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I understand that voltage is relative to ground, so that can be negative. However, I'm currently looking at current-sensors (ACS712 current sensor) and in the performance characteristics table, it specifies the Optimized Accuracy Range. In the case of this sensor, it's being specified in Amps, ranging from -5 A to +5 A

The table

I can't find anything explaining how you could have a negative amperage. As far as I know, electric current is the rate of flow of electric charge within a (part of a) circuit.

How could the flow of charge sensed by the sensor be negative?

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    \$\begingroup\$ It's from a point of reference. So positive current would be flow with the direction of the arrow showing Ip and negative current would be against the direction of the arrow showing Ip. Often positive current flow is into the IC, and negative flow is out of the IC, but that can depend on the pin. \$\endgroup\$
    – Puffafish
    Sep 9, 2019 at 7:38
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    \$\begingroup\$ Think of a battery with a charger. When charging, current flows into the battery. Lets call that positive current. When the battery provides current then current flows out of the battery, lets call that negative current. The current sensor can measure both charging and discharging current. \$\endgroup\$
    – Steve G
    Sep 9, 2019 at 7:40
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    \$\begingroup\$ @SteveG, The direction of current through a battery that you'd call "positive" might depend on whether you were the designer of a battery charger or, of a battery-powered appliance. \$\endgroup\$
    – Solomon Slow
    Sep 9, 2019 at 17:04
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    \$\begingroup\$ Current that flows in the opposite direction to the nominated positive direction. \$\endgroup\$
    – Chu
    Sep 10, 2019 at 0:36
  • \$\begingroup\$ Consider a waterfall. Negative current is the water falling upwards. Sounds like nonsense, until you involve a pump. The minus sign is just a modifier on your direction. \$\endgroup\$
    – Mast
    Sep 10, 2019 at 7:06

8 Answers 8

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understand that voltage is relative to ground,

I prefer to disagree. A voltage is against a reference point. Often that reference point is ground but not always.

Taken the above into account your current is defined the same way.

Take a pin/port of a component or circuit. You can now define the current going into that port/pin as positive from which it follows that if current comes out of that port/pin the current is negative.

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  • \$\begingroup\$ My bad, I did mean Voltage is relative against *a* reference point, I think I'm kind of getting what you mean with your example. Thanks! \$\endgroup\$
    – Florian Humblot
    Sep 9, 2019 at 8:20
  • \$\begingroup\$ One small addition though, given your example, does that mean that in the current-sensor situation, a positive amperage would mean current going from the positive to the negative terminal, and the negative amperage would mean current going from the negative to the positive terminal with 0 A literally meaning no current? \$\endgroup\$
    – Florian Humblot
    Sep 9, 2019 at 8:23
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    \$\begingroup\$ If you have a multi-meter and a DC current flows from the positive (often RED) contact to the negative (often BLACK) contact, the meter will show a positive current. \$\endgroup\$
    – Oldfart
    Sep 9, 2019 at 8:56
  • \$\begingroup\$ Yeah, I guess I can just see it as though I was using a multimeter. Thanks for taking the time to answer my question :) \$\endgroup\$
    – Florian Humblot
    Sep 9, 2019 at 9:03
  • \$\begingroup\$ It is also worth noting that current ratings are often, but not always, symmetric. Some chips can sink current better than they can source it, and vice-versa. This comes up a lot with thermocouple readers and analog front ends. \$\endgroup\$
    – DeusXMachina
    Sep 10, 2019 at 14:28
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It means current can flow in any direction through the device.

Just like with AC mains voltage will alternate polarity over a load, the current flows either clockwise or counter-clockwise direction in the loop via load.

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Electric current, in a physical sense, is the rate of flow of electric charge indeed. But charge can flow in one direction or in the opposite direction. That's the reason for positive or negative current: it's a matter of how you set your reference.

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    \$\begingroup\$ NO, NO, NO. Electric current, (in the physical sense) is the sum of the movement of positive charges in the reference direction, and negative charges opposed to the reference direction. It happens that electrons have negative charge, so they move against the reference direction. There are plenty of positive charge carriers, protons, positive ions, holes, and negative charge carriers, electrons, negative ions, so we should not be chauvanistic about electrons. I don't normally edit answers to remove incorrect information, but in this case I have to make an exception. \$\endgroup\$
    – Neil_UK
    Sep 9, 2019 at 7:53
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    \$\begingroup\$ I'm not "chauvinistic" about electrons, and in fact I totally agree with what you're saying. I thought the OP was falling prey of the usual confusion between electron flow and conventional current, and that's the reason why I tried (and failed, it seems) to explain that conventional current is in fact a positive charge flow. I should have said "opposite to electron flow". \$\endgroup\$
    – Enric Blanco
    Sep 9, 2019 at 8:07
  • \$\begingroup\$ Electrons (negative charges) moves toward negative pole or voltage and makes appears that the holes (lack of electrons or positive) moves in the contrary direction. \$\endgroup\$
    – JWBG
    Sep 10, 2019 at 3:43
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It's current going in the opposite direction to the direction defined as positive, nothing more or less than that.

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Negative current is current flowing in the opposite direction to positive current, just like the axes on a graph have negative and positiva in opposite directions.

A sensor that can read negative and positive current could be used to mesaure rate of charging or discharing a battery. with one being a positive current and the other negative.

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Negative current is the flow of charges produced by a negative voltage.

You seem to think that current is the magnitude of the charge flow, like speed is w.r.t change of position. In fact, the current is a vector and it has a direction, like velocity. It's just that in a wire there are only two possible directions for the charges to flow, so the current becomes a scalar with a sign.

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  • \$\begingroup\$ I think your first sentence is misleading. You imply that there must be a "negative voltage" to have a negative current, but the polarity of the voltage and the direction of the current are in general independent. Current can be positive or negative in an ideal wire or superconductor without the existence of a voltage difference between two points on the conductor. \$\endgroup\$
    – Elliot Alderson
    Sep 10, 2019 at 12:39
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Quite simply this IC supplies a positive slope voltage as the output that can be directly translated to a current. If the voltage across pins 1+2 and 3+4 is negative or positive the device will still represent the current as a positive voltage.

The polarity of the voltage is what is significant here. The data sheet implies that if a negative or positive voltage is applied to the aforementioned pins then the IC can handle 5 amps regardless of the polarity.

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Let us repeat Oersted’s experiment with a long skinny Flamming valve tube, filament heater at one end and cathode at the other. Then put a much smaller magnetic compass near the middle of the tube.

Warm the filament up. Apply a voltage between the filament and cathode and note the compass needle deflect. Call that a measurement of positive current. Turn the tube off. Call the compass indication zero. Now turn the Fleming tube around and light it up with the same heater and cathode voltages. The compass needle will now defect in the opposite direction from what you previously called positive current. You have just measured negative current (Left hand rule for magnetic flux generated by electrons flowing in a vacuum)

Current flow in a wire is similar, but with a much larger number of electrons pushed in a direction at much tinier average velocity component parallel to the wire. Due to Benjamin Franklin accidentally swapping post-it notes on two Leyden jars (or something like that), electron flow is now called negative current.

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  • \$\begingroup\$ Some times is hard to explain to people who doesn't have the minimal electronic knowledge. The question reveals that state, but the data sheet indicates flow both sides to show charging and discharging. \$\endgroup\$
    – Fernando Baltazar
    Sep 10, 2019 at 6:01

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