What is resistance? We understand simply - Resistors are obstacles that obstruct the electrical current of a conductor, if an object conducts electricity well, small resistors, poor conductors, large resistors, insulators are extremely great.
Resistance of conductor:
The resistance of the wire depends on the material, length and cross section of the wire. calculated according to the following formula:
R = ρ.L / S
- In which ρ is the output resistance depending on the material
- L is the wire length
- S is the wire section
- R is the unit resistance of Ohm
Resistors in electronic devices.
a) Shape and symbol:
In resistive electronic devices is an important component, they are made of carbon and metal compounds depending on the mixing ratio, so people can create different kinds of resistors.
b) Unit of resistor
- The resistor unit is Ω (Ohm), KΩ, MΩ
- 1KΩ = 1000 Ω
- 1MΩ = 1000 K Ω = 1000,000 Ω
c) How to write the value of the resistor
- Small-sized resistors are scored by color lines according to a common convention of the world (see picture above).
- Resistors larger than 2W are usually recorded directly on the body. Examples are power resistors, ceramic resistors.
Great successor, value is recorded directly.
How to read resistance values.
International color convention
Color | Value | Color | Value |
Black | 0 | Green | 5 |
Brown | 1 | Blue | 6 |
Red | 2 | Violet | 7 |
Oranges | 3 | Gray | 8 |
Yellow | 4 | White | 9 |
|
| Yellow emulsion | -1 |
|
| Silver | -2 |
Resistors are usually denoted by 4 color rings, the exact resistor is denoted by 5 color rings.
* How to read 4-color resistor values:
How to read 4 resistor colors
- Round 4 is the last ring always yellow or silver, this is the ring of the resistance of the resistor, when reading the numerical value, we ignore this circle.
- Opposite the last round is round 1, followed by round 2, number 3
- Rounds 1 and 2 are dozens and units
- Round 3 is a multiple of base 10.
- Value = (round 1) (round 2) x 10 (exponential ring 3)
- It is possible to calculate round 3 as the number of "0" numbers not added
- The color of the emulsion is only in the wrong number or round 3, if the 3rd round is the emulsion, the exponent of base 10 is a negative number.
* How to read the resistance value of 5 color rings: (exact resistor)
- Round 5 is the last round, which is the round of the wrong number, and the 5 color rings, the color has many colors, thus making it difficult for us to determine where is the last round, but the last round always has a distance. a little further.
- Opposite the last round is round 1
- Similarly, reading the numerical value becomes 4 color rings but here round 4 is a multiple of base 10, round 1, number 2 and number 3 are hundreds, tens and units.
- Value = (round 1) (round 2) (round 3) x 10 (ring 4)
- It is possible to calculate the number 4 as the number of zero "0" added
Practice reading resistor values.
- When different resistors are in the third color ring, we see this multiple color ring often changing from silver to green, equivalent to < 1 Ω to MΩ.
In the above figure, the resistance values are common in practice, when the color ring 3 changes, the upper resistance values increase and decrease 10 times.
Exercise - Quickly guess the number before the answer appears, when all the values you've guessed before the result appears, your knowledge in this section is fine!
Common resistor values.
We cannot get a resistor of any value, manufacturers only give about 150 common resistor values, the following table is the color and value of common resistors.
Classification of resistors.
Resistors: Resistors are usually resistors with small capacities from 0.125W to 0.5W
Public resistors: These are resistors with greater output from 1W, 2W, 5W, 10W.
Ceramic resistors, thermal resistors: As the other way of calling resistors, this resistor has ceramic sheath, and when it operates they release heat.
Output of resistors.
When a resistor is applied to a circuit, the resistor itself consumes a power P calculated according to the formula
P = U .I = U2 / R = I2.R
- According to the formula above, the power consumption of the resistor depends on the current flowing through the resistor or depending on the voltage on the two resistors.
- The power consumption of the resistor is completely calculated before the resistor is inserted into the circuit.
- If a resistor has a nominal output less than the power it will consume, the resistor will be burned.
- Usually people install resistors into circuits that have a nominal output > = 2 times the power that it will consume.
In the diagram above shows: Vcc source is 12V, resistors have a value of 120 số but have different capacities, when the switches K1 and K2 close, the resistors consume a power of
P = U2 / R = (12 x 12) / 120 = 1,2W
- When K1 is closed, because the resistor has a higher output than the power consumption, the resistor does not burn.
- When K2 is closed, the resistance is less than the power consumed, so the resistor is burned.
Variation, philosophy:
Variable is a tunable resistor that changes the value, denoted by VR as follows:
The transformer is usually assembled in the machine for the repair and calibration of the technician.
Pressure philosophy: Pressure philosophy is similar to variable but has more need to adjust and often arranged in front of the machine for users to adjust. For example - Pressure Philosophy Volume, Bass, Treec etc., philosophy means to extract part of the voltage from the input depending on the degree of correction.
The resistor is connected in series
Serial resistors have the same value as the sum of the component resistors combined.
Rtd = R1 + R2 + R3
- The current flowing through series resistors is equal and equal to I
I = ( U1 / R1) = ( U2 / R2) = ( U3 / R3 )
- From the above formula, we find that the voltage drop on the series resistors is directly proportional to the return value.
Resistors are parallel.
Parallel resistors with equivalent Rtd values are calculated by the formula
(1 / Rtd) = (1 / R1) + (1 / R2) + (1 / R3)
- If the circuit has only 2 parallel resistors then
Rtd = R1.R2 / ( R1 + R2) - The current flowing through resistors in parallel is inversely proportional to the resistance value.
I1 = ( U / R1) , I2 = ( U / R2) , I3 =( U / R3 ) - The voltage on the resistors in parallel is always the same
Mixed resistors
Use a mixture of resistors to create a better resistor.
- For example, if we need a 9K resistor we can get 2 15K resistors in parallel then connect in series with 1.5K resistor.
Application of resistors
Resistors are present everywhere in electronic devices and as such resistors are important components that are indispensable, in electrical circuits, resistors have the following effects:
- Control the current through the load accordingly, For example, there is a 9V light bulb, but we only have 12V power, we can connect the bulb to the resistor to reduce the 3V pressure on the resistor.
- As shown above, we can calculate the value and output of the resistor as follows: The lamp has a voltage of 9V and output 2W so the consumption current is I = P / U = ( 2/9) = Ampere is also the current flowing through the resistor.
- Since the source is 12V, the 9V bulb should have a voltage drop on R of 3V, so we deduce that the required resistance is R = U / I = 3 / (2/9) = 27/2 = 13.5 Ω
- The power consumption on the resistor is: P = U.I = 3. (2/9) = 6/9 W so we must use the resistor with the output P > 6/9 W
- The resistor is turned into a voltage divider to obtain a desired voltage from a given voltage.
TFrom the 12V power supply above via the voltage divider R1 and R2 we take out the voltage U1, the voltage U1 depends on the two resistors R1 and R2.
U1 / U = R1 / (R1 + R2) => U1 = U.R1(R1 + R2)
Changing the R1 or R2 value we will collect the desired U1 voltage.
- Polarized for active transistors.
