What is negative voltage?

  • Just a general electronics question: What is negative voltage, like -5 Volt?

    From my basic knowledge, power is generated by electrons wandering from the minus to the plus side of the power source (assuming DC power here). Is negative voltage when electrons wander from + to -?

    Why do some devices even need it, what is so special about it?

    You will do better to think about electronics as a kind of applied math, and forget about the electrons entirely.

    Yeah, don't think about electrons. Engineers use conventional current instead ("just a convention"), since charge can flow in both directions at the same time. http://amasci.com/amateur/elecdir.html And power is not "generated" by electrons. Energy is carried in electric and magnetic fields. http://amasci.com/elect/poynt/poynt.html

    Do the electrons wander, or do the holes they want to fall into wander? Might be a question for the Electrical Philosophy SE site?

  • Kellenjb

    Kellenjb Correct answer

    10 years ago

    Someone may have better words to explain this than me, but the big thing you have to remember is voltage is a potential difference. In most cases the "difference" part is a difference between some potential and ground potential. When someone says -5v, they're saying that you are below ground.

    You also need to keep in mind that voltage is relative. So like I mentioned before, most people reference to "ground"; but what is ground? You can say ground is earth ground, but what about the case when you have a battery powered device that has no contact to ground. In this situation we have to treat some arbitrary point as "ground". Usually the negative terminal on the battery is what we consider from this reference.

    Now consider the case that you have 2 batteries in series. If both were 5 volts, then you would say you would have 10 volts total.

    But the assumption that you get 0/+10 is based off of "ground" as being the negative terminal on the battery that isn't touching the other battery and then 10V as being the location of the positive terminal that isn't touching the other battery. In this situation we can make the decision that we want to make the connection between the 2 batteries be our "ground" reference. This would then result in +5v on one end and -5v on the other end.

    Here is what I was trying to explain:

    +10v   +++   +5v
           | |
           | | < Battery
           | |
    +5v    ---   0v
           | |
           | | < Another Battery
           | |
    0v     ---   -5v

    Thanks. Just wondering though: Why would devices "choose" to require -5 Volt instead of +5 Volt? If I have a device that's powered by a "normal" Power Supply, why wouldn't the manufacturer just require +5 Volt? (Some LC Screens requite negative voltage)

    It's specific to the device. Read up on how LCD's work at a technical level and I bet it'll explain it.

    @michael, LCD screen contrast provides the reference for each of the cells. It may be rated for common 3.3V or 5V for easy integration, but the cells may need more of a drop in order to read them (molecules need to be twisted). Often 5V LCDs can use 0V contrast, even without an integrated (-ve) charge pump.

    Very helpful description! I never really fully grasped this before, but this helps a lot.

    @MichaelStum: Many LCD controllers (especially those patterned after the Hitachi 44780) require a 3.3V-5V supply to operate the logic, and 3.3V-12V to power the glass. In such situations, it's generally easier to have one rail in common between the two supplies than to have four independent rails. Hitachi found it more convenient to have the two supplies share a positive rail than a negative one. Since the magnitude of the LCD drive voltage may be higher or lower than the magnitude of the logic voltage...

    ...the voltage on the negative LCD supply pin may be above or below the voltage on the negative logic supply pin. Note that from a logic-design perspective, the negative logic supply is "ground", but from the standpoint of wiring nomenclature, designating the positive rail as "ground" makes clearer the fact that the two negative rails are independent, and either one may be higher or lower than the other.

    So if voltage is relative, why can't I choose the + terminal as ground and flip the polarity on circuits?

    I was trying to understand this in the context of a development board with a positive voltage supply (Vdd) and negative voltage supply (Vss). Can you help me make the leap from this answer to that context?

  • Imagine you're measuring the height of a car. You could take a tape measure and measure the distance from the ground to the roof of the car. "The roof of this car is 4 feet above the ground."

    You could also stand on the roof of the car and dangle the same tape measure down to the ground. "The ground is 4 feet below the roof of this car."

    Voltage works the same way. The negative sign is just a convention, in the same way that the car has the same height, regardless of which way you measure it. Flip your multimeter leads, and the negative sign will disappear.

    Ground is just a convention, too. You can pick any single point in a circuit and label it ground, but we choose specific points out of convention.

    @michaelStum, @pingswept, I always like to remind people that there is potential energy from height if you take physics. (mgh) is potential energy, if I pick the atmosphere as my reference we all suddenly have negative potential energy.

    Good points, the both of you.

    Gravitational potential energy is referenced negative by setting it to zero at infinity. Electrical potential energy is negative between two charges of opposite sign and positive between charges of the same sign. Say there's an electrical potential drop from 5V to 3V. A positive test charge q would add -2q to its potential energy (becoming less positive), while a negative test charge -q would accelerate in the other direction (+2V, from 3V to 5V), but also adding -2q to its potential energy (becoming more negative). In both cases potential energy decreases.

