What is polarity?

Question

hi can somebody help me and tell me what polarity is and how to test it as an electrician, i no its something to do with the terminals, but other than that i no nothing, can somebody please help, i have my test comming up soon and need to know. please simplify it as well as you can so i understand it clearly.

Answer 1

Polarity is relevant only in direct current as alternating changes polarity (usually every 1/50th of a second if you're talking about mains).

There are two terminals in an electric circuit, depending on whether you're talking to scientists, techies or engineers they have different names.This is going to take some explaining, so bear with me.

Firstly, there is positive. This may also be called V in , V+, Vs, or a host of other names. Usually this is denoted by a red wire.

Secondly negative, which can also be called ground, 0volts, V- or return. It's normal in a complex circuit to have a common return rail.

In conventional physics we assume that the current flows from positive to negative. This is a hangover from the early days. It doesn't though, it flows from negative to positive, but since everything's been designed under the former assumption we keep it.

Polarity is the way of describing the flow of current and is important. In simple terms, a positive polarity means the current flows (conventionally) forward, negative backward.

A circuit containing diodes will allow current to flow only one way and this is to avoid damaging other parts of the circuit with a wrong current flow, thus you have to have the correct polarity or the circuit will receive no power. If you have a 741 op amp unprotected in the circuit and you put current the wrong way through it, you can expect smoke (as I found out last week when being careless with one of my prototypes).

As an experiment, try doing the following:

Using a terminal block, connect a resistor of about 330 ohms (at least 0.6 Watt rated) in series with an LED (I use a 5mm one). The shortest leg of the LED is negative, the longer positive. Connect the positive leg of the LED to the resistor.

If you can't tell which leg is longer, there are two more ways - if you can see inside the LED the big fat chunky bit is negative and the little thin one positive. If the LED is round, the negative has a flat on the side.

WARNING- Don't do any of the following with a high voltage. You might get away with about 15, I only ever do it with nine.

put one side of the supply on the leg of the resistor that's not attached to the LED and the other on the leg of the LED that's not attached to the resistor, (which will be the shortest of course). If the LED doesn't light, do it the other way round.

Whichever wire is on the leg of the resistor when it lights will be the positive, the other negative.

Polarity is usually marked on the device as a pictogram. If you have anything that works off a transformer, look at the label on the casing. I'm looking at a transformer (sometimes known as a PSU or an AC/DC adaptor) for a BT Freestyle cordless phone as I write this. On it, the pictogram has (reading from left) a little circle with a minus sign in it, which has a line leading to a broken circle. Inside this broken circle is a dot, and from the dot another line leads to a circle on the right with a plus sign in it. This means the polarity is (centre) positive.

On the lead coming from the transformer to the device you'll find a barrel  at the end, and there'll be an inside and outside of the barrel electrically isolated from each other. If the polarity is positive, as described above, the inside or centre of the barrel will be positive, the outside negative.

You can test this with a multimeter. If you've got it set up correctly,  as you'll know black is common return (i.e) negative and red is supply (positive), so if you put the red in the middle and the black on the outside of a centre positive barrel, and the supply voltage is X, the metre will read X volts.

If you swap them over, putting the black probe in the middle will cause the meter to read  minus X (-X) volts.

Hope that's helpful.

Addition: Output from a transformer itself will always be AC and polarity will change. To get DC and thus a non-changing polarity, you need to rectify the output from the transformer.This is usually built in to a plug in adaptor like the BT one I mentioned above.

You can read about rectifiers here:

http://en.wikipedia.org/wiki/Rectifier

Answer 2

Simply said. It's POSITIVE and NEGATIVE. The flow of current is from (+) to (-). Imagine a water going down a stream. You can't reverse it.

Only DC(direct current) voltages has a polarity. RED is a color code for (+). BLACK for (-).

AC(alternating current) voltage has not polarity "in layman" so you can reverse the two terminals.