# DC and AC on a single wire

#### kalemaxon89

Joined Oct 12, 2022
44
Let's assume that I have to receive 100V input to a pcb.
If DC, should I expect one cable or two cables?

If two, then should one be grounded?

Can it be just one? (which automatically refers to gnd)

I don't know if this question goes in the pcb section or here, I think it's a basic physics/electrotechnics concept... so I thought it correct to put it here. The admin can move it if he thinks it is correct.

Joined Jul 18, 2013
25,998
Define GND, earth GND or power common? Chassis ?

Last edited:

#### MrChips

Joined Oct 2, 2009
27,168
Let us assume that you chose 100V as an arbitrary value.
But it is not always arbitrary.

Let us pick a specific practical power to be delivered, e.g. 10A @ 24VDC.
You are delivering 10A x 24V = 240W of electrical power.

You need two wires. It is not necessary to ground either wire.

Should you ground one of the wires? That is a completely separate question and the answer depends on the specific situation and application.

#### crutschow

Joined Mar 14, 2008
30,818
Any current going to a circuit always requires a return path for that current.
Whether that return goes to ground on not depends upon the circuit the current is going to.

#### WBahn

Joined Mar 31, 2012
27,471
Let's assume that I have to receive 100V input to a pcb.
If DC, should I expect one cable or two cables?

If two, then should one be grounded?

Can it be just one? (which automatically refers to gnd)

I don't know if this question goes in the pcb section or here, I think it's a basic physics/electrotechnics concept... so I thought it correct to put it here. The admin can move it if he thinks it is correct.
By "cables", do you mean wires/conductors?

With damn few exceptions, any electrical circuit requires two paths in order to form a complete circuit.

Whether one of them is "grounded" depends on lots of other things -- not the least of which is what is meant by "ground"?

How does something "automatically" refer to gnd (and, again, what is "gnd")?

As for the question that is, at least somewhat, implied by the thread title, yes, you can have both AC and DC on the same wire -- that's done all the time.

#### kalemaxon89

Joined Oct 12, 2022
44
Let us assume that you chose 100V as an arbitrary value.
But it is not always arbitrary.

Let us pick a specific practical power to be delivered, e.g. 10A @ 24VDC.
You are delivering 10A x 24V = 240W of electrical power.

You need two wires. It is not necessary to ground either wire.

Should you ground one of the wires? That is a completely separate question and the answer depends on the specific situation and application.

By "cables", do you mean wires/conductors?

With damn few exceptions, any electrical circuit requires two paths in order to form a complete circuit.

Whether one of them is "grounded" depends on lots of other things -- not the least of which is what is meant by "ground"?

How does something "automatically" refer to gnd (and, again, what is "gnd")?

As for the question that is, at least somewhat, implied by the thread title, yes, you can have both AC and DC on the same wire -- that's done all the time.
If one day I were to make a PCB where I am asked to simply handle 100V DC input (from I don't know where) with a resistor divider ... I should expect two wires (+ and -) entering the pcb ... right?

That is, I'm wondering if a DC signal can travel on one wire, or does it necessarily need two.
As I understand it, all electrical power (AC and DC) has at least one 'live wire' and one common or ground wire.

#### MrChips

Joined Oct 2, 2009
27,168
If one day I were to make a PCB where I am asked to simply handle 100V DC input (from I don't know where) with a resistor divider ... I should expect two wires (+ and -) entering the pcb ... right?

That is, I'm wondering if a DC signal can travel on one wire, or does it necessarily need two.
As I understand it, all electrical power (AC and DC) has at least one 'live wire' and one common or ground wire.
The 'live wire' and 'common or ground wire' even though in common usage is a misnomer.

You need two conductors from the supply to the load in order to create a continuous current loop.
In the circuit diagram shown below, both conductors are 'live'.

#### panic mode

Joined Oct 10, 2011
2,303
do you understand the concept of an electrical circuit? circuit is something forming a closed loop. voltage is used to specify potential difference between two points. you cannot have potential at one point without telling what is that relative to?

#### WBahn

Joined Mar 31, 2012
27,471
If one day I were to make a PCB where I am asked to simply handle 100V DC input (from I don't know where) with a resistor divider ... I should expect two wires (+ and -) entering the pcb ... right?

That is, I'm wondering if a DC signal can travel on one wire, or does it necessarily need two.
As I understand it, all electrical power (AC and DC) has at least one 'live wire' and one common or ground wire.
You need two connections to have a complete circuit. In most instances, there is nothing magical that makes one of them "live" and the other "common" or "ground". Consider a 9 V battery. When you connect that to a circuit, you need to connect both terminals otherwise nothing happens -- the battery just sits there looking stupid. If you must call one of them "live", then flip a coin to choose which one.

There ARE instances where these terms (live, common, ground) are extremely relevant, the most common one being power systems in which one conductor truly is connected to physical earth ground at some point, but if you are just bringing in a 100 V DC input to your board, that is likely not the case. But you may still have to be very aware of how things are connected external to your board if your board, in fact, connects to ANY other things off the board. The common case where this is an often-overlooked issue is when someone connects oscilloscope probes to a circuit without taking into consideration the fact that nearly all scope probes are referenced to the oscilloscope's ground connection to the building's power supply.