    @eryksun: While it's true that potential energy is measured by taking zero at infinity, if one thinks in terms of height (a more familiar concept, and the original one above), one could recognize that while one could define absolute altitude (distance to the Earth's center of mass), it is in most contexts more helpful to define elevation relative to something that's a little closer [e.g. the "natural" height of the terrain]. If e.g. a house is built in a flat area, one may describe the floor of the basement as having an elevation of -2 meters relative to the surrounding terrain.

    @supercat: My comment was about how the change in electrical potential (V) works for both positive and negative charges (coulombs). If you place a positive charge q at the 5V side, it accelerates to the 3V side (say it's in vacuum). It's kinetic energy increases and potential energy (U) decreases. It makes sense that the dV is -2V since `dU = q * dV`. A negative charge -q set at the 3V position accelerates to the 5V position. Again the potential energy has to decrease. The dV is +2V, which makes sense because `dU = -q * dV`.

  • oh why not ..

    To say you have a voltage of +5V at point A means that point A is 5 volts more positive than your chosen ground.

    To say you have a voltage of -5V at point B means that 'ground' is 5 volts more positive than point B.

    The sign just tells you the polarity of the voltage, with respect to the ground node.

  • Voltage is a difference in potential. If I connect terminal A of a device to potential of 30 volts and terminal B to a potential 20 volts. The potential from A to B is 10 volts, but the potential from B to A is - 10 volts.

    Think of it a tall building To go from floor 30 to floor 20 you go down 10 floors.

  • The first part of this question has already been answered very well.

    As for the second part, you could take the lowest supply terminal from the power supply and call that 0V, then every other voltage would be positive. However, that would be very inconvenient for many circuits. For example, common power supplies for an op-amp circuit are +12V and -12V. You could relabel the -12V output as "ground", then the old ground would be +12V and the old +12V would be +24V. Also, all your signals would be referenced to +12V and you would have to take that into account any time you measure things. Also, a lot of the power is used between the highest and middle output (in effect the charge in the top output originally came from the middle output and wants to go back there), same with the lowest output. All in all it is easier to label the middle output 0V (ground) and work with positive and negative voltages.

    All this is ignoring earth grounding issues. In real life often the ground output on the supply is literally grounded, and thinking of the whole Earth as being at +12V would just be weird.

  • Some OP-Amps, for example the veteran 741, require their supply as two voltages, one positive and the other negative with respect to the ground or zero level of the signal input and output.In this context it is natural to speak of the supplies being +15v and -15v (these are the values commonly quoted for the 741)

    An alternating voltage, such as the a.c mains supply, swings positive and negative with respect to the neutral line, which is very close to the earth potential, so "neutral" is regarded as at zero voltage.

  • Voltage between two physical points \$A\$ and \$B\$ is defined as

    \$ V_{AB} = - \displaystyle \int\limits_{A}^{B} \vec{E}.d\vec{\ell}, \$

    where \$\vec{E}\$ is the electric field on the path of the integration.

    \$V_{AB}\$ becomes negative or positive (or just zero) according to this integration. For example, if you swap the points \$A\$ and \$B\$, the sign changes.

  • If for example your power supply shows ground, 5v, -5v then your ground terminal is positive to the -5v, you get 5v from the ground that way. If you use the 5v and -5v terminals, then you will be using the -5v as the ground and you will get 10v from the 5v terminal If say there was also a 3v terminal, you would get 8v from the 3v terminal using the -5v as ground. Simple question, simple answer people. I probably know less about this than everyone else who commented here.

  • Well, just to chuck in my two cents, let's say you have an ungrounded device. With +10 volts, you're expecting the current to come of the battery, through the widget, and then... where? It's only 10 volts, so arcing through the air to ground isn't really possible, but it could go through the case into the hand of the user, or the charge could simply stay on the far end of the widget. So now you have +10 volts on one side and +8 volts or something on the other, relative to ground. The wiget only sees a 2 volt difference though.

    With a a +5V and a -5V the current is both pushed into the widget and pulled out of the widget.

    I like your approach, but if you only connect one side of a circuit to 10V and leave the other floating(both relative ground) you will find that the entire device should measure 10V, as you have just charged it to 10V, if it only went to 8V on the fare side you have a 2V drop and current is flowing. This would only happen if your measurement device was loading it.

  • Power is not generated by electrons wandering around. In fact, an electron might wander all over the place with no voltage at all applied. Energy cannot be created or destroyed. It comes from someplace, and it goes someplace. For example, when you turn the crank of a generator, you are not really generating power; you are merely transferring the power that plants absorbed from the sun before you ate and digested them. You should market such a generator as "powered by thermonuclear fusion".

    The question is "What is negative voltage?" This answer does not address the question.

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Content dated before 6/26/2020 9:53 AM