magnetic field to create the counter EMF so the current in the center is the same as the current near the surface.

When the current is changing with AC, the longitudinal electric field near the surface can't be the same as the longitudinal electric field near the center because we have a changing magnetic flux.
 
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