Single-chip temperature control

The UAA2016 from Motorola lends itself admirably to making a proportional mains-operated temperature control. The UAA2016 is intended to drive triacs using the Zero Voltage technique, which allows resistive load power regulation without RFI. Its main application is the precision regulation of electrical heating systems such as panel heaters or irons, and it operates directly on the AC power line. A built-in digital sawtooth waveform allows for proportional temperature regulation over a 1°C band around the set point. There is a programmable temperature reduction function for energy savings, and a sensor failsafe inhibits output pulses when the sensor connection is broken for security. Application of predetermined temperature (i.e. defrost) is also possible. In applications requiring high hysteresis, its value can be adjusted to within 5°C of the set point. All of these features are implemented with a minimal number of external components.

A resistor with negative temperature control (NTC), R3, which must have a value of about 100 (12 at 25 °C, serves as a sensor. In the diagram, the heating element is a 100 W light bulb, which generates enough heat for a greenhouse or a climatic chamber (for testing electronic equipment). The triac can switch up to 3 A if it is fitted with a heat sink.

The properties of the regulator arc determined by the resistors at pins 1-4. Resistors R1 and R2 establish over -which range the temperature may be set to P1. With values shown, the range is 0-70 °C.

Closing S1 causes the temperature to drop by about 2 °C. The degree to which the temperature drops is determined by R4: a value of 100 K Ω results in a reduction of about 5°C.

Resistor R5 determines the hysteresis of the regulator: here, it is about 150 mV. How much that is in °C depends on the type of NTC and the temperature: an NTC has a non-linear temperature characteristic.

Do bear in mind that the entire circuit is connected to the mains and that, therefore, the regulators must be constructed with due care to safety regulations. It should be installed in a well-insulated, earthed if the metal case.

Single-chip temperature control Schematic diagram


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