Making a Simple Temperature Controller Using Adjustable Electric Contact Mercury Thermometer

There are many types of instruments used to measure and control temperature, including pointer-type, digital display-type, and electric contact-type. These temperature measuring and control instruments are all factory-produced, complex in structure, difficult to repair if broken, and very expensive.

This article introduces a temperature control instrument made with only two common electrical components. This inexpensive instrument accurately controls temperatures and can be used to measure and control the temperature of media (gases, liquids, or solids) within a temperature range of 0°C to 300°C. Because it uses only a few components, it's easy to manufacture.   

The working principle of the instrument is shown in the attached figure.

In the figure, Pt is an adjustable electric contact glass mercury thermometer used to measure and control temperature; K is a JTX-3c AC relay. This temperature control instrument can be used to control a thermostat. For example, if the thermostat needs to be controlled at 120°C, first adjust the electric contact thermometer Pt to 120°C and lock it in. Close switch S. The thermostat's electric heating wire Rf is energized and heated, and the red signal light H1 illuminates, indicating that the thermostat is heating up. When the temperature inside the thermostat reaches 120°C, thermometer Pt is energized, closing the attraction coil of electromagnetic relay K. Its normally closed contacts K1 and K2 are disconnected, de-energizing the electric heater Rf and stopping heating. Signal lights H1 go out and H2 illuminate, indicating that the thermostat is in a constant temperature state. When the temperature drops, thermometer Pt disconnects, relay K de-energizes, and Rf is re-energized and heated. This cycle repeats, keeping the thermostat temperature constant at the set value of 120°C.

Electronic Components: The adjustable electric contact glass mercury thermometer (PT) uses a WXC-11t, 0°C to 300°C; the relay (K) uses a JTX-3C, AC 220V, and an operating current of 11.5mA for the pull-in coil. This relay has three pairs of output changeover contacts. For high load currents, the output contacts can be connected in parallel, capable of carrying a 4kW load current for electric heaters.