There are three choices:
(i) Build Version One on this page. (small and awkward circuit board)
(ii) Build Version Two or Version Three (easier construction using Veroboard-style circuit board).
(iii) Buy the kit from the ATA (Melbourne, Australia)


Build this simple Mini Maximiser

Greatly improve the performance
of solar-powered pumps and motors
by building this
simple mini-maximiser.


by Alan Hutchinson, Plasmatronics, Melbourne (this Web version with his permission)

One of the more common uses of solar electric (photovoltaic) panels is driving motors and in particular for water pumping.

The motor is connected directly to the solar panels and no storage battery is used. This arrangement works well but suffers from one major weakness. Because solar panels are effectively constant current sources they cannot deliver extra power for starting like a battery can. In fact, due to the low resistance of motors when stopped, the panels can only deliver a small fraction of their normal power during start up. This means that there is very little power available at the time it is most needed to get the motor running. Fortunately, this problem can be easily solved by using an electronic device to match the panel to the motor so as to allow the panel to deliver its full power into the motor even when the motor is stalled. A variety of such devices are available on the market such the "Maximiser" and various forms of maximum power point trackers. These are quite sophisticated devices costing some hundreds of dollars. This is OK for a large pump setup but for a small one or two panel system it is a bit expensive. Here is a simple mini- maximiser that you can build yourself and will perform almost as well as some of the more expensive ones.

How it works

First let's take a closer look at the problem. The motor driven diaphram pump used here for testing the maximiser is a 12V motor nominally rated at 6amp. The pump would not start even when unloaded when driven from a single 50W panel (an Arco M55). With 3.3Amp from the panel flowing through the motor, 1.2 volt was measured across the motor. Hence the power going into the motor is only 1.2 x 3.3 = 4 Watt. If the panel was being operated at its peak power point (about 17.5V), it could have delivered 55W rather than the paltry 4W it actually delivered.

So the problem is that we need to get the panel to supply its energy at 17V but somehow we need to deliver it to the motor at a lower voltage. More expensive maximisers use switchmode down converters to perform this function of acting like a DC transformer. This maximiser is much simpler.

The circuit diagram shows that it is basically a large capacitor across the panel which can be connected by an electronic switch to the motor. When the switch is off, the capacitor will be charged up by the panel. As the open circuit voltage of the panel is usually above 20V, the capacitor can charge to quite a high voltage. When the capacitor voltage rises above an adjustable set point (in the range 10.5-17 volts) the electronic switch (the FET) turns on. This connects the motor across the capacitor and will draw a lot of current from the capacitor into the motor. This will cause the capacitor to discharge rapidly. When the capacitor voltage falls about half a volt below the set point, the FET will switch off again and allow the capacitor to charge up again. This cycle then repeats indefinitely. It is basically a free running oscillator with the rate of charge determined by the panel charge current and the rate of discharge determined by the amount by which the motor current exceeds the panel current. Yes, it is possible to get more current flowing into the motor than is coming out of the panel!)

So what have we got happening? The panel is delivering a steady 3 amp at close to the set point voltage. If the set point is set close to the maximum power point of the panel, the panel will be delivering as much power as it can. On the motor side, the motor will be driven with pulses of high current which is excellent for providing the high torque necessary to get it moving. When its stalled, the pulses will be large but of short duration. As the motor picks up speed, the voltage across it will rise due to the back EMF and the pulses will be at a lower current but of longer duration. This means that the frequency of the oscillator will fall as the motor gets going. In this example, the frequency when running was about 1Khz and the switch was off for about 1/3 of the time.

What it did

The improvement in performance due to the maximiser was amazing. Without it, the pump motor would not even run unloaded, even if it was turned over by hand. With the maximiser, it started up and ran well. With the set point voltage at 13V, the panel current measured 3.2A (input power 41W) and we had it pumping water up 6 metres.

Circuit details

The two large electrolytic capacitors are charged by the solar panel and then quickly discharged into the load by the FET switch.

A BUK455-60A is used. for the electronic switch. This will block 50V and has an on resistance of 0.03 Ohm. Because of the low on resistance it is not necessary to provide a heat sink. The SF52 is a very fast recovery diode (normal diodes are not good enough) that is used as a catch or freewheeling diode. This provides a path for the motor current to flow through when the switch is off. This protects the FET from high voltage spikes when it switches off.

