Temperature-Based Water Pump Control for Solar Systems
In most small solar power systems that employ a boiler, it’s essential for the water circulation pump to activate only when the collector (the solar panel) temperature surpasses that of the water in the vessel. To address this requirement, a dual-sensor monitor is introduced, facilitating the fulfillment of this condition. One sensor is positioned on the collector, while the other is placed on the water vessel. The control system discussed here incorporates two adjustments: one for determining the temperature difference that triggers the pump to start operating and another for specifying the temperature difference that prompts it to switch off. These settings are independent, but it’s crucial that the switch-off level remains lower than the switch-on level.
The calibration is straightforward in degrees Celsius, thanks to the precisely consistent voltage gradient at the wiper of the potentiometers (or presets) responsible for establishing the on/off temperatures, which equals 0.1 V °C-1. The two ‘Type LM334’ temperature sensors are adjusted to provide a temperature gradient of 1 pA/°C. When the sensors record unequal temperatures, they generate a current flow at their connection point. The voltage across R1 is directly proportional to the measured temperature difference, enabling the configuration of the on/off control’s switching thresholds with the assistance of two presets: ‘on’ preset (P4) adjusted to, let’s say, 3°C, and ‘off’ preset (P3) set to 1°C.
Presets for Temperature Control Range
The temperature control range of the two presets is approximately 5°C. Unlike voltage-based sensors like PTCs or NTCs, the sensors utilized here provide a current output rather than a voltage. This design choice eliminates thermocouple effects that might arise due to temperature fluctuations along the connecting cables between the sensors and the circuit. Had voltage-type sensors like PTCs or NTCs been employed, the circuit’s complexity would have increased due to the need for compensation. An alternative to the LM334, the AD590, may be used. However, note that the AD590 doesn’t require an adjustment preset or resistor.
Relay Operation and Pump Speed Control
Relay Re2 manages the activation and deactivation of the water pump, while a secondary relay, Re1, follows after Re2. The inclusion of Re1 is discretionary and offers the option to momentarily switch the pump to a higher speed, a feature needed in some solar heating systems to enhance initial water flow or fill the system.
The circuit is calibrated by setting equal sensor currents at equal sensor temperatures. The sensor current equals:[273 + Ta)} μA,
where Ta is the ambient temperature in degrees Celsius. Thus, at a room temperature of 20 °C, presets P) and P2 are adjusted until the current flow through each sensor is 293 μA. A few microamps more or less will not make much difference, as long as the sensor currents are equal. It is best to first adjust one sensor only. Start by connecting a microammeter between ‘A’ and ground, and adjust P1. Next, adjust the other preset until the voltage across R1 is zero. It will be clear that these initial adjustments require that the two LM334 be at the same temperature.
The current consumption of the on/off control is about 11 mA, plus about 35 mA for each relay. The dimensions of the printed circuit board are geared to the size of the box mentioned in the parts list. The potentiometers are fitted with the spindles on the track side of the board.
- R1 = 100 kΩ
- R2:R3 = 470 Ω
- P3:P4 = 220 Ω linear potentiometer
- R4:R6= 10 kΩ
- R5 = 1 MΩ
- P1:P2 = 500 Ω multiturn preset
- C = 1 μF MKT
- C2 = 10 μF 10V radial
- C3 = 100 μF 10V radial
- C4 = 470 μF 25 V radial
- D1: D2 = 1N4001
- D3 : D4 = LED
- D5:D6 = LM334
- IC1 = 7805
- IC2 = TLC272
- IC3 = 4013
- T1 :T2 = BC547B
- K1 = 2-way PCB terminal block, pitch 5 mm.
- K2: K3 = 3-way PCB terminal block, pitch 7.5 mm.
- Re1; Re2 = GBR 10.2-11.12 (12V coil, content 250V @8A).
- Enclosure: approx., size 155×61×90 mm;
- e.g., Retex Gibox Type RG3.
- PCB Ref: 914007.