Load controller   Back to Electrics
 
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Generating power is one thing, using it is another. Various conditions need protection against, such as over charging the battery, draining it too flat and overspeed rotation of the turbine. There are two situations that can damage the batteries. One is overcharge, where current is fed into a fully charged battery. This results in the voltage climbing, and hydrolosis, whereby water is split into hydrogen and oxygen. This is undesirable as it reduces the amount of electrolye, can damage the batery plates, and produces flammable gases.

The other situation is undercharge, where the battery voltage is allowed to fall to a point where a deposit can form on the lead plates (via sulphation) which can permanently prevent the battery from accepting a full charge.

The basis of protecting against these is a voltage detection circuit. This needs to be adjustable to turn a dump load on at high voltages, and to switch off the load at a low voltage. Some hysteresis is also needed, and so four settable voltage thresholds are required: dump load on, dump load off, load (e.g. lights) off, load on. I have designed a circuit which is made available to those attending courses.

On the left is a graph of voltage against time, showing how the circuit could be used to control a dump load. As soon as the voltage rises above the high threshold, the load is turned on. This draws current, and so (as long as the load is big enough) the voltage falls. Once it falls below the lower threshold, the load is switched off, and the voltage rises. This cycle is repeated until the wind blows less strongly.

Similarly, the second set of potentiometers can be used to control the loads, such as LED lights. For this, the lower threshold (for a 24V system) should be around 24V, and the upper (switch on) threshold about 25V.

ASSEMBLING THE VOLTAGE DETECTION CIRCUIT

Here is the component layout (click for a larger image) and some photos of the recommended sequence of assembling the circuit. There are several extra holes to allow for various options and additions, but these photos show only the essential components.

The left hand connecter is for battery power (upper one positive), and the right hand side connecter is for the two outputs.

The upper set of two potentiometers controls the upper output, and is usually used for the dump load. The lower set of potentiometers usually controls the load, and there are two options: For low current loads, there is space to mount a N-FET on the PCB - an IRFZ48 is ideal. The lower (third) leg of this FET is then attached to negative, and the middle (second) leg is attached to the return wire of the load (the positive of the load is directly connected to the battery). For higher current loads, this FET should drive a relay (or set of relays) which provide power to the loads when active.



Using a toothbrush to scrub the bottom of the board as soon as you have finished soldering will make it clean and neat looking.

Tip: R2 can get squeezed between the test point for P3 and the connecter to the left. It will make it fit more easily if you push R2 up towards D1 when you install it.

Here is the circuit diagram:

CALLIBRATION

Once the circuit is assembled, follow these steps to set it up

1. Measure these two voltages:
- Battery input (pin 1 of incoming wire connector), e.g. 25V
- Between R1 and R2, e.g. 2.47V
Now divide the first by the second e.g. 25/2.47 = 10.121

2. Choose the voltage at which you want to start dumping, say 28.5V
Divide by the above factor, e.g. 28.5/10.121 = 2.82V
Turn the pot P4 until the voltage at the test point next to it reads this voltage

3. Now choose the voltage at which you want to stop dumping, say 27.9V
Divide by the above factor, e.g. 27.9/10.121 = 2.76V
Turn the pot P3 until the voltage at the test point next to it reads this voltage

4. Now choose the voltage at which you want to turn off the load, say 23.5V
Divide by the above factor, e.g. 23.5/10.121 = 2.32V
Turn the pot P1 until the voltage at the test point next to it reads this voltage

5. Now choose the voltage at which you want to turn the loads back on, say 25V
Divide by the above factor, e.g. 25/10.121 = 2.47V
Turn the pot P2 until the voltage at the test point next to it reads this voltage

Then it should be ready to go!

If you have access to a variable voltage power supply, you can verify the circuit is working as follows:

1. Connect the voltage supply, and dial up the voltage to 29V.
2. Measure output A - it should be high (6V).
3. Now slowly turn down the voltage, and as it passes 27.9V output A should go low (0V).
4. Check output B - it should be high.
5. Turn down the voltage slowly to 23.5V, and as it passes 23.5V, output B should go low.
6. Turn the voltage up again slowly, and as it passes 25V output B should go high again.

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