1-Wire Temperature Control

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Controller Algoritam    Controller Setup    List of the Components
The 1-Wire network protocol was developed by Dallas Semiconductor ( now Maxim ) as a way to interface to range of small 1-wire devices.Every 1-Wire device contains a unique identifier preset at the factory. The devices are networked together by a single wire that acts as a physical medium to communicate between a host and a large number of devices attached along the wire. It is also capable of powering these devices through the same wire. In practice, a second wire is actually needed to provide a return to ground and complete the circuit.This project uses two 1-wire devices DS18B20 and DS2405.DS18B20 is a digital thermometer with 0.5C accuracy over a -10C to +85C range.It has an operating temperature range of -55C to +125C.The second device DS2405 Addressable Switch is an open drain N-channel transistor that can be turned on or off by matching the 64-bit factory-lasered registration number within each part. The 8-Bit CMOS microcontroller of Microchip PIC16F628A continuously reads the process temperature ( PVar ) from the DS18B20 sensor and depending on the desired temperature setpoint ( SPoint ) value of the process, manipulates the DS2405 addressable switch,using ON-OFF control algorithm. In this algorithm,the controller will switch the output relay only when the temperature crosses the setpoint.For heating control,the output relay is on when the temperature is below the setpoint, and off above setpoint (fig.1).For colling control, the output relay is on when the temperature is above the setpoint, and off below setpoint (fig.2).
Heating control:    Colling control:
Heating control
fig. 1
     Colling control
fig. 2
Since the temperature crosses the setpoint to change the output state, the process temperature will be cycling continually, going from below setpoint to above, and back below. In cases where this cycling occurs rapidly,and to prevent damage to relay contacts ,a bandwidth ( hysteresis ) is added to the controller operations. For this reason, a timer function can also be used.This function sets the relay output to ON when a preset time (timer setting) elapses after PV has reached the setpoint.
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