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polarity 

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Polarity is a term used in electricity, magnetism, and electronic signaling. Suppose there is a

constant voltage , also called an electric potential or electromotive force (EMF), between two objects or

points. In such a situation, one of the objects or points (poles) has more electrons than the other. The

pole with relatively more electrons is said to have negative polarity; the other is assigned positive

polarity. If the two poles are connected by a conductive path such as a wire, electron s flow from the

negative pole toward the positive pole. This flow of  charge carriers constitutes an electric current . In

physics, the theoretical direction of current flow is considered to be from positive to negative by

convention, opposite to the flow of electrons.

The movement of electric charge carriers inevitably produces a magnetic field. Conversely, any magnetic

field is the result of the motion of charge carriers. In a permanent magnet, a magnetic field is produced

by the composite motions of electrons in geometrically alignedatom s. A magnetic field is characterized

by poles called north and south. Magnetic polarity refers to the orientation of these poles in space.

In digital communications, data is composed of short-duration pulses called bit s (binary digits). There

are two possible states for each bit: logic 0 (also called low) and logic 1 (also called high). In a closed

circuit, these logic elements are represented by direct currentvoltages. A high-speed data signal varies

rapidly between the low and high states. Common values are approximately +0.5 volts for low and +5

volts for high. In some cases different values are used, for example, -3 volts for low and +3 volts for high,

or -5 volts for low and -0.5 volts for high. If both voltages have the same polarity, the signal is called

unipolar; if the voltages have opposite polarity, the signal is called bipolar.

Understanding Polari ty Markings

In order to select the proper converter for your electricaldevice, you need to be sure that the polarity is correct.Polarity is generally described as either "tip positive" or"tip negative" and is often indicated using the symbologyto the right. You will need to know the polarity for both theconverter and for the power jack on your device.

If the converter has a fixed tip, the polarity should beindicated either on the lable or in the manual or

instructions. If the converter has a changeable tip, polarity should be indicated by markings on

the tip and the connection point.The electrical device will generally have the polarity marked very near to the jack itself. If thereis not a polarity marking, you should check the manual or instructions for power requirements. Ifthe information is still not found, you should contact the device's manufacturer for assistance.

Note: Devices which require a proprietary adapter or converter may not list this information atall, as using a non-approved adapter or converter could

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Electrical polarity (positive and negative) is present in every electrical circuit. Electrons flow from

the negative pole to the positive pole. In a direct current (DC) circuit, one pole is always negative,

the other pole is always positive and the electrons flow in one direction only. In an alternating

current (AC) circuit the two poles alternate between negative and positive and the direction of the

electron flow reverses.

Positive and negative[edit] 

In DC circuits, the positive pole is usually marked red (or "+") and the negative pole is usually

marked black (or "−"), but other color schemes are sometimes used in automotive and

telecommunications systems.

In cars[edit] 

On a car battery, the positive pole usually has a larger diameter than the negative pole.

Modern cars have a "negative earth" electrical system. In this case the negative terminal of the

battery is bonded to the vehicle's chassis (the metallic body work) and the positive terminal provides

the "live" wire to the various systems. However, some older cars were built with a "positive earth"

electrical system, in this case the positive terminal of the battery is bonded to the chassis and the

negative terminal for the live.

 AC systems[edit] 

In AC systems the two wires alternate polarity many times per second. In this context, "polarity"

refers to the order in which the neutral and phase wires are connected. This is meaningful only withrespect to ground potential. Reverse polarity could be dangerous.

In domestic mains wiring the neutral is commonly earthed at substations and generators. There is

one wire that is designated "hot" and colored black while the other wire is designated neutral,

colored white, and is bonded to earth at the source. The power to the wires alternates in polarity and

varies in potential from 0 - 120 volts depending at which point in the generation cycle you analyze. At

one point in time, the white wire will be at negative with reference to the black wire while 180° later in

the cycle the opposite is true. There is actually a point in the cycle where the wires have no potential

difference to each other as the alternation passes through the zero potential point while moving to

reversal. The frequency of this cycle is 50 or 60 times per second, depending on which country youare in.

