26-06-2012, 03:59 PM
PROJECT NON SOLAR CHARGER
SOLAR CHARGER.docx (Size: 51.51 KB / Downloads: 34)
OBJECTIVE
The aim of this project is to cater for the other end of the range. We are looking at charging a 12v battery, using the cheapest set of solar cells and the cheapest inverter.
The problem with charging a battery from a solar panel is the SUN! It doesn't shine all the time and clouds get in the way! Our eyes adjust to the variations in the strength of the sun but a solar panel behaves differently.
As soon as the sun loses its intensity, the output from a solar panel drops enormously. Not only does the output current fall, but the output voltage also decreases. Many of the solar panels drop to below the 13.6v needed to charge a 12v battery and as soon as this occurs, the charging current drops to ZERO. This means they become useless as soon as the brightness of the sun goes away.
Our project cannot work miracles but it will convert voltages as low as 3.5v into 13.6v and keep delivering a current to the battery. Obviously the current will be much lower than the maximum, when the sun "half-shines" but the inverter will take advantage of all those hours of half-sun.
The other advantage of the inverter is the cost of the panel. You don't have to buy a 12v panel. Almost any panel or set of solar cells will be suitable. You can even use a faulty 12v panel. Sometimes a 12v panel becomes damaged or cracked due to sun, rail, heat or shock. If one or two of the cells do not output a voltage (see below on how to fix faulty panels) the cells can be removed (or unwired) and the gap closed up. This will lower the output voltage (in fact it may increase the voltage, the faulty cells may have reduced the output to zero) but the inverter will automatically adjust.
The aim of this project is to achieve a 13.6v supply at the lowest cost. That's why the project has been released as a kit. The equivalent in made-up form is 3 times more expensive yet doesn't have some of the features we have incorporated in our kit. We have used a more efficient output circuit than the closest rival design and the driver transistor is the latest "low-voltage" type. These two factors increased the efficiency by 20% over the rival.
Circuit Operation:
The circuit is a single transistor oscillator called a feedback oscillator, or more accurately a BLOCKING OSCILLATOR. It has 45 turns on the primary and 15 turns on the feedback winding. There is no secondary as the primary produces a high voltage during part of the cycle and this voltage is delivered to the output via a high-speed diode to produce the output. The output voltage consists of high voltage spikes and should not be measured without a load connected to the output. In our case, the load is the battery being charged. The spikes feed into the battery and our prototype delivered 30mA as a starting current and as the battery voltage increased, the charging current dropped to 22mA.
The transistor is turned on via the 1 ohm base resistor. This causes current to flow in the primary winding and produce magnetic flux. This flux cuts the turns of the feedback winding and produces a voltage in the winding that turns the transistor ON more. This continues until the transistor is fully turned ON and at this point, the magnetic flux in the core of the transformer is a maximum, but is not EXPANDING FLUX. It is STATIONARY FLUX and does not produce a voltage in the feedback winding.
BLOCKING OSCILLATOR
The operation of the circuit has been covered above but the term BLOCKING OSCILLATOR needs more discussion. By simply looking at the circuit you cannot tell if the oscillator is operating as a sinewave or if it is turning on and off very quickly. If the circuit operated as a sinewave, it would not produce a high-voltage spike and a secondary winding would be needed, having an appropriate number of turns for the required voltage.
A sinewave design has advantages. It does not produce RF interference and the output is determined by the number of turns on the secondary. The disadvantage of a sinewave design is the extra winding and the extra losses in the driving transistor, since it is turned on and off fairly slowly, and thus it gets considerably hotter than a blocking oscillator design.