China Education Equipment Procurement Network News: According to recent news from Alibaba, LM35 is a widely used temperature sensor. Because it uses internal compensation, the output can start from 0 ℃. The device uses a plastic package TO992, operating voltage 4 ~ 30V, so at first glance, it seems that it is LM335 without calibration.

Within the above voltage range, the current drawn by the chip from the power supply is almost constant (about 50μA), so the chip itself has almost no heat dissipation problem. Such a small current also makes the chip particularly suitable in certain applications, such as in battery-powered applications, the output can be taken out by the third pin, without calibration at all.

Currently, there are two models of LM35 available. The output of LM35DZ is 0 ℃ ~ 100 ℃, and the output of LM35CZ can cover -40 ℃ ~ 110 ℃, and the accuracy is higher. The accuracy of both chips is higher than that of LM335, but the price is also slightly higher.

In the simplest application, the LM35 can be connected as shown in Figure 1. It only provides a set of DC power, such as PP3 battery power, plus a DVM (digital voltmeter) for display. The sensor can even be installed in the "probe", extending the DVM into an electronic direct reading thermometer for measuring the temperature of the heat sink.

One disadvantage of the LM35 when using a single power supply is that it cannot indicate temperatures as low as zero degrees. It is said that the LM35 can measure a voltage of 20mV, which is equivalent to 2 ° C (in some cases, it can even measure a voltage of 0 ~ 2mV!), But when indicating a temperature of zero degrees or lower, it is best to provide it again A negative power supply and a pull-down resistor.

A "simple thermometer circuit that achieves this goal is shown in Figure 2. Resistors R1 and R2 divide the power supply voltage to obtain a voltage reference value. This voltage is buffered by IC1, and its output is used as the LM35 and the meter's head." Ground.

Resistor R3 is used as a pull-down resistor. When the output is below zero volts, this resistance value is about 20kΩ per volt. In this example, it only needs to be less than 1V, so the resistance is 18kΩ. This can make LM35 cover 0 ℃ ~ 100 ℃ (LM35DZ) or -40 ℃ ~ 110 ℃ (LM35CZ).

When the load of the LM35 is capacitive, the output may oscillate, so it is often necessary to connect a resistor in series with the load to avoid the occurrence of oscillation. This is the function of R4 in Figure 2, and its resistance value of 1kΩ is sufficient. This simple circuit can be used to obtain an improved and more accurate universal thermometer, which is of great use in the laboratory.

Analog thermometer

The analog thermometer uses a dual operational amplifier, and the second op amp is used as the output buffer of the LM35 and turns it into the current of the meter (MEI).

The two R4 values ​​are applicable to the two situations of 100μA and 1mA current required for the full scale of the meter at 100 ° C. When using other meters, as long as the value of R4 is adjusted, it is not difficult to meet the requirements of the full scale. You can even choose the appropriate R4 to get the Fahrenheit temperature scale indication.

Resistor R5 is used to limit the maximum current to prevent possible damage to the meter head. Although this circuit can work stably without any decoupling capacitors, we recommend that you add capacitors C1 and C2 (capacitors connected by dotted lines).

When the power supply is 9V, if IC1 selects operational amplifier LM358, the current required by this circuit is about 1 ~ 2mA. If IC1 selects OP296G, a low-power operational amplifier, the operating current can be reduced to 300 μA. The lower offset voltage of OP296G can further improve the accuracy of the thermometer.

There are many ways to make the circuit read below zero degrees. For example, you can use a meter with a center value of zero, or add a polarity switch.

In many cases, if the ammeter can remove the surface, this may artificially increase the forward bias of the meter reading by 10% to 20%. Finally, you can adjust the value of R4 to get a small voltage to provide the required bias, but doing so will increase the complexity of the circuit, because you may need to increase the reference voltage and an additional amplifier.

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