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What is a resistor

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Subido el 5 de octubre de 2014 por Samuel E.

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What is a resistor

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What is a resistor? 00:00:00
A good definition might be this one. 00:00:02
A resistor is a passive electrical component with a primary function to limit the flow 00:00:04
of electric current. 00:00:09
The standard symbols for resistors are given below. 00:00:11
The zigzag on the left is the American standard and the one on the right is the international 00:00:14
standard by the IEC. 00:00:18
To explain the definition more clearly, we use the example of water that flows through 00:00:21
a tube. 00:00:24
The flow of water is similar to the electrical current in an electrical circuit. 00:00:26
The pressure difference that causes the water to flow can be compared to a voltage difference 00:00:30
which causes the flow of electrical current. 00:00:34
If we create a resistance in the flow of water, the current will reduce. 00:00:38
We can do this for example by making the tube more narrow at a certain place. 00:00:42
A resistor is pretty much the same. 00:00:46
The resistor has a higher resistance than the connecting leads and causes a reduced 00:00:48
electrical current. 00:00:52
We can see this in the water pipe that a pressure drop is created because of the narrow part 00:00:54
in the middle. 00:00:58
The pressure on the left is bigger than on the right. 00:00:59
The resistor has a similar effect, here a voltage drop is created. 00:01:02
The relation between the electrical current, voltage and resistance is described by Ohm's 00:01:07
law. 00:01:12
Mr. Ohm was a German scientist that discovered in 1827 that electrical resistance is equal 00:01:13
to voltage divided over current. 00:01:19
In this formula resistance is in ohms, voltage in volts and current in amps. 00:01:22
A simple circuit with a battery and a resistor can explain this. 00:01:27
The voltage source causes a current which is limited by the resistor. 00:01:31
The voltage source is 2 volts and we want a current of 4 amps. 00:01:36
So what resistance should the resistor have? 00:01:40
According to Ohm's law, the resistance is equal to the voltage divided over current, 00:01:43
or 2 over 4 is equal to 0.5 ohms. 00:01:47
An example of an application of this simple network is a basic LED circuit. 00:01:51
Suppose we want to light a red LED with a 9V battery. 00:01:56
The LED has a specified maximum current of 30mA. 00:02:01
If we directly connect the battery, the LED might burn out instantly. 00:02:05
The LED practically doesn't create resistance, so the current will get much higher than the 00:02:10
30mA. 00:02:14
To prevent this we can place a resistor at the positive lead between the battery and 00:02:16
the LID. 00:02:20
The resistor should have a resistance which is just high enough to reduce the flow of 00:02:21
current to 30mA. 00:02:25
Using Ohm's law again, we know that the resistance is equal to voltage divided over 00:02:28
current or 9V divided over 0.03A is 300 Ohms. 00:02:32
If we now connect also the negative lead and create a circuit, the LID emits a nice red 00:02:39
light. 00:02:44
Using a resistor for an LID circuit is just one application, but there are numerous other 00:02:45
applications and purposes for resistors. 00:02:50
To give a complete overview would be absolutely impossible in this short video. 00:02:53
There are a lot of different resistor types, all with their own applications, characteristics 00:02:58
and construction. 00:03:01
Fixed resistors have a constant resistance value and they are the most common type. 00:03:03
When people talk about a resistor, they most probably mean a fixed resistor. 00:03:08
The picture shows an actual carbon film resistor. 00:03:12
Fixed resistors are available in axial and SMD packages. 00:03:15
Variable resistors have an adjustable resistance value. 00:03:20
Most variable resistors are adjusted by mechanical movement. 00:03:23
When they are used as a variable voltage divider, they are called potentiometers. 00:03:27
When they are used as variable resistance to control the current in the circuit, they 00:03:32
are called rheostats. 00:03:36
Digital potentiometers are controlled electronically instead of by mechanical action. 00:03:38
A third category are resistors which have a varying resistance dependent on a physical 00:03:43
quantity such as light, temperature or voltage. 00:03:47
They are often used as measurement devices. 00:03:51
Another breakdown of resistor types can be made according to resistance material and 00:03:54
construction. 00:03:58
Wire wound resistors are the oldest type which are still used today. 00:03:59
They are constructed by winding a resistive wire around a non-conducting core. 00:04:03
They can have very low resistance values and can be produced fairly accurate. 00:04:08
Furthermore, they are very durable. 00:04:12
A disadvantage is the parasitic reactants for higher frequencies. 00:04:14
Carbon composition resistors are constructed with a mixture of a non-conducting ceramic 00:04:19
and fine carbon particles. 00:04:23
They are also very old, and used to be the most common resistor type a few decades ago. 00:04:26
Although their properties are inferior to other types, regarding for example tolerance, 00:04:30
they are still in demand for certain applications. 00:04:35
For example, they have the ability to withstand high energy pulses. 00:04:37
Carbon film resistors are widely used today. 00:04:41
They are made out of a non-conducting core with a thin carbon film around it. 00:04:44
Carbon film resistors have a higher accuracy than carbon composition resistors, but have 00:04:49
inferior properties compared to metal or metal oxide film. 00:04:54
Metal film resistors have a similar construction as carbon film resistors but have a metal 00:04:58
layer instead of a carbon film. 00:05:02
They have a better accuracy, a lower temperature coefficient and a fairly good stability. 00:05:04
Metal oxide film resistors are even more durable and have a higher temperature resistance and 00:05:09
reliability than the metal film resistors. 00:05:13
Foil resistors have a resistive element of a thin metallic foil of several micrometers 00:05:16
thick. 00:05:21
They have the highest available precision and stability today. 00:05:22
Most actual leaded resistors have a marking with colored bands to indicate the resistance 00:05:26
value and tolerance. 00:05:30
This resistor is a carbon composition resistor with four color bands. 00:05:32
The first band gives the first digit of the resistance value. 00:05:36
The second band gives the second digit. 00:05:39
The third band indicates a multiplication factor and the fourth band gives away the 00:05:41
tolerance of the resistor. 00:05:46
You can try to memorize the meaning of each band and color, but you can also use the resistor 00:05:48
color code chart to decipher the code. 00:05:53
At resistorguide.com you can even find an automatic calculator to decipher the code 00:05:56
for you. 00:05:59
Using the chart we see that the first red band has a value of 2. 00:06:02
The second blue band is 6. 00:06:05
The third band is grey which means a multiplication factor of 10.000. 00:06:08
The golden fourth band means a tolerance of 5%. 00:06:12
We know that the resistor has a value of 2.6 megaohms with a tolerance of 5%. 00:06:16
You can find a lot more information about the color code, other resistor types or resistor 00:06:23
properties at resistorguide.com 00:06:28
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Idioma/s:
en
Autor/es:
resistorguide
Subido por:
Samuel E.
Licencia:
Reconocimiento - No comercial - Sin obra derivada
Visualizaciones:
34
Fecha:
5 de octubre de 2014 - 17:49
Visibilidad:
Público
Centro:
IES JOAQUIN ARAUJO
Duración:
06′ 31″
Relación de aspecto:
1.78:1
Resolución:
640x360 píxeles
Tamaño:
11.05 MBytes

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