In the US, national standards call for the hot side of the line to be a black wire, or in the case of 120-

240 VAC, the second hot wire (2nd phase) is to be red. The neutral side of the line (transformer

secondary center tap) is to be white. The neutral wire is required to be bonded to earth at the service

entrance point of a building. This is primarily for the prevention of lightning injury to occupants and

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damage to the building from fire caused by a lightning strike on the incoming lines. There is also a

ground conductor that must be run along with any circuit wires to provide for ground bonding of all

metal housings on equipment and also for bonding outlet boxes to ground potential. That ground

wire is never connected to the neutral line except at the main service entrance panel. It can be bare

(or insulated if colored green) and can be either copper or aluminum. Along with augmenting thelightning strike protection, another important safety factor is provided by bonding this ground wire to

any metal appliance housing. This makes it impossible for the metal housing to ever become live

with reference to ground since it would constitute a short circuit and would trip the circuit breaker.

This ground conductor will never carry any current under normal operation. Its primary purpose is to

carry fault currents.

Polarity Symbols:

Many electrical devices use direct current (DC) power, often provided by an AC (alternating current) 

to DC adapter , also known as a wall wart or power brick. The polarity of the adapter cord must

match the polarity of the device, meaning that the positive contact of the plug must mate with the

positive contact in the receptacle, and the negative plug contact must mate with the negative

receptacle contact. Since there is no standardization of these plugs, a polarity symbol is printed on

the case indicating which type of plug is needed.

The commonly used symbol denoting the polarity of a device or adapter consists of a black dot with

a line leading to the right and a broken circle (like the letter "C") surrounding the dot and with a line

leading to the left. At the ends of the lines leading right and left are found a plus sign (+), meaning

positive, and a minus sign (-), meaning negative.

The symbol connected to the dot (usually the symbol found to the right) denotes the polarity of the

device or adapter. Thus a device with a plus sign at the end of the line leading from the center dot is

said to have "positive polarity" and requires an adapter that has positive polarity. A device with a

minus sign at the end of the line leading from the center dot has "negative polarity" and requires an

adapter of negative polarity.

Centre Positive 

Indicates that the centre (tip) of the output plug is Positive (+) and the barrel of the output plug is

Negative (-).

Centre Negative 

Indicates that the centre (tip) of the output plug is Negative (-) and the barrel of the output plug is

Positive (+).

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Power Supply Units Abbreviations and Symbols

The rating plate of an Extra Low Voltage Power Supply Unit (ELVPSU) shows various symbols and

abbreviations representing ratings, class, insulation, polarity and other electrical safety and EMC

compliance details.

DC Adaptor Polarity Symbols

The polarity symbol on the rating plate of an AC to DC power supply indicates if the centre (or tip) of

the output plug is positive (+) or negative (-). It is important to use a power supply having the correct

polarity for the host device. Reverse polarity may result in malfunctioning or damaged equipment.

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Centre Positive.

Indicates that the centre (tip) of the output plug is Positive (+) and the barrel of the output plug is

Negative (-).

Centre Negative.

Indicates that the centre (tip) of the output plug is Negative (-) and the barrel of the output plug isPositive (+).

What is Polarity?

In the realm of electronics, polarity indicates whether a circuit component is symmetric or not. A

non-polarized component – a part without  polarity – can be connected in any direction and still

function the way it’s supposed to function. A symmetric component rarely has more than two

terminals, and every terminal on the component is equivalent. You can connect a non-polarizedcomponent in any direction, and it’ll function just the same. 

A polarized component – a part with polarity – can only be connected to a circuit in one direction. A

polarized component might have two, twenty, or even two-hundred pins, and each one has a unique

function and/or position. If a polarized component was connected to a circuit incorrectly, at best it

won’t work as intended. At worst, an incorrectly connected polarized component will smoke, spark,

and be one very dead part.

