A Solar Panel Power Point
Tracker
DISCLAIMER : - The drawings, proceedures and words are for information only. No claims are expressed or implied as to the safety, usefulness, or accuracy of this information. I will not accept any liability for any damages caused to people or property from the using of this information or from any associated links. Your actions are your responsibility - VERIFY and CHECK information out before proceeding, and don't attempt anything without the required skills, if you cannot agree to this, leave this page now . . . . . . . Chris.
A photovoltaic cells internal resistance will vary greatly with the insolation
level , and to some degree temperature. If we consider a fixed insolation level
falling on the PV cell we could plot the voltage verses current performance and
find a curve like the one shown below.
If the product of the voltage and current were now computed for all values we would now have the power output for the cell under the entire load conditions, with that fixed insolation level. As can be seen, there is a point where the PV cell develops a maximum power. This is called the "maximum power point" . ( and why they are called maximum power point trackers {MPPT}) So to fully utilize the PV cell we need to operate it at this point with all insolation levels. Looking at figure 2 it may be noticed that the PV maximum power voltage is only within a couple of percent for differing insolation levels .
This means that if a unit could built that would
maintain the cells output voltage at a fixed value, "say - at the most
predominate insolation and cell temperature", the PV cell would operate at
close or near it's maximum efficiency for all insolation levels. The unit below
will do this for you, and keep your 12v battery nicely charged. It uses a cheap
switching regulator IC, ( LM 78s40 ) , and a few other inexpensive
parts.
OPERATION:
The chip senses the preset voltage level of the PV array ( Vmp ) via the 27K
and 2K pot and is compared with the reference voltage available at pin 8 (1.3v
reference). When the volatge at pin 9, (comparator + input) is larger than that
at pin 10, (- input, and held at 1.3 volts), the internal oscillator is gated
through to an internal flip flop to drive the internal output transistors. This
then switches the power fet on so current flows through the the load and the
inductor. Power is drawn from the array and the paralleled storage capacitor
which provides more current than the array alone could deliver. When the array
voltage drops below the preset voltage the fet is turned off again and the
energy stored in the inductor discharges through the schottky diode into the
"load". The PV array also charges up the storage capacitor ready for
the next cycle. The unit operates at about 12 KHz, so keep your leads short.
The internal oscillator's frequency is set by a capacitor on pin 12, this
oscillator is also current controlled, (at pin 14). This feature (we don't use
here) allows the chip to modify it's duty cycle from it's nornal 6:1 ratio to
higher ratios depending on this "current sense" line. The switching
FET needs to be matched to your array power, and adequately heatsunk. Ensure
that a "load" is across the MPPT output whenever it is in operation.
The circuit configuration is a buck stepdown converter, with switching taking
place in the negative line, other output configurations may be possible if you
wish too, (ie, Boost converters) .The second comparator (pins 4, 6 and 7), and
internal shottky diode (pins 1 and 2) are not used in this circuit too. The
floating input lines should be tied to supply rails to assure chip stability
too. Other FETS can also be used such as a BUK456-60, or an IRF540 etc. The
diode shown in circuit between the inductor and the output is a blocking diode
to prevent the battery discharging through the solar panel or MPPT electronics.
I have built this on "vero board" without experiencing any problems
in oscillator stability too.
The solar array should be mounted to utilise as much
sun as possible, ( you may wish to build a " solar tracker" perhaps),
facing north and angled at the same latitude as your location for best results.
An additional regulator ( see my regulators page ) may be needed to stop your
battery from overcharging ( excessive gassing will deplete the electrolyte
level !, and no good for the battery - see my batteries page too) An additional
regulator will depend on how finely tuned your loads, charging devices and
storage capacity are.
This could also be used as the basis of a unit for driving pumps, or other
equipment, directly from the solar panel, without the need of storage
batteries. Used in this manner it is known as a " linear current booster
" (LCB) and is a means of delivering higher currents for longer to a
"pump" say. When insolation levels are low these devices can
providing 20 to 30 % more pumping time ( depending what part of the world you
live in). Additional circuitry could be used to switch out the pump, when
insolation levels were "very" low, or if the pump was running
"dry". (use the spare comparator perhaps?) A back emf diode would
also be wise for inductive loads such as a pump motor, and ensure you match the
solar panels watt rating to that of your load 's power requirements, (or stick
a shunt voltage regulator across the MPPT's o/p so you don't "cook"
your motor etc. on days of high insolation).
Hope you find this useful. . . . . .
Good luck Chris.
