② In order to eliminate the influence of changes in the resistance of connecting wires, the three-wire connection method must be used

As the name suggests, the resistance value of a thermal resistor changes with temperature, such as using platinum or copper wires with good linearity as resistors. Industrial thermistors generally use Pt100, Pt10, Pt1000, Cu50, Cu100. The temperature measurement range of platinum thermistors is generally -200 to 800 degrees Celsius, while copper thermistors are -40 to 140 degrees Celsius.

If a thermistor is made of platinum wire, its graduation is called Pt100. That is to say, its resistance value is 100 ohms at 0 degrees, 18.52 ohms at minus 200 degrees, 175.86 ohms at 200 degrees, and 375.70 ohms at 800 degrees.

For example, a thermal resistor made of copper wire, with a graduation number of Cu50. It has a resistance value of 50 ohms at 0 degrees and 71.400 ohms at 100 degrees.

The thermal resistance formula is Rt=Ro (1+A * t+B * t * t); Rt=Ro [1+A * t+B * t * t+C (t-100) * t * t * t], where t represents the temperature in Celsius, Ro is the resistance value at zero Celsius, and A, B, and C are all specified coefficients. For Pt100, Ro is equal to 100.

1. Use diode half wave rectification to change the effective value of AC power, achieving the goal of changing heating power and heating speed; This method has the most applications, and its advantage is that the circuit structure is simple and easy to operate.

2. Voltage transformation, using a small transformer for voltage transformation, has the advantage of being adjustable in multiple stages, but the disadvantage is that the circuit structure is complex and the production cost is high.

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