Fixed resistance is a resistor with constant resistance value. Fixed resistance is also called resistance for short. We know that the resistance of a conductor depends on the length of the conductor, the cross-sectional area of the conductor, and the material (i.e. resistivity: P = RS / L, which is related to temperature).
For a constant resistance, it is obvious that its length and cross-sectional area are constant. When making constant resistance, in order to keep the resistance nearly unchanged under the condition of temperature change, the material whose resistivity is almost independent of temperature is usually used as conductor (winding the set resistance). That is to say, the resistance of the material is almost independent of the temperature change. Generally, some alloy materials are chosen.
The so-called constant resistance refers to a resistor with a certain resistance value. It is a component used more in electronic instruments. There are many kinds of constant resistance, and with the development of science and technology, new types of resistance are constantly emerging. Here, only a few common and commonly used constant resistance are introduced.
There are signs on the constant resistance. The mark code of resistance usually consists of four parts: model, nominal resistance, rated power, resistance error grade or allowable error.
The resistance models are as follows:
What is the effect of constant resistance?
1. Enlargement (or protection) effect.
1. In the refitting of ammeter, the head or low-range ammeter can be connected in series with a resistor, which can be refitted into a voltmeter. The fixed-value resistor shares the surplus voltage, enlarges the range of the ammeter, and protects the meter at the same time.
2. In the refitting of ammeter, the head of the meter or the low-range voltmeter is connected with a resistor in parallel, which can be refitted into an ammeter. Among them, the constant resistance shares the surplus current, enlarges the measuring range of the meter and protects the head of the meter.
3. The constant resistance is connected in series with the measuring object.
2. Equivalent to ammeter.
1. Act as ammeter: Fixed resistance is parallel to voltmeter and can be used as ammeter.
In addition, if a circuit is used to measure the EMF and internal resistance of the power supply, the main current can be obtained by dividing the indication of the voltmeter by the resistance value of the resistance box, which is equivalent to an ammeter.
2. A voltmeter with known internal resistance is equivalent to an ammeter.
3. The combination of voltmeter and constant resistance can measure current.
3. Equivalent to Voltmeter
1. Acting as a voltmeter: If the constant resistance is connected in series with an ammeter, it can be used as a voltmeter, similar to the modification of the voltmeter. But there are also different situations.
2. A ammeter with known internal resistance is equivalent to a voltmeter.