Free Electronics Circuits

Friday, August 22, 2008

Free Electronic Parts -- Resistors

The resistor's function is to reduce the flow of electric current.
This symbol is used to indicate a resistor in a circuit diagram, known as a schematic.
Resistance value is designated in units called the "Ohm." A 1000 Ohm resistor is typically shown as 1K-Ohm ( kilo Ohm ), and 1000 K-Ohms is written as 1M-Ohm ( megohm ).
There are two classes of resistors; fixed resistors and the variable resistors. They are also classified according to the material from which they are made. The typical resistor is made of either carbon film or metal film. There are other types as well, but these are the most common.
The resistance value of the resistor is not the only thing to consider when selecting a resistor for use in a circuit. The "tolerance" and the electric power ratings of the resistor are also important.
The tolerance of a resistor denotes how close it is to the actual rated resistence value. For example, a ±5% tolerance would indicate a resistor that is within ±5% of the specified resistance value.
The power rating indicates how much power the resistor can safely tolerate. Just like you wouldn't use a 6 volt flashlight lamp to replace a burned out light in your house, you wouldn't use a 1/8 watt resistor when you should be using a 1/2 watt resistor.

The maximum rated power of the resistor is specified in Watts.
Power is calculated using the square of the current ( I2 ) x the resistance value ( R ) of the resistor. If the maximum rating of the resistor is exceeded, it will become extremely hot, and even burn.
Resistors in electronic circuits are typicaly rated 1/8W, 1/4W, and 1/2W. 1/8W is almost always used in signal circuit applications.
When powering a light emitting diode, a comparatively large current flows through the resistor, so you need to consider the power rating of the resistor you choose.

Rating electric power

For example, to power a 5V circuit using a 12V supply, a three-terminal voltage regulator is usually used.
However, if you try to drop the voltage from 12V to 5V using only a resistor, then you need to calculate the power rating of the resistor as well as the resistance value.

At this time, the current consumed by the 5V circuit needs to be known.
Here are a few ways to find out how much current the circuit demands.

Assemble the circuit and measure the actual current used with a multi-meter.

Check the component's current use against a standard table.
Assume the current consumed is 100 mA (milliamps) in the following example.
7V must be dropped with the resistor. The resistance value of the resistor becomes 7V / 0.1A = 70(ohm). The consumption of electric power for this resistor becomes 0.1A x 0.1A x 70 ohm = 0.7W.
Generally, it's safe to choose a resistor which has a power rating of about twice the power consumption needed.

Resistance value

As for the standard resistance value, the values used can be divided like a logarithm. ( See the logarithm table )
For example, in the case of E3, The values [1], [2.2], [4.7] and [10] are used. They divide 10 into three, like a logarithm.
In the case of E6 : [1], [1.5], [2.2], [3.3], [4.7], [6.8], [10].
In the case of E12 : [1], [1.2], [1.5], [1.8], [2.2], [2.7], [3.3], [3.9], [4.7], [5.6], [6.8], [8.2], [10].
It is because of this that the resistance value is seen at a glance to be a discrete value.
The resistance value is displayed using the color code( the colored bars/the colored stripes ), because the average resistor is too small to have the value printed on it with numbers.
You had better learn the color code, because almost all resistors of 1/2W or less use the color code to display the resistance value.

Metal film resistors

Metal film resistors are used when a higher tolerance (more accurate value) is needed. They are much more accurate in value than carbon film resistors. They have about ±0.05% tolerance. They have about ±0.05% tolerance. I don't use any high tolerance resistors in my circuits. Resistors that are about ±1% are more than sufficient. Ni-Cr (Nichrome) seems to be used for the material of resistor. The metal film resistor is used for bridge circuits, filter circuits, and low-noise analog signal circuits.

Variable Resistors

There are two general ways in which variable resistors are used. One is the variable resistor which value is easily changed, like the volume adjustment of Radio. The other is semi-fixed resistor that is not meant to be adjusted by anyone but a technician. It is used to adjust the operating condition of the circuit by the technician. Semi-fixed resistors are used to compensate for the inaccuracies of the resistors, and to fine-tune a circuit. The rotation angle of the variable resistor is usually about 300 degrees. Some variable resistors must be turned many times to use the whole range of resistance they offer. This allows for very precise adjustments of their value. These are called "Potentiometers" or "Trimmer Potentiometers."

In the photograph to the left, the variable resistor typically used for volume controls can be seen on the far right. Its value is very easy to adjust.
The four resistors at the center of the photograph are the semi-fixed type. These ones are mounted on the printed circuit board.
The two resistors on the left are the trimmer potentiometers.

This symbol

is used to indicate a variable resistor in a circuit diagram.

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Friday, August 22, 2008

Free Electronic Parts -- Resistors

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Rating electric power

Resistance value

Carbon film resistors

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Variable Resistors