Basic Solar Heating Controller
Why use a controller?
A solar water heating controller is a gadget used in conjunction with a solar water heating panel array to supply hot water to your taps. In the simplest systems, water circulates by itself. However, if you install a pump it means greater chotce over where you position the panel, plus improved thermal efficiency. The controller is a simple thermostat that only switches on your pump to carry water from the heater when it's hot enough to be of use. This section tells you how to make and install one from standard electrical components.
The controller we describe can be used for most solar heated water systems. Let's took first at how it works and then at making the control box, fitting the sensors, choosing the temperature difference, testing procedure, installation, and some additional technical detail.
How it works
Two sensors are mounted, on the hot water tank and on the solar panel. They measure the difference in temperature between the two. When the panel is sufficiently hotter, they tell the controller automatically to switch on the pump. When the tank has heated up it will switch the pump off. There is also a manual override switch.
Building the control box
Most of the components are fitted on to a printed circuit board. This is shown in Figures 3 & 4 below. Figure 2 shows the circuit diagram. The board was designed to fit inside a plastic case measuring 150mm by 80mm by 50mm, although any reasonable size may be used; you could adapt something or build it out of wood. It should be possible to solder together and construct the controller, and put it in the case, with careful reference to the diagrams and photograph. However the following points are offered to avoid some of the mistakes that might occur.
Three rubber grommets are fitted to cable holes in the lid of the case, and the LEDs and switches are fitted in the lid. Take care to position these lid-mounted parts so that there's enough clearance space when the case is put together. When fitting the diodes and the capacitor C4, make sure that their polarity is correct and that the integrated circuit IC1 is the right way round. Similarly, when completing the interwiring between the components, pay careful altention to the polarity of the LEDs, to make sure they are connected the right way round.
Check the wiring of switch S1 since this will carry mains voltage. When the components of the board are all securely soldered in place, lower the board into the case you have made and connect a three core mains cable to the three way terminal block. The cable should be fitted with a 3 amp fused plug.
Installing the sensors
First, make up the two sensor leads, one for the solar panel and one for the hot water tank. Use twin core 7/0.25mm cable. The type with one side marked with a coloured stripe or ribbing is ideal. At this stage each lead may be about 2 metres long. If necessary, extend them with more cable laler; or calculale first the distance you want to position the controller from the panel and tank.
The sensors themselves are 1N4148 diodes. Simply solder them to the end of each lead as shown in Figure 5. Seat the bare wire and joints with epoxy putty against water. This will harden in a few minutes.
Thread the two sensor leads through the grommet on the end of your box and connect them into the terminal block as shown in Figure 4.
Choosing the temperature difference
The size of the temperature difference between the tank and the panel that switches the pump on or off is controlled by the component called R9. This is a feedback resistor of the 741 amplifier. You can choose its value to give a temperature difference as shown below:
| Value | Temperature Difference |
| 3M3 | 10°C |
| 4M7 | 7°C |
| 10M | 3.5°C |
You may wish to experiment with installing different resistors at different values to find the one that works best for your conditions. Alternalively you could fit a variable resistor, and having found the optimum position, leave it there.
Fig. 2: Complete circuit diagram.
Fig. 3: Full sized printed circuit board master paltern.
Fig. 4: Component layout and interwiring details.
