How come negative power supply has negative voltage only

Question

So I have seen power supplies that produce e.g. -48V DC (used e.g. in telecom but probably not only). For sure, that power supply is floating and in no way is negative terminal connected to earth. So, what doesn't make sense to me is why we need to specify minus sign on power supply? What does it actually means?

Current flows from positive to negative (or actually from higher potential to lower). So, if we have negatiive power supply - ok, current will flow from 0V to -48V, so it would be reversed. But then, if you have +48V PS - you swap terminals and got -48V. What's the actual difference? Is it sink vs source of current, or is it related with human safety? For me from engineering point of view it somewhat doesn't make sense unless you need both positive and negative voltage rails.

I do get this notion with ATX power supply, where we have +12V rail and -12V though

Upd 1 found this link which indcate why telecom uses negative voltage but that doesn't explain what it really is. https://deltaelectronicsindia.com/faq-items/why-does-telecom-use-negative-voltage/

Update 2 Another thing came to my mind as result of thought experiment... So we have polarized DC MCBs. Those should be connected correctly in terms of polarity, otherwise those can catch fire because arc won't be properly extinguished. So, how do you properly connect those? Something tells me that 0V should be connected to "+" terminal and -48V for "-"

Answer 1

A voltage source is negative or positive according to how it is measured. It has no inherent polarity of its own.

Only the terminals can have a polarity relative to another reference.

The telecom industry uses the positive terminal of a 48-volt battery as the reference (0 volts) making the other terminal -48V.

ATX power supply, where we have +12V rail and -12V though

These voltages are relative to the 0 volt rail which happens to be connected to ground.

If the black wire of a voltmeter is connected to -12 rail with the red wire connected to ground. A value of +12V, a positive value, would be indicated.

Whenever voltage and its polarity is discussed or measured, it is always with respect to a reference node which is usually chosen to be sensible for the associated system.

Answer 2

In telecom the positive terminal of the battery or power supply is grounded.

That makes everything powered by the -48V negative or 0 at most relative to ground, which is superior in preventing the more damaging electrochemical reactions should the circuits get wet and current leaks to ground.

Answer 3

A terminal of a power supply is marked 'positive' or 'negative' depending on which terminal is taken as the reference terminal (marked 'zero').

Here's a pictorial (with batteries) for easier understanding.

Answer 4

The importance of +48V vs -48V in the telco context is entirely related to the complete solution deployment.

The whole idea of -48V comes from the +ve terminals on the battery banks being grounded to reduce galvanic corrosion on external phone lines, which are powered at 48V.

Why this then impacts supplies is tied to the two main grounding schemes:

• IBN (Isolated Bonding Network)
• MBN/CBN (Mesh or Common Bonding Network).

The IBN model has each rack of kit "floating" (literally in insulated pads), with + & - power terminals on equipment not connected to chassis, and rack ground wires all going back separately to a common grounding bar. This was designed to limit ground loops and mains hum on analog phone lines. It delivers on this goal, but maintaining galvanic isolation on all the interfaces can be a pain, and the long ground connections have a high inductance which makes them less useful for sinking high frequency EMC.

In the CBN/MBN model, the alternative approach is taken, and the goal is to form a dense mesh of ground connections, to reduce loop impedance for HF EMC. Critically, this generally includes equipment containing a jumper/link which connects chassis ground to the "0V"/"+ve" terminal. A jumper is used so the same kit can be deployed in an IBN environment.

(This is a bit like the AC PME setup, where N & E come from a common conductor at each supply head.)

It is this default grounding of the +ve/"0V" terminals in equipment, that enforces the concept of the -ve terminals being "-48V", because every element (equipment or supplies) has to either be floating or agree that it is the + terminal that is to be grounded.

It also impacts on colour coding of wires used in exchanges, where the US telephony convention is that 0V/gnd is red (as +ve) and -48V is black (for -ve), whilst in Europe the convention is to use blue for 0v/gnd, and grey for -48V (as opposed to brown that is the default colour for a positive DC wire).

Having said that, with the exception of where the grounding link sits, there is no real difference in the supplies.

Answer 5

So, if we have negatiive power supply - ok, current will flow from 0V to -48V, so it would be reversed. But then, if you have +48V PS - you swap terminals and got -48V.

There's cases where you cannot swap terminals. For example, you have a power supply that plugs into a backplane where ground is a specific pin on a fixed connector. Here you'd have to specify whether you want a +48 or -48 V model. Internally, the manufacturer might have merely swapped the output, but the end-user can't do that.

Another case is in test and measurement. You might not want the labor of swapping the connectors when measuring the I-V curves of a batch of diodes, for example. Four-quadrant and bipolar power supplies allow you to "swap the terminals" electronically by simply programming in a positive or negative voltage.

Answer 6

If you only have a black box with two terminals and nothing else, whether you mark something 0V/+48V or -48V/0V is arbitrary.

A car battery is like that. But already an AA battery has an enclosing shell that somewhat naturally (because it might touch other things) is dubbed 0V, with an isolated tip being the +1.5V electrode.

A mains-operated power supply will tend to have one electrode that is better suited for sinking and sourcing disturbances that are referenced towards the main voltage.

And once you have more than two terminals, there will be one terminal that will be used for sourcing and sinking disturbance currents via "blocking capacitors" from arbitrary other locations. A simple 2-terminal unregulated DC power supply may be symmetric; but once you stabilize voltages, you will use one terminal as the reference (0V) and the other as the regulated terminal.

Whenever the inherent symmetry of the two-terminal black-box is breached (like when a mobile phone goes off and injects RF into it, or the capacity to the ground may play a role) and the sourcing and sinking capabilities of the two terminals stop being treatable as a compulsory matched pair, the difference between 0V/+48V and -48V/0V may surface.