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Managing Variable and Constant Temperature Circuits

The radiator uses the principle of a variable temperature circuit


I have a complicated relationship with coffee. Apparently, the caffeine-rich bean is a double-edged sword. While a cup of fresh-brewed Americano will clear my mind on a fuzzy day, drinking a cup of coffee on other occasions will send my mind to a shutdown. 

Due to how my body reacts differently to caffeine, I couldn’t maintain a constant routine of having a cup of coffee daily, regardless of how I’m attracted to its rich aroma. On a different note, variable and constant temperature circuits allow temperature-system dependent to be regulated in a precise manner, just as how I’ve controlled my caffeine intake. 

What Are Variable And Constant Temperature Circuits

In its primary form, both variable and constant temperature circuits are electronic components that are brought together to control the output based on the feedback temperature. Both circuits will require temperature sensors and output control in the form of analog signals, pulses, or actuators. 

A constant temperature circuit regulates the load temperature at a specific set point. By having a temperature sensor at the load, the algorithm of the temperature controller will adjust the output control so that the load temperature is constantly sustained. Irrespective of external factors, the load temperature will remain the same. A typical home air conditioner is a good example of a constant temperature circuit. 

The variable temperature circuit functions differently, although it may have an almost similar mechanism. Instead of having a single temperature sensor, the variable temperature circuit requires two sensors. One is placed at the load while the other sensor is usually used for sensing ambient temperature.

Instead of ensuring the load temperature remains constant, the variable temperature circuit regulates the load based on the ambient temperature. This is often done through a specific table or formula that is mapped within the controller’s algorithm. In other words, the load temperature is dependent on the ambient temperature. 

Real-Life Application Of Variable And Constant Temperature Circuits In Boiler System

It is easy to imagine variable and constant temperature circuits as miniature prototypes on a piece of PCB. However, actual applications of such circuits are on a much greater scale. Both variable and constant temperature circuits are commonly used in the boiler system.

The boiler is a type of water heating system that disperses hot water to radiators, radiant panels and other devices around a building. The hot water is channeled through pipes valves are used to control the flow of the heated water. After passing through the radiators, the much cooler water is then directed back to the boiler. 


Boiler system in control of the temperature of an electrical system

Variable and constant temperature circuits are applied in boiler systems.


Radiators are usually controlled by a variable temperature circuit. The actual term that refers to the control is a variable temperature valve or VT valve. The logics controlling the valve adjust the flow rate to ensure the radiator temperature corresponds to external temperature. For example, an external temperature of -5°C will result in the outlet flow being adjusted to 80°C. As the external temperature decreases, the outlet flow temperature will be decreased accordingly.  

Constant temperature circuits are usually found in radiating panels that heat up large surfaces such as floor or wall. The temperature on the panels is sustained at a specific temperature. In such setups, the constant temperature valves are used. 

Important Considerations When Designing Variable And Constant Temperature Circuits

The boiler system is all about valves, pumps, and pipes. However, it requires electronics to control the water flow to regulate load temperature accordingly. Boiler systems often use thermostats to detect the temperature.

Conventional thermostats rely on using two different pieces of metal that expand at different rates. Modern thermostat involves the use of sensors, controllers and wired or wireless connection to the boiler controller. Generally, digital thermostats are more accurate than mechanical thermostats.  

Designing with mechanical thermostats is straightforward as the metal pieces is basically a switch that turns on and off. However, there are more challenges when dealing with digital thermostat. Besides getting the sensor right, you’ll need to deal with potential interference to the analog output of the temperature sensor. 


Digital thermometer connecting the house through an IoT architecture

The digital thermostat is part of a modern boiler system.


You’ll also need to be aware of how the valves are controlled. Some valves are triggered by connecting the live wire to the AC supply while others are controlled by analog voltage. Both types of valves pose their respective design challenges.

Connecting a valve to an AC supply is usually done via a relay or solid-state-drive. It’s important to get the current rating of the relay right. A flyback diode is also required at the coil of the relay to prevent voltage spike when the relay is released. 

If you’re driving an analog-controlled valve, you’ll need to ensure the output voltage is at the correct level. This can be achieved by using a combination of an RC circuit and a Mosfet, or including a DAC chip to the design. Either way, it is important to ensure the valve control signal is free from electrical interference. 

When working through any circuit design with thermal requirements, the suite of design and analysis tools available through Cadence are your best choice. Your task is made easier if you have a user-friendly PCB design software like OrCAD. Setting proper design rules helps to minimize the chances of noise coupling. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts