motion machine! Why not take a circular raceway (or
better yet, an oval raceway) and raise one end. Take a
generator and battery, connect them together, and put
them on a cart. When the cart goes downhill, it will
charge the battery; when the cart starts uphill, the
generator will become a motor and drive the cart uphill.
But I digress ... back to the use of relays. Relays are
good as they have very low contact resistance. However,
the contacts do arc and the setup is mechanical. (Not
good for speed nor endurance.)
Silicon devices such as triacs, SCRs, transistors, FETS,
etc., have the advantage there is not much to wear out
and they can work at high speeds. They will burn out,
however, if not properly heatsunk. Where does this heat
come from? Silicon devices like diodes and transistors
often have a voltage drop of something equal to . 8 volts.
The FET I’m using is a P-MOS IRF5305PBF which has a
resistance of .06 ohms from source to drain when
running in the saturated mode. One watt will produce
14. 3 calories per minute since a calorie is equal to
raising 1 gm (or at 1 ml) of water 1oC in one minute. So,
14. 3 calories would raise 14. 3 gms of water one degree.
A tablespoon of water is about 15 gms.
When 12 amps are drawn with a resistance of .06
ohms, the voltage drop across the FET is . 72 volts:
. 72 volts 12 amps = 8. 7 watts or ≅ 120 calories
If we take a tablespoon of water at 25oC (room
temperature) and place it on the transistor, in one
minute it will raise the tablespoon of water to 34oC
(93oF). In seven minutes, the water would boil. Now, you
should understand why we need to heatsink silicon
devices. A great website for the calculation of heat
transfer to a heatsink is at www.mustcalculate.com.
To measure the voltage of the battery, I used a
PIC16F675 which has a 10-bit A/D (analog-to-digital)
converter. This allows you to get down to /.0049 volts
per bit (five volts/1,024 bits). A voltage divider using 1%
resistors reduces the voltage down 1: 10, so 14.00 volts
cut-off voltage will be 1.400 volts, or 286 bits. The turn-on voltage is set to 13. 5 volts which equates to 1.35
volts or 275 bits. The PIC is powered at five volts with a
7805L using the battery voltage; see the schematic.
A P-MOSFET is used to turn the solar panel on and
off. The one used here is capable of sinking 31 amps.
Keep in mind, though, the heatsink is the key. MOSFETs
have the advantage of a low resistance when they are
run in the saturated mode. A second N-MOSFET is used
as a voltage translator as the five volts from the PIC will
not totally turn off the P-MOSFET.
The P-MOSFET is run either fully on or off. It is
connected to the positive lead of the battery from the
positive lead of the solar panel. The PIC measures the
voltage on the battery by bringing the FET high, thus
disconnecting the solar panel from the battery. It then
determines if the battery needs to be charged by
measuring its voltage.
If it does need charging, it connects the positive line
of the solar panel to the positive terminal of the battery.
If not, it continues to monitor the voltage until the
battery voltage goes below 13. 5 volts. It then connects
the positive side of the solar panel to the positive
terminal of the battery and allows its voltage to charge
To prevent the unit from drawing excessive power,
I measure the voltage across Q2 that controls the
24 March 2014
■ Solar controller schematic.