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On/Off control in action: Temperature control   |   Pressure control   |   Zone control

On/Off control for HVAC control

On/Off control is one of the simplest methods for controlling temperature. The basis of this style of control is that full power is supplied to the system until the desired setpoint is reached. Once the pressure feedback reaches and surpasses the setpoint, the power is completely turned off to the heater. 

When the pressure drops below a bandwidth setpoint, the power will again turn back on until the setpoint is again reached. This cycle of turning on and off repeats continuously to control the pressure within the desired bandwidth over time.

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On/Off method of temperature control
Figure 1: On/Off method of HVAC control

How on/off control works

On/off control is the most basic and inexpensive method to control HVAC. It operates on the principle of supplying full power to a heater when the temperature or pressure is below the desired setpoint.

Once the temperature or pressure rises and surpasses the setpoint, power is then turned off until it drops below the hysteresis setpoint. Power is then turn on to restore it to the desired setpoint. 

If hysteresis is not implemented the system can, and most likely will chatter. Chattering occurs when the setpoint and feedback are very close to one another and causes relays to turn on and off very quickly which is hard on the components. 

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An illustration of the on-off method of temperature control
Figure 2: An illustration of the on-off method of temperature control 

Implementing on/off temperature control

The easyE4 nano-programmable logic controller (PLC) makes it very simple to implement this technique.

The software used to program the device contains a function block called “Three Step Controller” that can be used for on-off control. Also, it only takes minutes to program since the function block contains all the important parameters such as setpoint, feedback, and hysteresis values.

Likewise, the PLC has built in relay outputs that can handle up to 8 amps, meaning the device can either directly power heaters or it can pilot contactors for high power heaters.

XH/-XH:  Distance X from switching point
H1:  Hysteresis 1 for XH
H2:  Hysteresis 2 for -XH
Y(t):  Operating points for Q1/Q2
Q1:  Switch output X = positive
Q2:  Switch output X = negative

Operating ranges
X> XH:  Q1 switches one to < (XH-H1)
X< -XH:  Q2 switches one to X> -XH+H2)

Programming on-off control using the easyE4 nano programmable logic controller (PLC)
Figure 3: Programming on/off control using the easyE4 nano PLC
easyE4 Nano PLC small image 250 x 277

Explore on/off control capabilities of the easyE4 Nano PLC

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