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 An assortment of polarized components: batteries, integrated circuits, transistors, voltage regulators,

electrolytic capacitors, and diodes, among others. 

Polarity is a very important concept, especially when it comes to physically building circuits. Whether

you’re plugging parts into a breadboard, soldering them to a PCB, or sewing them into an e -textile

project, it’s critical to be able to identify polarized components and to connect them in the correctdirection. So that’s what we’re here for! In this tutorial we’ll discuss which components do and don’t

have polarity, how to identify component polarity, and how to test some components for polarity.

Consider Reading

If your head’s not swimming yet, it’s probably safe to read through the rest of this tutorial. Polarity is

a concept which builds on some lower-level electronics concepts and reinforces a few others. If you

haven’t already, consider checking out some of the below tutorials, before you read through this one.

  How to Use a Multimeter 

  What is a Circuit 

  Voltage, Current, Resistance, and Ohm’s Law 

  How to Use a Breadboard 

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Diode and LED Polarity 

Diodes only allow current to flow in one direction, and they’re always polarized. A diode has two

terminals. The positive side is called the anode, and the negative one is called the cathode.

The diode circuit symbol, with the anode and cathode marked. 

Current through a diode can only flow from the anode to the cathode, which would explain why it’s

important for a diode to be connected in the correct direction. Physically, every diode should havesome sort of indication for either the anode or cathode pin. Usually the diode will have a line near

the cathode pin, which matches the vertical line in the diode circuit symbol.

Below are a few examples of diodes. The top diode, a 1N40001 rectifier, has a grey ring near the

cathode. Below that, a 1N4148 signal diode uses a black ring to mark the cathode. At the bottom are

a couple surface mount diodes, each of which use a line to mark which pin is the cathode.

LEDs

LED stands for light-emitting diode, which means that much like their diode cousins, they’re

polarized. There are a handful of identifiers for finding the positive and negative pins on an LED. You

can try to find the longer leg, which should indicate the positive, anode pin.

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Or, if someone’s trimmed the legs, try finding the flat edge on the LED’s outer casing. The pin

nearest the flat edgewill be the negative, cathode pin.

There might be other indicators as well. SMD diodes have a range of anode/cathode identifiers.

Sometimes it’s easiest to just use a multimeter to test for polarity. Turn the multimeter to the diode

setting (usually indicated by a diode symbol), and touch each probe to one of the LED terminals. If

the LED lights up, the positive probe is touching the anode, and the negative probe is touching the

cathode. If it doesn’t light up, try swapping the probes around. 

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The polarity of a tiny, yellow, surface-mount LED is tested with a multimeter. If the positive lead

touches the anode and negative touches the cathode, the LED should light up.  

Diodes certainly aren’t the only polarized component. There are tons of parts out there that won’t

work if connected incorrectly. Next we’ll discuss some of the other common polarized components,

beginning with integrated circuits.

Integrated Circuit Polarity 

Integrated circuits (ICs) might have eight pins or eighty pins, and each pin on an IC has a unique

function and position. It’s very important to keep polarity straight with ICs. There’s a good chance

they’ll smoke, melt, and be ruined if connected incorrectly.

Through-hole ICs usually come in a dual-inline package (DIP) – two rows of pins, each spaced by

0.1" wide enough to straddle the center of a breadboard. DIP ICs usually have a notch to indicate

which of the many pins is the first. If not a notch, the IC might have an etched dot in the casing near

pin 1.

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 An IC with both a dot and a notch to indicate polarity. Sometimes you get both, sometimes you only

 get one or the other. 

For all IC packages, pin numbers increase sequentially as you move counter-clockwise away from

pin 1.

Surface-mount ICs might come in QFN, SOIC, SSOP, or a number of other form-factors. These ICs

will usually have adot near pin 1.

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 An  ATmega32U4 in a TQFP package, next to the datasheet pinout. 

Electrolytic Capacitors 

Not all capacitors are polarized, but when they are, it’s very  important not to mix their polarity up.

