male header), so that a
ribbon cable can be used
to connect those three
lines to a breadboard
circuit.
USB Connector
The USB connector in
the UPS schematic is a
standard USB Type A
receptacle. Its two data
pins (pins 2 and 3) are not
used in the circuit; I just
snipped them off before
soldering the connector in
place. In order to power
the Pi from the UPS, you
will need a standard USB cable with
a Type A male connector on one end
and a Micro B plug on the other end
to connect to the Pi.
Heatsink
Although it isn’t shown in the
schematic, the LM2940 requires a
heatsink in order to be able to
provide sufficient power to the Pi.
We’ll discuss heatsinks in more detail
in the next section.
Constructing the
Stripboard Circuit
As I mentioned earlier, I
originally built two different versions
of the UPS stripboard circuit — one
that included all four sensing
resistors, and a second one that only
included R1 and R2 so that I could
experiment with changing the values
of R3 and R4 on my breadboard
circuit. In the remainder of this
article, we're going to discuss the
results of those experiments, so
Figure 3 presents the second layout
(only R1 and R2 are included).
However, I realize that not everyone
will want to run the same
experiments, so full page versions of
both layouts are available for
downloading at the article link.
In the layout of Figure 3, the
black line (running up and down
from the left side to the right side of
the stripboard) represents the shape
of the heatsink that I used for this
project. It may seem unusually large,
so I probably should explain why I
chose that specific heatsink.
Before I constructed either of the
two layouts I have already
mentioned, I built a simpler
stripboard circuit that only included
the 12V supply, the voltage regulator,
and the necessary support circuitry. I
did that because I wanted to make
sure the LM2940 could supply
sufficient current to power the Pi
without over-heating. In that circuit, I
just used one of several smaller
heatsinks that I had on hand.
As it turned out, the LM2940
was able to do the job, but it ran so
hot that I wasn’t able to touch it for
more than a fraction of a second. I
was concerned that the excessive
heat could eventually destroy the
power supply, so I began a search for
a more massive heatsink. My main
requirement was that all the
components should fit within the
width of a standard stripboard, and
the heatsink I finally chose (Digi-Key
#HS410-ND) works well for that
purpose.
I also decided to use a thermal
pad (Digi-Key #BER168-ND) because
it can significantly improve a
heatsink’s ability to dissipate heat. I
have been continuously powering my
Pi from the UPS for more than two
weeks now, and the heatsink is only
warm to the touch.
Figure 4 is the Parts
List for the UPS project.
Except for the batteries,
battery holders, heatsink,
and thermal pad, all the
required parts are available
on my website (www.JR
Hackett.net). In order to
conserve space in this
article, the detailed
assembly instructions for
both versions of the UPS
are available at the article
link. However, there’s one
aspect of the construction
process that I do want to
mention.
The current-carrying capacity of
a PCB (printed circuit board) trace
depends on the width and thickness
of the trace. On a stripboard, the
traces are about 0.08” wide, but they
also contain numerous holes which
are about 0.04” wide, leaving about
0.04” as the effective width of each
trace. I did a fair amount of online
searching, but I haven’t been able to
determine the thickness of the
stripboard trace. (It probably varies
from one manufacturer to another.)
I also did some searching to
gather opinions as to whether it’s
safe to use a stripboard to source a
full amp of current, but all I found
was a considerable amount of
disagreement!
Rather than risk the possibility of
constructing an unreliable (or even
self-destructing) power supply, I
decided to “beef up” the power and
ground traces that supply power from
the main 12V source and from the
battery pack. If you refer back to the
bottom view of the stripboard layout
in Figure 3 and focus on the traces in
columns J through M, you can see
that I added bare jumper wires to
those traces wherever the power and
ground lines could be carrying a full
amp of power.
Of course, you may decide this
precaution is not necessary.
However, if you do decide to take
the same approach, the provided
assembly instructions include the
June 2014 11
ITEM DESCRIPTION
Batteries, 7 Alkaline D-Cells None
Battery Holders ( 2 pieces)** None
Capacitor, 10 µF, Electrolytic C1
Capacitor, 100 µF, Electrolytic C2
Connector, Power 12 Volt Power In
Connector, USB USB-A Receptacle
Diode, 1N5818 D1 & D2
Header, Male, 2 pins Battery
Heatsink, Digi-Key #HS410-ND None
Resistor, 68K, 1/4 Watt R1
Resistor, 22K, 1/4 Watt R2
Resistor, ??K, 1/4 Watt R3 (Varies — see text)
Resistor, ??K, 1/4 Watt R4 (Varies — see text)
Thermal Pad, Digi-Key #BER168-ND Optional
Stripboard, 19 traces with 17 holes None
Voltage Regulator LM2940 (5V)
**RadioShack #270-396 or similar ■ FIGURE 4. Parts List.
