I'm converting a battery powered device to be powered off a USB connection. I think I have everything figured out except one thing.

I understand there is a GND wire inside the USB cable for power, and also the Vbus wire (I think Vbus is also called the +5v wire). I'm lead to believe one is like the positive wire, and the other is like the negative wire. If I just look at the two wires, I would think the Black one is the negative wire, so I would hook it to the negative terminal of the formally battery powered device (after doing proper connections with a resistor of course :) ). I would also think that the Red wire is the positive wire and hook it to the devices positive terminal. I would think all of that strictly based off colors and experience with other things like batteries and solar (maybe I was wrong then) .

However after doing research, the names of these two wires makes me think the opposite. The Black wire is actually Ground, which makes me think it's actually like the "return path" for the electricity (so positive). The Red wire is referred to as +5v, which makes the think that is where the "source" of electricity is coming from (so negative).

Which of these wires are the "source" (negative), and which is the "return" (positive) ? Thank You everyone!!

EDIT: { Thank You everyone for commenting and answering! All of your replies are helping a lot!!! I guess I always try to think about electron flow, and how electrons flows from an area that is really negative to an area that is less negative. I always thought a batteries negative terminal was the source, and the positive terminal was the return. To hear CURRENT flows from what we call and label the positive terminal is really shocking to me and made me take a step back, yet it makes a couple other things make sense.

I find it sad that the most fundamental and basic electrical principles and ideas can actually be quite confusing. }

  • 7
    \$\begingroup\$ 1: you cannot trust colors inside of cables. 2: '+' means positive, just like in math. 3: Your usage of "source" and "return" are unlike anything I've seen in 30 years in industry; I believe you are using them to mislead yourself. 4: Black is probably negative (and ground), red is probably positive (+5V), but see (1), and check with a voltmeter. \$\endgroup\$
    – TimWescott
    Commented Nov 27, 2018 at 2:01
  • \$\begingroup\$ I've got to wonder if this guy's trolling, Benjamin Franklin got the polarity of electricity wrong. and now it has confused this guy. Ben Frank! \$\endgroup\$ Commented Nov 27, 2018 at 7:00
  • 1
    \$\begingroup\$ @Jasen , I assure you that the question is real and a serious question. HOWEVER, my username Ben Frank is not my real name, and is 100% inspired by Benjamin Franklin. I respect your skepticism! (Also thanks for your answer below :)) If I wanted to troll I'd troll the chat in call of duty, which I do do sometimes :) \$\endgroup\$
    – Ben Frank
    Commented Nov 29, 2018 at 1:27

3 Answers 3


Unfortunately, when we talk about current flow it is opposite of the direction that the electrons move. It can cause a lot of confusion.

Here is an example circuit with a battery and a lamp. We usually use the following terminology:


  • You will almost never find anyone in the industry talking about electron flow. It is always just "current", flowing from the positive ("+") terminal to the negative ("-") terminal.
  • Because of this terminology, we say that the positive terminal is the "source" of the current.
  • Voltages are measured between two points. Imagine that the battery is 1.5V. If you connect your voltmeter with it's red probe on "positive" and the black probe on "negative", it should read 1.5V. If you reverse the probes, you will see "-1.5V".
  • In an electrical schematic, the battery's negative terminal is generally referred to as "ground" or "return". All other voltages in the circuit are measured relative to this node. It is sometimes referred to as "zero volts", or "0V", to make it more intuitive.
  • \$\begingroup\$ This answer is clear and easy. Ben Frank, and probably the original one too, was confused by "current flow" and "electron flow". When too much theory kills theory! :) \$\endgroup\$
    – Fredled
    Commented Jan 28, 2019 at 17:18
  • \$\begingroup\$ And, here's the xkcd :) \$\endgroup\$
    – bitsmack
    Commented Jan 28, 2019 at 19:49

This confusion comes from too much knowledge about electrons and their sign of charge they carry in Physics. In Electrical Engineering the "negative" terminal was never meant as "source". The "positive" terminal is the "source", electrical current is assumed to flow from positive terminal to negative terminal (even if electrons are flying in opposite way), thus where the term "return current" is originated. In most DC-powered cases the negative terminal has traditionally black color and is associated with (-) of a equivalent battery source, and a red wire is associated with positive (+) terminal. So your initial consideration was correct. But it is always a good idea to check twice, with a multimeter.

  • \$\begingroup\$ Thank You! This makes me have a new question now. Would I place the proper resistor between the red positive wire and the devices positive terminal? \$\endgroup\$
    – Ben Frank
    Commented Nov 27, 2018 at 3:15
  • \$\begingroup\$ @BenFrank, no, no resistor is needed. \$\endgroup\$ Commented Nov 27, 2018 at 3:40
  • \$\begingroup\$ I didn't put this in the question, but the device I am powering runs on 2.5 volts, and I believe USB supplies 5 volts. I have some 51 ohm resistors that should get me close enough to 2.5 volts. Wouldn't I need to use them? (Thanks for putting up with the amateur I am! ) \$\endgroup\$
    – Ben Frank
    Commented Nov 27, 2018 at 3:44
  • \$\begingroup\$ @BenFrank. why do you believe that 51 Ohm will provide you with 2.5V? Your device likely has a variable consumption, so your result could be anywhere from zero to +5V. You need a DC-DC voltage regulator, not just a resistor. \$\endgroup\$ Commented Nov 27, 2018 at 3:50
  • \$\begingroup\$ I think I'm going to shut up soon. I'm learning that there is a lot to electronics that I don't know :) .Anyways my device is powered on two D batteries in series. Each battery is 1.225 volts (not 1.5). So the device runs on 2.45 volts. I rounded it up to 2.5 (sorry). I thought a resistor of that value would do it because math, and a buddy told me he came to a 55 ohm resistor. \$\endgroup\$
    – Ben Frank
    Commented Nov 27, 2018 at 4:03

However after doing research, the names of these two wires makes me think the opposite. The Black wire is actually Ground, which makes me think it's actually like the "return path" for the electricity (so positive).

There's a couple of miamprehensions there.

Electricity isn't a clear concept... usually the term is used to mean elecric power or electric energy, and these quantities usually only flow in one direction, from the energy source to the consumer - they don't return.

If you mean "electric current" then yes that must return. but for historical reasons most engineers use what is called "conventional current" and consider electric current to flow from positive to negative. and so ground is considered the return. It was in fact a man with a similar name to yours Benjamin Franklin who decided which type of electric charge to call positive ans which to call negative. It wasn't until the later discovery of the electron that it was realized that he had chosen the wrong polarity.

The US Navy, teaches their technicians "electron current" which does flow from negative to positive the same direction that the actual electrons flow.

The Red wire is referred to as +5v, which makes the think that is where the "source" of electricity is coming from (so negative).

Actually it's the source of conventional current, and the return for electrons.

The black is the opposite the return for conventional current and the source for electrons.

Which of these wires are the "source" (negative), and which is the "return" (positive) ? Thank You everyone!!

The + in +5 indicates that it is the positive.

the GND is the negative and in most computers it's also connected to the safety ground via the main board and the power supply.

you should probably fit your resistor in series with the +5, but that probably won't make much difference in most cases.

I hope it's a simple device like a flashlight or a hand-held fan, a more complicated device may have variable current draw and not respond well to a resistive dropper. in that case a voltage regulator module like "mini360" may be a solution.


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