Sign of electric current - Page 1
Forum section: electronics / beginners

author: eeguy
author_rank: Regular Contributor
timestamp: October 02, 2020, 01:39:23 am
content: Hi, in first year introduction course, we define a convention for positive current flow. During calculations of mathematical model of circuits, a negative current means current flowing in the opposite direction. In real life, can current be negative? If we measure the current flowing through a DC motor when it is turning clockwise and then counter-clockwise, will we see both positive and negative values? I think so but somebody said that current is always positive. Who is correct?

author: greenpossum
author_rank: Frequent Contributor
timestamp: October 02, 2020, 01:45:45 am
content: What is your aversion to "negative" current? Positive and negative are mathematical concepts. As you know, the charge carriers are electrons so electron flow is actually opposite to current flow.

author: Cerebus
author_rank: Super Contributor
timestamp: October 02, 2020, 01:51:09 am
content: Quote from: greenpossum on October 02, 2020, 01:45:45 amHe doesn't have one, someone else does. Read the question again.

author: Cerebus
author_rank: Super Contributor
timestamp: October 02, 2020, 01:59:53 am
content: Quote from: eeguy on October 02, 2020, 01:39:23 amYou're right, with the caveat that a motor (presumably a DC one) isn't a good example. The way to make the current in a DC start flowing out, rather than in, is to turn the motor faster than the driving voltage would naturally turn it (making the motors internal 'back EMF' greater than the 'forward EMF' across its terminals). In general, motors become generators when you start 'pushing' mechanical work into them, rather than taking mechanical work out. Like I say, a motor is a poor (i.e. overcomplicated) case to make a simple example from.The really obvious place you'll see positive and then negative current all over the place is in AC, either a signal or power. With mains power lines for 1/100th or 1/120th of a second the current flows in one direction, and then for the next 1/100th or 1/120th of a second it flows in the other, ad infinitum. Which direction you call 'positive' and which 'negative' is, as you've alread correctly noted, a matter of convention.

author: IanB
author_rank: Super Contributor
timestamp: October 02, 2020, 02:05:39 am
content: Quote from: eeguy on October 02, 2020, 01:39:23 amYou can do the experiment yourself to check this out. It is always better to do experiments if you are not sure about something.Here is the experiment. Get a small DC motor, some wires and a battery. The motor can be any small motor such as found in toys or hand held fans.Connect the motor to the battery and see which way it spins. Now reverse the connections on the battery, and see which way it spins now. If it is a small DC motor it should spin in the opposite direction.The direction a DC motor spins depends on the direction of the current flow. If the motor reversed direction, it means the current also reversed direction.Current has a magnitude and a direction. If a current has a magnitude of 3 amps it is always 3 amps, whichever way it is flowing. It will have the same heating effect either way.However, if you define one direction as positive, then you can say the current in that direction measures +3 amps. If you reverse the direction, then it becomes -3 amps.By convention, current is positive if it is flowing from plus to minus, or from a more positive potential to a less positive potential.

author: ledtester
author_rank: Super Contributor
timestamp: October 02, 2020, 02:15:39 am
content: When you analyze a circuit using Kirchoff's laws you have to pick an orientation for each segment between nodes. This orientation determines which direction represents positive/negative current flow.So one person could decide that electron flow from one node to another represents positive current flow while another person adopts the view that it represents negative current flow. This is fine as long as both of them stick to their choice of orientations for all of the their calculations.This is why in circuit diagrams a current quantity usually has an arrow associated with it to indicate the direction of positive current flow for that segment in the circuit -- e.g.:So -- yes there can be negative current flow if current happens to flow in the opposite direction that you choose to represent positive current flow in that segment of the circuit.

