Rheostat Basics: Types, Principle and Functions

Catalog

Ⅰ Sliding rheostat

As a special kind of resistor, the sliding rheostat is widely used in ordinary physical tests. Many circuits use the sliding rheostat to control the circuit, and it can be used to control the current and voltage changes in the circuit.

1 Working principle

The sliding rheostat is one of the commonly used devices in electricity. It changes the resistance by changing the length of the resistance wire of the connected circuit, thereby gradually changing the current in the circuit. The resistance wire of the sliding rheostat is generally nickel-chromium alloy with a high melting point and high resistance. The resistance rod is generally metal with low resistance. Therefore, the longer the resistance wire, the greater the resistance, and the shorter the resistance rod, the lower the resistance. The resistance wire is coated with an insulating layer, wound on the insulating tube, and its two ends are connected to the A and B terminals. The sliding piece P is connected to terminal C through a metal rod. When the sliding piece moves to different positions, the length of the resistance wire between the two terminals A and C are different, so that the resistance in the connected circuit can be changed. A general rheostat is composed of a wire with a larger resistance and a device that can change the contact point to adjust the effective length of the resistance wire. The adjustment function of the sliding rheostat in the circuit is reflected in the current limiting and partial pressure connection.

Sliding Rheostat 

2 Function and Application

Main functions:

(1) Protection circuit. Before the switch is closed, adjust the sliding piece P of the sliding rheostat to maximize the resistance of the sliding rheostat connected to the circuit.

(2) Change the size and direction of the current in the circuit by changing the resistance of the connected circuit part, thereby changing the voltage at both ends of the conductor (appliance) connected in series with it. When connecting the sliding rheostat, it is required: "one up and down, the focus is on the bottom", the metal rod and the resistance wire each use one terminal; the actual connection should be based on the requirements to choose the two terminals of the resistance wire.

(3) Change the voltage. When exploring Ohm's law (), it plays a role in changing the voltage at both ends of the electrical appliance connected in series.

(4) Use voltammetry to measure resistance, based on the deformation formula of Ohm's law:

Applications:

The knob for adjusting the volume on the audio; the knob for adjusting the brightness of the light on the desk lamp; the knob for adjusting the brightness of the display on the computer; the knob for adjusting the temperature of the electric iron. In addition, the fuel gauge on the car, the weighing instrument, etc.

3 Structure and materials

Structure of sliding rheostat

Structure of sliding rheostat: 1. Wiring post 2. Sliding plate 3. Coil 4. Metal rod 5. Porcelain tube. Principle of sliding rheostat: The resistance of the metal rod is small, and the current flows through the metal wire along with the dicing sheet, which changes the length of the resistance wire connected to the circuit and also changes the size of the resistance. The material of the resistance wire of the sliding varistor is generally constantan wire or nickel-chromium alloy wire. The constantan wire or nickel-chromium alloy wire is wound on the insulating cylinder, and the two ends are led out with lead wires. The sliding piece of the varistor contacts the resistance wire and can be adjusted to the distance between the two ends, thereby changing the resistance from the metal rod to the two ends of the resistance wire, which constitutes a sliding rheostat. There is also a sliding varistor that is "plated" on an insulating substrate with resistive materials (such as carbonaceous materials), and the resistance is adjusted by a sliding piece in the middle.

Ⅱ Resistance box

The sliding rheostat can gradually change the resistance of the connected circuit and play the role of continuously changing the current size, but it cannot accurately test the resistance value of the connected circuit. If you need to know the resistance of the resistor connected to the circuit, you can use a resistance box. Therefore, the resistance box is a rheostat that can adjust the resistance and can display the resistance value. Compared with the sliding rheostat, the sliding rheostat cannot indicate the resistance value of the connected circuit, but it can continuously change the resistance in the connected circuit. The resistance box can indicate the resistance value connected to the circuit, but the resistance value change is discontinuous.

resistance box

Use of resistance box

When in use, connect the two binding posts to the circuit and adjust the dial to get any resistance between 0~nx (n x, n is the number of knobs) ohm. Multiply the indication of the indicator point corresponding to each dial by the multiple marked on the dial, and then add them together to get the resistance value of the access circuit. The sliding rheostat can be used as a current limiter or a voltage divider in the circuit. How do you choose these two different forms? This is first determined by the needs in the circuit. For example, sometimes the load voltage is required to have a large change, and sometimes it is necessary to be able to make fine adjustments. Which circuit can meet these requirements? We need to study the output characteristics of the two circuits.

Ⅲ Potentiometer

The potentiometer is a resistor element with three lead-out terminals and the resistance value can be adjusted according to a certain change rule. The potentiometer is usually composed of a resistor and a movable brush. When the brush moves along the resistor body, the resistance value or voltage that has a certain relationship with the displacement is obtained at the output end. The potentiometer can be used as a three-terminal component or a two-terminal component. The latter can be regarded as a variable resistor.

Potentiometer

The role of the potentiometer-adjust the voltage (including DC voltage and signal voltage) and current.

The structural characteristics of the potentiometer-the resistor body of the potentiometer have two fixed ends. By manually adjusting the shaft or sliding handle, changing the position of the movable contact on the resistor body will change the position between the movable contact and any fixed end The resistance value, which changes the size of the voltage and current.

The potentiometer is composed of a resistor body and a rotating or sliding system. When a voltage is applied between the two fixed electric shocks of the resistor body, the position of the contact on the resistor body can be changed by rotating or sliding the system, and the voltage that has a certain relationship to the movable contact point can be obtained between the movable contact and the fixed contact. It is mostly used as a voltage divider when the potentiometer is a four-terminal element. The potentiometer is basically a sliding rheostat, there are several styles. They are generally used in the speaker volume switch and laser head power size adjustment. It is widely used in electronic equipment for volume control in speakers and receivers.

Principle 

The pulse potentiometer is the same as the ordinary potentiometer with three pins. but inside the pulse potentiometer, connected to pins 1 and 2 are two metal static pieces of different lengths, and the one connected to pin 3 is a circle Metal rotor with 12 or 24 teeth. When the pulse potentiometer rotates, there are four states: pin 3 is connected to pin 1, pin 3 is connected to pin 2 and pin 1; pin 3 is connected to pin 2, pin 3 is connected to pin Pin 2, and pin 1 is all disconnected.

Potentiometer circuit

In actual use, we generally ground pin 3 as the data input terminal. And pins 1 and 2 are connected to the I/O port of the microcontroller as the data output terminal. As shown in the figure, connect pin 1 to P1.0 of the microcontroller, and pin 2 to P1.1 of the microcontroller. When the pulse potentiometer rotates left or right, P1.0 and P1.1 will periodically generate the waveform shown. If it is a 12-point pulse potentiometer, it will generate 12 sets of such waveforms, 24 points pulse potentiometer will generate 24 groups of such waveforms; a group of waveforms (or a cycle) contains 4 working states. Therefore, as long as the waveforms of P1.0 and P1.1 are detected, it can be recognized whether the pulse potentiometer is rotating to the left or right.

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