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Keysight Technologies
Gauging Temperature Accuracy Using the
U3606B Multimeter | DC Power Supply




                             Application Note
Introduction
Temperature plays an important role in maintaining the overall functionality of an
electrical appliance and the design of these controller circuits is getting increasingly
stringent. This makes it important to test the integrity of components and the overall
the controller circuit. This article illustrates the unique features of the Keysight
Technologies, Inc. U3606B multimeter | DC power supply that can be used to evaluate
the functionality of a temperature controller board.
03 | Keysight | Gauging Temperature Accuracy Using the U3606B Multimeter | DC Power Supply - Application Note



How a Temperature Controller Circuit Works




Figure 1. Temperature controllers found on refrigerators


Figure 2 illustrates a temperature controller circuit that uses a LM339 comparator. A
LM339 chip contains four comparators, but this specific circuit only uses one of them.
The operation of the circuit is simple. When the voltage at positive input (pin 5) is higher
than negative input (pin 4), the output of LM339 (pin 2) will turn On. The thermistor in
this circuit acts as a voltage divider. Its resistance decreases when the temperature
drops. This brings more voltage on the positive pin of the LM339. Notice there is also
another voltage divider in the circuit which is constructed on a variable resistor and
connected to the negative pin of the LM339. In most cases, when the temperature
setting on an analog refrigerator is changed, it is actually the variable resistor that is
being adjusted.

When the output of the LM330 turns On, it turns On the transistor which powers up
the thermostat circuit. The thermostat circuit then switches the cooling and heating
elements On and Off to maintain the temperature within the correct range. An LED is
usually connected to the thermostat circuit to serve as an indicator and it illuminates
when the thermostat circuit is turned On.


+


      Thermistor
                                   

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