Ceramic capacitors – the small (1µF and less), commonly yellow guys – are not polarized. You can

stick those in either way.

Through-hole and  SMD 0.1 F ceramic capacitors. These are NOT polarized. 

Electrolytic caps (they’ve got electrolytes), which look like little tin cans, are polarized. The negative

pin of the cap is usually indicated by a “-” marking, and/or a colored strip along the can. They

might also have a longer positive leg.

Below are 10µF (left) and a 1mF electrolytic capacitors, each of which has a dash symbol to mark

the negative leg, as well as a longer positive leg.

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Applying a negative voltage for an extended period to an electrolytic capacitor results in a briefly

exciting, but catastrophic, failure. They’ll make a  pop, and the top of the cap will either swell or burst

open. From then on the cap will be as good as dead, acting like a short circuit.

Other Polarized Components 

Batteries and Power Supplies

Getting polarity right in your circuit all starts and ends with getting the power supply connected

correctly. Whether you’re project’s getting power from a wall-wart or a LiPo battery, it’s critical to

make sure you don’t accidently connect them backwards and apply -9V or -4.2V to your project

accidently.

 Anyone that’s ever replaced batteries knows how to find their polarity. Most batteries will indicate the

positive and negative terminals with a “+” or “-” symbol. Other times it might be red wire for positive

and a black wire for negative.

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 An assortment of batteries. Lithium polymer , coin cell , 9V alkaline, AA alkaline, and   AA NiMH . Each

has some way to represent positive or negative terminals.  

Power supplies usually have a standardized connector, which should usually have polarity itself.

A barrel jack, for example, has two conductors: outer and inner; the inner/center conductor is usually

the positive terminal. Other connectors, like a JST, are keyed so you just can’t connect them

backwards.

For extra protection against reversing power supply polarity, you can add reverse polarity

protection using a diode, or a MOSFET.

Transistors, MOSFETs, and Voltage Regulators

These (traditionally) three-terminal, polarized components are lumped together because they share

similar package types. Through-hole transistors, MOSFETs, and voltage regulators commonly come

in a TO-92 or TO-220 package, seen below. To find which pin is which, look for the flat edge on the

TO-92 package or the metal heatsink on the TO-220, and match that up to the pin-out in the

datasheet.

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 Above, a  2N3904 transistor  in a TO-92 package, note the curved and straight edges. A 3.3V

regulator  in a TO-220 package, note the metal heatsink on the back.  

Etc.

This is just the tip of the polarized-component iceberg. Even non-polarized components,

like resistors, can come in polarized packages. A resistor pack – a grouping of five-or-so pre-

arranged resistors – is one such example.

 A polarized resistor pack. An array of five 330Ω resistors, all tied together at one end. The dot

represents the first, common pin. 

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Fortunately, every polarized component should have some way to inform you which pin is which. Be

sure to alwaysread the datasheets, and check the case for dots or other markers.

Electrical Polarity TestingDomestic alternating current (AC) electrical circuits consist of two current-carrying conductors. One is

 positive or "hot" and has a constantly changing potential difference -- difference in electrical charge --

compared with the Earth, and the other is neutral or "cold" and has about the same potential difference as

the Earth. 

Reversed Polarity

  Domestic appliances are designed to keep the positive conductor as far away from users as

 possible. Reversing the polarity -- that is, reversing the positive and neutral conductors --

significantly increase the risk of electric shock from an appliance.

Testing

  Alternating current can be tested with a multimeter or volt ohm meter. Testing with a multimeter

typically involves touching positive and negative leads from the meter to the wiring to be tested.

A flashing negative sign ("-") before the numerical reading on the meter indicates that the

 polarity is incorrect.

Correction

  If you find an outlet whose polarity is reversed, you should turn off your domestic electricity

supply before inspecting the wiring. Positive wires, which are typically red or black, should be

connected to the brass terminal, while neutral wires, which are typically white or gray, should be

connected to the lighter-colored, chrome terminal.