author: eeguy
author_rank: Regular Contributor
timestamp: October 02, 2020, 03:47:46 am
content: Quote from: IanB on October 02, 2020, 02:05:39 amThanks for the suggestion. I recall doing that experiment before. Reversing the connection to the battery reversed the direction of rotation.What I get confused is that on the specs, the current range of some current sensors is positive (e.g. 0-5A) while that of the others have range of both positive and negative values (e.g. +/5A). Does that mean current sensors that have only positive current range listed will only be able to measure the current when they are flowing in one (whichever way is defined as positive) direction? For example, if a DC motor repeatedly rotates +/30 degrees, does that mean such sensors can only measure the current when they are flowing in one direction and therefore not as good or accurate (as we only get half the measurements) as those sensors that have current range from negative to positive (e.g. +/- 5A)? Given that in real life, current can be both positive and negative, what is the use of sensors that only take positive range of current?

author: ejeffrey
author_rank: Super Contributor
timestamp: October 02, 2020, 03:54:48 am
content: Quote from: eeguy on October 02, 2020, 01:39:23 amWell, generically current is a vector -- it has a magnitude and direction. But if you are considering a wire or something similar that confines you to quasi-1D, then a vector is the same as a signed number.

author: IanB
author_rank: Super Contributor
timestamp: October 02, 2020, 04:47:42 am
content: Quote from: eeguy on October 02, 2020, 03:47:46 amWell, the normal name for a current sensing instrument is an ammeter.In the days before digital meters, ammeters had an analog dial with a needle. Many ammeters had the needle resting at one end of the scale, and increasing current would move the needle round towards the other end ("full scale"). If you tried to reverse the current on these meters the needle would go backwards and hit the stop.Some ammeters had the needle sitting in the middle of the scale and could be deflected left or right. These might be used in a Wheatstone bridge to balance the current to zero.Digital meters generally don't care, and will simply show reversed current with a minus sign in front of the value.

author: eeguy
author_rank: Regular Contributor
timestamp: October 02, 2020, 07:23:27 am
content: What is the difference between digital current sensor circuits that have only positive range vs. those that have both +/- range?

author: MosherIV
author_rank: Super Contributor
timestamp: October 02, 2020, 07:38:48 am
content: If it only measures positive current then it can only sense/measure current in 1 direction.It it measures +/- current then it can measure current in both directions.Edit: Most digital current meters measure the voltage drop across a shunt (calibrated, low value resistor with low temperature coefficient).So it ether measures the voltage only in 1 direction or both.

author: eeguy
author_rank: Regular Contributor
timestamp: October 02, 2020, 01:03:30 pm
content: Quote from: MosherIV on October 02, 2020, 07:38:48 amThanks. Given that current travels in both directions, why manufacturers made less accurate meters that only measure in one direction?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 02, 2020, 01:36:59 pm
content: Quote from: eeguy on October 02, 2020, 01:03:30 pmThere are so many applications. For example, if you have a load that cannot supply current back to the supply, you can only have current in one direction, thus unidirectional meter suffices. If analog, you'd never see the needle go to one of the sides, this would be wasted display area. If digital, you are wasting half of the range of the analog-to-digital converter circuit. Hence unidirectional design sometimes makes sense.As for less and more accurate meters, this is a separate issue from being uni- or bidirectional. More accurate meter generally costs more to design and manufacture.

author: Cerebus
author_rank: Super Contributor
timestamp: October 02, 2020, 01:47:29 pm
content: I see that there's several people here confusing ameterwith asensor.When the OP says a 'digital current sensor' I suspect that he's talking about something like an integrated Hall effect sensor IC which measures the current through an isolated conductor with a Hall effect sensor and an ADC, and spits out binary the other side. These are common in motor control and other process control systems where isolated current measurements are the order of the day because you don't want to galvanically connect a 400V high side current sense resistor to your 3.3V control logic. A much older technique (AC only) is to use a current sense transformer around the conductor whose current one wants to measure.As far as integrated current sensors go there are two types available, ones that can only register positive current (i.e. unidirectional sensors) and those that will measure current in both directions. e.g. The Allegro ACS722 series has parts like the ACS722LLCTR-10AU-T which will measure 0-10A, and the ACS722LLCTR-10AB-T which will measure ±10A. (The Allegro parts happen to be analogue rather than digital, producing an output voltage proportional to the current sensed.) Which you choose to use is purely based on the application requirements.