An LM311 comparator is used to compare the working capacitor voltage (reduced by the adjustable voltage divider) with a reference voltage supplied by the 6.2 volt zener diode. The output of the LM311 drives the FET switch. The 100K resistor supplies positive feedback round the LM311 and creates about 0.5V of hysteresis between the switch on and switch off voltages. The 100pF cap. speeds the switching and suppresses transition oscillations. (The speed of the maximiser's oscillations could be doubled by increasing the 100K resistor to 220K) If a 24V version is required, the extra three components shown should be inserted to limit the LM311 supply to 15V, and the top 3K3 divider resistor should be replaced with a 12K (one of the 470 Ohm resistors replaces D1) About 2.5W is wasted in the operation of the maximiser.

Construction

This is not a difficult device to make. It takes about 1 hour to put together. To make it easier, the circuit is constructed on a piece of predrilled circuit board supplied in the kit. The components are mounted on the opposite side from the copper tracks according to the layout diagram. Take care to get all the diodes,the LM311, the FET, and the large capacitors round the right way. Solder the components on, taking care not to create bridges between tracks.

click here to display cct-dn.gif for printing.

Note: view in 800 x 600 screen mode to see these images side-by-side. To print the whole image of copper tracks, click on it, and print the image "cct-dn.gif" on a separate page.

Bend the wires from the large capacitors and the FET along the copper tracks to reinforce the tracks if two panels are to be used.

When assembly is finished, clean off the flux and check that each component is in the correct place, that it is the right way round and that there are no bridges between tracks.

Testing

Connect a 12V battery to the solar terminals with a 220 Ohm resistor between the battery+ and the solar+ terminals. The resistor is to limit the current in case something is wrong) Don't connect anything to the motor terminals.

Measure the voltage across the 220 Ohm resistor with a multimeter. If everything is OK, then it should be roughly 2.2V. If this is OK, remove the 220 Ohm resistor and connect battery+ direct to sol+. Now check the voltage across the 6.2V zener diode (Z1). This should be roughly 6.2 volts. Set the trimpot to its mid point and measure the voltage between the trimpot centre leg and the sol- terminal. This should be about 6V. Turn the pot fully anticlockwise, then measure the voltage across the 15V zener diode (Z2). It should be about 11V indicating that the switch to the motor is turned on. Turn the pot fully clockwise. The voltage across the zener should now be less than lV indicating that the switch is off. If you can get it to perform like this, then try it on the real thing.

Parts List (Kit version only)
1 Power FET BUK455 60A FET1
1 SF52 superfast diode D2
1 LM311N comparator IC IC1
1 6.2V 400mW zener diode Z1
1 15V 400mW (or 1W) zener diode) Z2
1 1N4148 diode D1
3 3K3 1/4W metal film resistors R1,R2,R3
1 1K5 1/4W metal film resistor R4
1 100K 1/4W metal film resistor R5
1 2K horizontal trimpot TP1
1 0.1 uF monolithic ceramic capacitor 50V C2
1 100 pF ceramic capacitor 50V C1
1 4way PCB mount terminal block CONN1
2 470 uF 50V RB electrolytic capacitors C3,C4
1 MAXI Circuit board
(for a 24V version these extra components are required)
1 15V 1W zener diode Z3
2 470R 1W resistors R6 & replace D1
1 12K 1/4W resistor R2 (alternate value)

Operation

Connect the solar panel(s) to the solar+ and solar- terminals. Connect the motor to the motor- and motor+ terminals. When the motor is stalled, you may hear a high pitched whine as the maximiser is pouring maximum current into the motor to try and get it moving. As the motor speeds up, the pitch of the whine drops. You can also hear it change pitch as the amount of light falling on the panel changes.

The reason for the adjustment pot is to allow the operating point of the panel to be varied according to the situation. If you are trying to get pumping under overcast conditions or in the morning or evening, then the operating point needs to be low enough so that the peak panel voltage is high enough to charge the capacitor up to the switch threshold. In other words, if the operating point is set too high, the panel voltage can never get above it and the maximiser won't oscillate. In the test situation described above, we found that with the operating point set at about 13V the pump would start with about 14W of input power (ie about 1/4 of full output). A setting in the range of 12-14 volt seems to be appropriate. If there is sufficient power from the panel to bring the motor voltage up above the set operating point then the switch will simply remain on. (ie if there is enough power the maximiser will retire gracefully until its services are needed again.)

If you put the circuit in a box then be careful to allow holes for adequate ventilation especially with 2 panels. This simple circuit extends the operating range of motors and pumps run directly off solar panels in a cheap but effective way.

Our unit worked first time and if you are careful with the construction, there is no reason why yours shouldn't either.

This text is from the kit form of the Mini Maximiser

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