Circuit knowledge

This circuit is used for optimum regulation of the flow of hot water in a central heating system.
It measures the water temperature, and arranges for a particular valve or pump in the system to be switched on to achieve a user-defined temperature distribution in the home. Residual heat in the central heating system can thus be used to lower the cost of fuel. Fig. 1 shows that water in temperature range I can be used for the central heating and the storage vessel, while that in range II is also suitable for directing to the boiler. In most cases, it is not recommended to re-use water with a temperature below 30 °C. The circuit arranges for an alarm to be activated when the water temperature falls below 5°C, or exceeds 95 °C. The circuit diagram of the central heating control appears in Fig. 2. Relays Re1 and Res are activated upon measuring the maximum and minimum permissible temperature, respect- ively.

The temperature sensor is a Type LM35, which has a scale factor of +10 mV/°C. Its output voltage is amplified in At and fed to the non-inverting inputs of comparators A2-A6. The presets at the inverting input of each of these is used to set the toggle voltage, i.e., the temperature at which the relevant relay is switched on or off. The relay drivers are open-collector power buffers with built-in freewheeling diodes to afford protection against inductive surges.

The use of the Type ULN2003 makes it possible to use relays with a coil voltage of upto 50 V without the need for additional interfacing. Each temperature setting has a hysteresis of about 2 °C. Transistors T1-Ta serve to disable the previously energized pump or valve upon detecting a water temperature that falls within another, predefined, range. In this manner, only one relay is activated at a time. It stand to reason that the temperature sensor, IC1, must be mounted such that it is in thermal contact with the water in the heating system. Make sure that the device is well-insulated, and that it does not cause leakage. The temperature range settings for the presets are shown opposite.