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What is a resistor?

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A good definition might be this one.

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A resistor is a passive electrical component with a primary function to limit the flow

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of electric current.

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The standard symbols for resistors are given below.

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The zigzag on the left is the American standard and the one on the right is the international

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standard by the IEC.

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To explain the definition more clearly, we use the example of water that flows through

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a tube.

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The flow of water is similar to the electrical current in an electrical circuit.

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The pressure difference that causes the water to flow can be compared to a voltage difference

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which causes the flow of electrical current.

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If we create a resistance in the flow of water, the current will reduce.

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We can do this for example by making the tube more narrow at a certain place.

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A resistor is pretty much the same.

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The resistor has a higher resistance than the connecting leads and causes a reduced

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electrical current.

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We can see this in the water pipe that a pressure drop is created because of the narrow part

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in the middle.

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The pressure on the left is bigger than on the right.

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The resistor has a similar effect, here a voltage drop is created.

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The relation between the electrical current, voltage and resistance is described by Ohm's

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law.

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Mr. Ohm was a German scientist that discovered in 1827 that electrical resistance is equal

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to voltage divided over current.

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In this formula resistance is in ohms, voltage in volts and current in amps.

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A simple circuit with a battery and a resistor can explain this.

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The voltage source causes a current which is limited by the resistor.

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The voltage source is 2 volts and we want a current of 4 amps.

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So what resistance should the resistor have?

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According to Ohm's law, the resistance is equal to the voltage divided over current,

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or 2 over 4 is equal to 0.5 ohms.

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An example of an application of this simple network is a basic LED circuit.

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Suppose we want to light a red LED with a 9V battery.

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The LED has a specified maximum current of 30mA.

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If we directly connect the battery, the LED might burn out instantly.

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The LED practically doesn't create resistance, so the current will get much higher than the

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30mA.

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To prevent this we can place a resistor at the positive lead between the battery and

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the LID.

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The resistor should have a resistance which is just high enough to reduce the flow of

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current to 30mA.

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Using Ohm's law again, we know that the resistance is equal to voltage divided over

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current or 9V divided over 0.03A is 300 Ohms.

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If we now connect also the negative lead and create a circuit, the LID emits a nice red

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light.

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Using a resistor for an LID circuit is just one application, but there are numerous other

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applications and purposes for resistors.

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To give a complete overview would be absolutely impossible in this short video.

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There are a lot of different resistor types, all with their own applications, characteristics

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and construction.

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Fixed resistors have a constant resistance value and they are the most common type.

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When people talk about a resistor, they most probably mean a fixed resistor.

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The picture shows an actual carbon film resistor.

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Fixed resistors are available in axial and SMD packages.

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Variable resistors have an adjustable resistance value.

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Most variable resistors are adjusted by mechanical movement.

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When they are used as a variable voltage divider, they are called potentiometers.

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When they are used as variable resistance to control the current in the circuit, they

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are called rheostats.

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Digital potentiometers are controlled electronically instead of by mechanical action.

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A third category are resistors which have a varying resistance dependent on a physical

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quantity such as light, temperature or voltage.

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They are often used as measurement devices.

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Another breakdown of resistor types can be made according to resistance material and

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construction.

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Wire wound resistors are the oldest type which are still used today.

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They are constructed by winding a resistive wire around a non-conducting core.

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They can have very low resistance values and can be produced fairly accurate.

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Furthermore, they are very durable.

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A disadvantage is the parasitic reactants for higher frequencies.

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Carbon composition resistors are constructed with a mixture of a non-conducting ceramic

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and fine carbon particles.

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They are also very old, and used to be the most common resistor type a few decades ago.

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Although their properties are inferior to other types, regarding for example tolerance,

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they are still in demand for certain applications.

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For example, they have the ability to withstand high energy pulses.

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Carbon film resistors are widely used today.

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They are made out of a non-conducting core with a thin carbon film around it.

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Carbon film resistors have a higher accuracy than carbon composition resistors, but have

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inferior properties compared to metal or metal oxide film.

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Metal film resistors have a similar construction as carbon film resistors but have a metal

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layer instead of a carbon film.

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They have a better accuracy, a lower temperature coefficient and a fairly good stability.

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Metal oxide film resistors are even more durable and have a higher temperature resistance and

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reliability than the metal film resistors.

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Foil resistors have a resistive element of a thin metallic foil of several micrometers

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thick.

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They have the highest available precision and stability today.

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Most actual leaded resistors have a marking with colored bands to indicate the resistance

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value and tolerance.

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This resistor is a carbon composition resistor with four color bands.

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The first band gives the first digit of the resistance value.

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The second band gives the second digit.

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The third band indicates a multiplication factor and the fourth band gives away the

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tolerance of the resistor.

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You can try to memorize the meaning of each band and color, but you can also use the resistor

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color code chart to decipher the code.

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At resistorguide.com you can even find an automatic calculator to decipher the code

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for you.

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Using the chart we see that the first red band has a value of 2.

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The second blue band is 6.

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The third band is grey which means a multiplication factor of 10.000.

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The golden fourth band means a tolerance of 5%.

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We know that the resistor has a value of 2.6 megaohms with a tolerance of 5%.

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You can find a lot more information about the color code, other resistor types or resistor

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properties at resistorguide.com