2 is 40. Vbe, from Figure 3, is 1.1
volts, leaving 0.4 amps for base
current. 40 X 0.4 = 16 amps, so 10
amps at least is possible.
I put a 47 ohm, one watt resistor
across Q2 to provide a trickle charge
to the battery when the charger is off.
When the battery is fully charged
(about 14 volts), the peak voltage
across R10 will be four volts, giving a
peak trickle current of 85 mA.
The size of the fuse in the
transformer primary can be determined as follows: The transformer
ratio is 10:1 so, the primary current is
one amp when the load is 10 amps.
Use a two amp fuse so it only blows
under conditions of transformer or
QI am a novice when it comes
to solar cells. I recently
bought six cells and started
playing with them. I noticed
that even though they are all from the
same company and the same make,
they produce different outputs. Is this
normal? Also, when I paralleled two
cells, if I did not put a diode in series
with each, I would get a lower value
than the average cell voltage. I was not
expecting these results.
Could you please shed some light
on this? Do you always need to use a
diode in parallel with each cell when
trying to build up the amperage? I guess
it goes without saying that you only
need one diode in series with two cells
in series for greater voltages.
Background: If you test the cells
by themselves (open circuit), four of
these solar cells are producing 15 to 17
VDC, with two only producing eight
volts. The short circuit current is 100
to 140 mA.
Also, could you explain how
to choose a solar cell for a project? I
understand if I am running four 12
volt fans at 100 mA, I would add up
the total amps: 400 mA for the project.
But from what I am seeing on my
meter, my voltage drop is a lot lower
than what I would expect for a parallel
circuit. Do I need to add a capacitor
AMost solar cells are made
from silicon and have an
output voltage of 0.5 volts.
What you have is a series array of about 30 cells, giving an output
of nominal 15 volts. There are several
ways to make a solar cell, but they all
act like diodes. You can connect them
in parallel and the output will be that
of the highest cell. I don’t know what
is causing the effects that you see.
Perhaps you bought some production
rejects. You should not need a diode
between units; the units themselves are
a diode. The power output of the solar
array must meet or exceed the power
required by the load. If you need 400
mA at 12 volts ( 4. 8 watts), the solar
array must exceed that because the sun
is not at maximum most of the time.
You can get good information from this
(The July issue of Nuts & Volts will
have extra “green power” coverage, so
PCB LAYOUT PROGRAM
QI would like to download a
PCB artwork program. Do
you know of any?
AI assume you mean a free PCB
layout program. A Google
search turned up these possibilities: www.ExpressPCB.
com, www.PCB123.com, www.free PCB.com, and www.cadsoft.de. The
first two are produced by companies
that hope you will use their facility to
make the PC boards. My reading of
reviews indicates that these are full featured. FreePCB is open source freeware
and the one review I saw indicated that
it was good. I included Cadsoft because
I am using their Eagle program. The free
version is limited in board size and parts
count but should be adequate for your
use. There is a good manual and online
help is excellent. NV