author: TimFox
author_rank: Super Contributor
timestamp: October 02, 2020, 02:02:09 pm
content: Quote from: ejeffrey on October 02, 2020, 03:54:48 amNo, current is ascalar, which can be positive or negative. Current densityJ(A/m2) is a vector. To obtain the current, you need to integrate the scalar product ("dot product") ofJdA, where the area vector is normal to the area, with a direction defined by the user. For a simple wire with uniform current density, use the area of a circle cutting the wire perpendicular to its direction. That choice of area vector orientation and the flow of charge through the wire can give either a positive or negative value for the current. Similarly, voltage (potential) is a scalar that can be positive or negative. The voltage gradient (E-field) is a vector, and the scalar-product integral ofEdsalong a path between two points gives the potential difference (voltage) between those two points.

author: eeguy
author_rank: Regular Contributor
timestamp: October 02, 2020, 02:39:01 pm
content: Quote from: Cerebus on October 02, 2020, 01:47:29 pmThanks for pointing this out. Yes, I have been talking about integrated current sensors. I got confused when I saw sensors that only accept positive current. So it is not true that current is only positive in the real-world. It is the manufactures who decided to make current sensors that accept only positive current.

author: eeguy
author_rank: Regular Contributor
timestamp: October 02, 2020, 04:07:08 pm
content: Quote from: Siwastaja on October 02, 2020, 01:36:59 pmWhat kind of loads cannot supply current back to the supply? Can you provide some examples?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 02, 2020, 04:25:31 pm
content: Quote from: eeguy on October 02, 2020, 04:07:08 pmMost! Think about light bulb. Heater. Your smartphone. Even some type of motors, like a series wound (universal) motor. These are just loads, not generators under any circumstance (ignoring very short reverse current spikes under edge conditions because they might have a relatively small amount of capacitance).It'seasierto list those thatcanbackfeed. A permanent magnet DC motor is a good example: if you rotate it faster than the equivalent speed of the applied external voltage, then it will work as a generator and feed the current to the opposite direction, compared to when it acts as a load. You can try this out: rotate a small DC motor by hand and you can measure both voltage, and current being generated if you short it or add a load like an LED. The LED will even light up from the generated current. The direction of this current is opposite to what it would be if you were going to make the motor turn, as a motor, by applying voltage to it.This is very relevant in electric vehicles, where the drive motor can be used either as a motor, discharging the battery pack, or as a generator during motor braking, charging the battery pack. The sign of the current changes.The choice, which direction is +, which is -, is completely arbitrary and up to you as a designer.

author: MosherIV
author_rank: Super Contributor
timestamp: October 02, 2020, 04:49:43 pm
content: QuoteThat depends on your point of reference.Current flows from the higher potential (Volts) to lower potential terminal.If by convetion, you measure volts between the 'positive' (higher potential) terminal and 'negative' (lower potential) terminal. Where the negative terminal is the point of reference. You would also measure the current flow from higher to lower, the current should always be positive in this case.Now, you can have negative Volts. The Voltage is lower than the point of reference.Since current always flows from higher potential to lower, the current will flow from the point of reference to the lower potential (negative). And with the same convention, the current is now negative.Hope I have not confused you.

author: andy3055
author_rank: Super Contributor
timestamp: October 02, 2020, 05:49:17 pm
content: This is how I would interpret it:Imagine a resistor R with one end at -V and the other end at -2V potential. Electrons will flow from the more negative end (-2V) to the less negative (-V) end. This is interpreted as the convention, where the current I will flow from the more positive point (-v) to the less positive (-2V) point. Now, if we do the math:V = IRI = V/RIf the ref point voltage is: -V and the measured point voltage is: -2V, as -V is greater than -2V,I = {-V-(-2V)}/R = {-V+2V}/R = V/RV/R is a positive component. It means that the assumption we made that the current I flows from -V point to -2V point is correct. Considering that a current is negative leads to ambiguity, as electrons can only flow in one direction (more –ve to less –ve). Treating the current as a positive quantity and the voltage as a +ve or -ve value depending on which point is more +ve (or more –ve) with respect to the other, makes it simpler to understand.