In most small solar power systems using a boiler, it is required that the water circulation pump is not switched on until the temperature of the collector (the solar panel) exceeds that of the water in the vessel. Here, a two-sensor monitor is presented that enables this condition to be met. One sensor is fitted on the collector, the other on the water vessel. The control shown here has two adjustments: one for the temperature difference at which the pump starts to operate, and one for the temperature difference at which it is switched off. Although these settings are independent, the switch-off level must be lower than the switch-on level. Calibration in degrees Celcius is simple because the gradient of the voltage at the wiper of the potentiometers (or presets) that set the on/off temperatures is exactly 0.1 V °C-1. The two temperature sensors ‘Type LM334 is adjusted to supply a temperature gradient of 1pA/°C. Unequal sensor temperatures, therefore, produce a current flow at their junction. The voltage across RI is directly proportional to the temperature difference measured. This enables the switching thresholds of the °°/off control to be set with the aid of two Presets: the_ on’ preset (P4) is adjusted to.
say. 3 °C, and the ‘off preset (P3) to 1 °C. The range of the two presets is about 5 °C. The sensors used here to supply a current rather than a voltage. This eliminates thermocouple effects caused by temperature changes on the connecting cables between the sensors and the circuit. If voltage-type sensors such as PTCs or NTCs were used, the circuit would have become more complex because of the required compensation. The AD590 may be used instead of the LM334. Note, however, that the AD590 does not require an adjustment preset or resistor.
Relay Re2 switches the pump on and off. A second relay. Re1 comes on after Re2. It is optional and may be used to switch the pump briefly to a higher speed, which is required in some solar heating systems to increase the initial water flow or to 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 is 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 nought. It will be clear that these initial adjustments require that the two LM334 are at the same temperature.
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 at the track side of the board.
Parts list
Resistors:
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
Capacitors:
C = 1 μF MKT
C2 = 10 μF 10V radial
C3 = 100 μF 10V radial
C4 = 470 μF 25 V radial
Semiconductors:
D1: D2 = 1N4001
D3 : D4 = LED
D5:D6 = LM334
Integrated circuits:
IC1 = 7805
IC2 = TLC272
IC3 = 4013
T1 :T2 = BC547B
Miscellaneous:
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.
