Home Appliance Temperature Sensor Function

Temperature sensor functions and parameters of soymilk machines, rice cookers, gas water heaters, and heated footbaths.
Example 1: For a Joyoung soybean milk machine, sometimes the motor starts turning to beat the beans before the water is heated. Sometimes the water is not heated at all, and the alarm goes off when the power is turned on. Soymilk machines have multiple working programs. Take the Thousand Beans procedure as an example: first inject cold water so that the water level reaches the scale line. After powering on, select the program and press the start button. The machine will first let the beans absorb water for a while, then start heating, and stop heating when the water temperature reaches 80°. The motor starts at a slow speed to stir the beans and then continues to heat. When the water temperature reaches 90°, the motor rotates rapidly to crush the beans, and then heating and crushing are performed alternately. After the beans are completely crushed, the machine heats intermittently at half power to prevent the soy milk from overflowing. During heating, if the soy milk comes into contact with the anti-overflow rod, the machine will stop immediately and the heating will stop. After the soy milk is made, the buzzer will beep 3 times.

Soy milk machine temperature sensor

The machine can sometimes boil water, the motor can rotate, and sometimes it can sound an alarm. This shows that the CPU is working normally, but the CPU may receive error information and malfunction. This machine only has a water temperature sensor and an anti-overflow detection rod. The relevant circuit is shown in Figure 1. When starting work, the anti-overflow detection rod and the ground are insulated. The voltage at point B is determined by the voltage divider of R3 and R4 and should be high level (>2.5V). When the soy milk comes into contact with the detection rod, the voltage at point B changes to low level (<2.5V) and the machine stops heating. If the voltage at point B is lower than 2.5V when the soymilk machine first starts working, the machine will sound an alarm. The measured voltage at point B is always 4.5V, indicating that this fault has nothing to do with the detection rod.

Gas water heater temperature sensor

The temperature sensor is a semiconductor component enclosed in a stainless steel tube. The measured voltage at point A is 23V and is unstable. Normally, point A is at a high level. As the water temperature rises, the voltage value gradually decreases. Unplug the temperature sensor plug and measure that the voltage at point A rises to 4.2V. Use a pointer multimeter Rx1k block to measure the resistance of the temperature sensor. The readings vary between 15k~20kΩ, which indicates that the sensor is leaking electricity. Remove a similar sensor from the scrapped soybean milk machine, measure its resistance to be 100kΩ (the ambient temperature is about 12°C), install it on the test machine, and eliminate the fault. At this time, the measured voltage at point A is 4V (temperature is about 12°C). When the voltage at point A drops to 2.5V, the machine stops heating. When the water temperature reaches 90C, the voltage at point A drops to 1.7V.

heater temperature sensor

Example 2: A Pentium computer-type rice cooker cooks rice. The top layer is filled with raw rice. Test the water boiling function and the water can be boiled normally, but it feels like it takes a long time. When you select the cooking function, you feel that the water in the machine boils less vigorously. It can be seen from the ammeter connected in series on the power line that when the intermittent heating program is entered after the water is boiled, the heating stops for a long time. The rice cooker has two temperature sensors, one is installed in the center of the heating plate to detect the temperature of the bottom of the pot; the other is installed inside the lid to detect the temperature of the upper part of the pot. If the water can boil, it means the sensor at the bottom of the pot is normal. The resistance was measured to be 90kΩ (room temperature 16°C). The resistance of the pot lid sensor is only 15kΩ, which is obviously too small. According to experience, these two sensors are generally of the same specifications. Since the author does not have a sensor of this specification on hand, I tried an 82kΩ resistor instead and then tested the machine to eliminate the fault. In computer-type rice cookers, the upper lid sensor is set to prevent rice soup from overflowing. Especially when cooking porridge, when a large amount of rice soup pours onto the pot lid, causing the temperature of the pot lid to rise, the sensor resistance becomes smaller. At this time, the CPU issues an instruction to stop heating to prevent the rice soup from overflowing. The resistance of the upper cover sensor of this machine is only 15kΩ. After detection, the CPU determines that the temperature of the upper cover is too high, so it reduces the heating time, resulting in longer cooking time and insufficient boiling intensity, causing the rice to become cooked. After emergency replacement with a fixed resistor, the user is told not to cook porridge, otherwise the rice soup will overflow.

Rice cooker temperature sensor

Example 3: A constant temperature gas water heater does not work. The moment it is powered on, the water temperature is displayed as 85°, and then an alarm sounds. The panel of the machine displays an over-temperature alarm, which is obviously caused by the deterioration of the temperature sensor. The sensor has been immersed in water for a long time and is similar in shape to the sensor of a soymilk machine. Observe carefully with a magnifying glass that there seems to be a slight gap in the sensor housing. Use a soldering iron to heat the sensor shell intermittently (to prevent the sensor from burning out) to dry out the moisture inside. After cooling, the resistance value is measured to be 30kΩ (room temperature is 25°C). First apply a layer of sealant on the surface of the sensor, and then put a plastic tube on it to prevent it from being waterproof. Wait for the glue to dry and put it back into the water heater. After testing, the water heater works normally.

Example 4: A footbath, not heated. Analysis and maintenance: The measured water temperature in the basin is 15°C, but the temperature display is 45°C. It is suspected that there is a problem with the temperature sensor R1. Try a 100kΩ potentiometer instead of R1, and slowly adjust the resistance of the potentiometer connected to the circuit so that the displayed temperature is the same as the actual water temperature. At this time, measure the resistance of the current connected circuit of the potentiometer, and then replace it with a fixed resistor of the same resistance to test whether the machine is heating properly. The measurement found that when the water level was higher than 309C, the displayed temperature was lower than the actual temperature, so R1 was appropriately reduced. Obviously, the temperature displayed at low temperature is slightly higher than the actual temperature, but this can compensate for the error at high temperature, and at the same time inform the user that there is a deviation in the temperature display, and it should be based on physical comfort when using it.
Summary: Temperature sensors all work in harsh environments of high temperature and high humidity, and their resistance is prone to decrease. It is probably caused by leakage due to immersion in water. In addition, the sensor resistance may become larger or open circuit, which may also cause the machine to stop working or sound an alarm. There are many resistance specifications for temperature sensors. If the normal resistance value of the sensor cannot be known after it is damaged, a 220kΩ potentiometer can be used to replace it during maintenance, and the resistance value connected to the circuit can be adjusted so that it can work normally. Furthermore, you can also consider replacing the panel temperature and power tube temperature sensors in the induction cooker. The appearance of this type of sensor is similar to the 1N4148 glass-encapsulated diode. At room temperature, the resistance value is about 50k~100kΩ.