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SWITCHBOX, LOAD CONTROL AND LIGHTING

A switch box is a basic requirement for any turbine. Click here for more about switch boxes.

Load control refers to avoiding damaging the batteries. Click here for information about control circuits.

A dump load 'dumps' excess power when the batteries are full, which controls the speed of the turbine. Click here for more about dump loads.

There are a few choices for lighting which runs directly off the battery. This is much more efficient than running lights through an inverter. Click here for more about DC lighting.

BATTERIES

Using two batteries allows working at 24V, which means the wire can be 25% of the thickness that would be required for a 12V system. Normal car batteries will work, but will not last long. Marine or deep cycle batteries are ideal. These are 'high' cycle 105 AmpHour batteries, which at the time were a good compromise between price and life time. Recently, the price difference between 'high' cycle and deep cycle is not that much, and deep cycle would be the better choice.

Its a good idea to make a wooden cover which keeps the batteries clean and prevents anything metal falling on them and causing a short. I heard a story about someone dropping a spanner onto a submarine battery bank. There was a bang and a flash, and no trace of the spanner was ever found - it was instantly vapourised!

MASTER PLAN

Here is the master plan drawn up before any of the above was done. Unfortunately it is far from fully realised so far.

RECTIFYING WITH SCHOTTKY DIODES


Standard bridge rectifiers have a voltage drop of about 0.6V per diode, but there are Schottky diodes with lower voltage drops.

There are other factors however, such as switching speed and reverse leakage, and the actual voltage drop varies depending on the current. So the only way to be sure about what improvement is possible was to make a test circuit and do some benchtesting.

This circuit has 6 Schottkys (STPS20H100CT). Each package has a pair of diodes, with the positive ends connected. Each pair is connected in parallel to double the maximum current to 20A per phase. Note how the diodes are mounted to allow attachment to a single compact block heatsink.

Testing on the bench and comparing this to standard SB356 rectifiers, the Schottky method produces about 5 Watts more power (over the range of about 150 - 200rpm).

Conclusion: it doesnt seem worth the bother.

VOLTAGE CONVERSION

  
Once you have a 24V system, you may want to use 12V appliances, most of which will blow or melt if connected to 24V.

This is a simple voltage control test circuit I have built to drive a 12V load from 24V. Problems are it is very sensitive to the pot, the voltage is not stable when a load is applied and the MOSFET gets quite hot.

All in all this circuit was an interesting exercise but is pretty useless practically.

What is needed is a DC-DC conversion using a buck, boost, or buck boost converter. This uses an inductor, a switch such as a FET, and a capacitor to convert DC voltages with very high efficiencies, up to 95%.