Fig. 5: Sensor construction and mounting
LIST OF COMPONENTS
| Resistors | Value | Cotour code |
| R1.R2 | 4K7 (two) | yellow, violet, red |
| R3,R13 | 100R (two) | brown, black, brown |
| R4,R5,R6 | 2K2 (three) | red, red, red |
| R7,R8 | 10K (two) | brown, black, orange |
| R9 | 4M7 (see note) | yellow, violet, green |
| R10 | 1K5 | brown, green, red |
| R11 | 330R | orange, orange, brown |
| R12.R14 | 2K2, 0.5Walt | red, red, red |
| R15 | 1K1Walt | brown, black, red |
| VR1 | 100R cermet preset | |
| Capacitors | ||
| C1 | 470nF capacitor polyester 100V | |
| C2,C3 | 100nF capacitor polyester 100V (two) | |
| C4 | 330uF capacitor radial electrolytic 40V | |
| C5 | 100nF suppression capacitor | |
| Diodes | ||
| D1 | 8V2 zener diode | |
| D2,D3,D4 | 1N4148 (three) | |
| D5,D6 | 1N4002 or 1N4007 (two) | |
| D7 | green LED (with clip) | |
| D8 | red LED (with clip) | |
| Others | ||
| TR1 | BC182L transistor | |
| IC1 | LM741 op-amp | |
| RLA | 10A single pole changeover relay, 24V coil | |
| T1 | 3 VA transformer, 240V primary, 20-0-20V secondary | |
| S1 | d.p.s.t. rocker switch, 240V AC 4A | |
| S2 | s.p.s.t. toggle switch, 30V DC1A | |
| FS1 | 1 amp fuse | |
| Misc | ||
| 3 way terminal block | ||
| 4 way terminal block (two) p.c.b. terminal pins (ten) fuse clips (two) | ||
| plastic case | ||
| rubber grommets (three) | ||
| printed circuit board | ||
| twin 7/0.25mm cable | ||
| epoxy putty and (maybe) silicone sealant | ||
Test procedure
You're ready to try it out. Plug in the unit and turn switch S1 on. If it's all connected properly, the red LEDs should light up. Now switch S2 into the auto position. Take care. The fuse clips and components near the 3-way terminal block are now live, as is the back of switch S1.
Next, ensure that the two sensors are near to each other, and therefore al the same temperature. Turning the preset dial [VR1] should cause the relay and the green LED to turn on and off. If it does, the next step is to find the correct point at which to set VR1. This is done by turning it until the relay just comes on. Then turn it back until it is just past the point al which the relay turns off. Five degrees of rotalion past the turn off point is about right. Leave it there.
You're now ready to see if a temperature difference between the sensors turns on the switch. Find something hot like a soldering iron and hold it near or against the panel sensor. The relay should click on after a few seconds. If you then leave the sensor to cool down for a few minutes the relay should turn off. If it does all this you've succeeded so far. Now disconnect the unit from the mains. You may now to install it.
Installation
Have another took at Figure 2 on page 3, which shows a typical solar heated system. The solar energy collected by the panels is pumped to the pre-heat cylinder. The pump is switched on by the solar panel controller. This pre-heated water is then drawn through the existing hot water tank.
The pump is connected to your controller on the terminals marked LI, N and E on the controller's four way terminal block. Take note of the fact that terminal L2 is live when the green LED is off.
Now you can position the controller box. It should be in a visible and accessible position so the switching of the pump can be monitored. It should also be somewhere where the temperature won't vary widely. Don't put it in the altic.
It's now time to fix the sensors. If you don't get this right, the whole system won't work. Firstly, don't run the leads near mains leads, because they cause interferance. Fit the sensors in place with epoxy putty or silicone sealant and cover them with insulation, such as polystyrene. Figure 5 shows this arrangement. The tank sensor should be mounted on the pipe running from the pre-heat cylinder to the solar panels, and as near as possible to the tank. The actual connection flange is an ideal place.
The panel sensor goes on the outlet pipe inside the box of the last solar panel in the sequence. After the putty has dried, you're now ready to switch on, and the whole system should work.
How It Works
This is for electronics buffs who wish to know the functions of each of the components in the circuit.
Diodes D2 and D3 act as sensors. A current is passed through each of them via resistors R1 and R2. One property of small signal diodes is that a change of temperature will cause their forward conduction of voltage to vary in a repeatable manner. This 'differential' voltage is applied to the inputs of the IC1, which is configured to act as a Schmitt trigger with positive feedback introduced by R9.
R3 and VR1 are used to balance the Schmitt trigger inputs initially so that a differential temperature change within the desired range will be detected.
If the solar panel diode, D3, becomes hotter than the tank diode, D2, the output voltage of the amplifier will rise. Resistor R9 ensures that the output switches cleanly between the supply rails. This turns on TR1 which activales the relay. The green LED, D7, indicates when this happens.
The power supply for the circuit comprises TR1, D5, D6 and C4, giving a full wave rectified and smoothed 30V nominal. This is then dropped through R15 and clamped by D1 at 8.2V to power the op-amp and provide a stable voltage drop across R1 and R2.
Capacitor C1 has been incorporated to reduce differential input noise. Any common mode noise is filtered by C2. C3 has been provided to reduce the effects of interference on switching transistor TR1.
C5 and R13 protect the relay contacts from the arcing that occurs when altempting to switch an inductive toad. They also provide suppression of any outside electrical interference.
