Issue
149 December 2002
Quad
Bench Power Supply
Start
•
The Analog Core • The
Zetex ZXCT1009
• An Ideal Isolator •
MCU and User Interface • Firmware
•
Sources and PDF
MCU
AND USER INTERFACE
As
with every other project I’ve worked on in the last
two years, I chose the Atmel AVR family for the MCU.
In this case, I went with the AT90S8535 for a couple
of reasons. I needed 23 I/O lines to handle the three
SPI channels, LCD, rotary encoders, and RS-232. This
ruled out the use of smaller AVR devices. I could’ve
used the slightly less expensive AT90LS8515, but I wanted
to allow for the possibility of adding a temperature-sensing
meter/alarm option to the circuit. The ’8535 has a 10-bit
ADC function that’s suitable for this purpose; the ’8515
does not.
The
’8535 MCU has 8 KB of ISP flash memory, which is just
about right for the necessary firmware. It also contains
512 bytes of EEPROM. I used a small amount of the EEPROM
to store default values for the three programmable power
supplies. That is to say, the power supply will power
up with the same settings that existed at the time its
Save Configuration push button was last pressed.
To
simplify construction, I decided to use a SIMM100 SimmStick
module made by Lawicel. The SIMM100 is a 3.5" ×
2.0" PCB containing the ’8535, power supply regulator,
reset function, RS-232 interface, ADC, ISP programming
headers, and a 30-pin SimmStick-style bus. I’ve used
this module for prototypes several times in the past,
but this is the first time I’ve actually incorporated
one into a finished project. Photo 2 is the manufacturer’s
picture of an assembled module. For this project, I
populated a bare SIMM100 PCB with only the components
that I actually needed.
The
MCU port signals needed to operate the three SPI channels
and interface the two rotary encoders come out through
the 30-pin bus. As you now know, I designed the ground-referenced
power supply PCB to include space to mount the SIMM100
module, as well as the IsoLoop isolators. The SIMM100
mounts at right angles to this PCB; it’s hard-wired
in place using 90° header pins. The floating power supplies
share a virtually identical PCB layout apart from being
smaller because of the lack of traces and circuitry
associated with the SIMM100 bus and IsoLoop isolators.
The
SIMM100 module has headers for the ISP programming cable
and RS-232 port. I used its ADC header to run the LCD
by reassigning six of the ADC port pins to general I/O
pins.
When
I buy in bulk, it’s inevitable that by the time I use
the last item in my stock, something better has taken
its place. After contacting Lawicel to request a .jpg
image of the SIMM100 for this article, I was introduced
to the new line of AVR modules that the company is developing.
Rather
than a SimmStick-based module, the new modules are 24-
and 40-pin DIP modules that are meant to replace Basic
Stamps. Instead of using PIC chips/serial EEPROM and
a Basic Interpreter, they implement the most powerful
members of Atmel’s AVR family—the Mega chips.
Mega
chips execute compiled code from fast internal flash
memory and contain much more RAM and EEPROM than Stamps.
Even though flash programming AVR-family chips is easy
through SPI, using inexpensive printer port programming
cables, these modules go one step further by incorporating
RS-232 flash memory programming. This makes field updates
a snap. Take a look at the new stAVeR40 module in Photo
3. I might have used this module instead of the SIMM100
had it existed when I started the project.
|
(Click
here to enlarge)
|
Photo
3—Lawicel’s new stAVeR40 module is a decent product.
I might have used it in place of the SimmStick had
it been available when I was designing my project. |
The
user interface I settled on consisted of a common 4
× 20 LCD panel along with two rotary encoders. One encoder
is used to scroll through the various power supply parameters,
and the other adjusts the selected parameter. The cost
of LCDs and rotary encoders is reasonable these days.
Being able to eliminate the substantial cost of six
DPMs and six 10-turn potentiometers was the main reason
for choosing an MCU-based design in the first place.
Photo 4 shows the front panel of the unit.
|
(Click
here to enlarge)
|
Photo
4—To the right of the output Johnson posts are the
switches that set the polarity of the floating supplies—as
well as the switch that disconnects all power supply
outputs—while leaving the unit still powered up. |
Inexpensive
rotary encoders come in two basic flavors: quadrature
encoded and 2-bit binary (Gray) coded. I’ve used the
quadrature-encoded style in the past, but the ones I
used for this project have a 2-bit output (with Gray
coding). With only 2 bits, the encoder can represent
only four different values, even though it has 32 detents
per rotation. With this in mind, it’s necessary for
the firmware to constantly poll both encoders and keep
track of the carry or borrow conditions that occur as
the encoder moves beyond a four-position range. The
main control loop in the firmware is executed every
few milliseconds, so keeping an accurate track of the
rotary encoder’s position is accomplished readily.
The
RS-232 port came as part of the SIMM100 module. Thinking
about the future, I envision adding some firmware code
to allow the bench supply to be remotely controlled
by a host PC, and to allow for the data logging of the
various voltages/currents over time.
I
haven’t provided you with a complete block diagram,
but I did incorporate a few features that don’t show
up on the individual schematics. Previously, I mentioned
adding an additional commercial 5-V, 3-A supply for
logic circuits. I also added a 3PST switch, with one
section in series with each supply’s positive output,
to allow all power supplies to be disconnected from
the load during power-up. A small DC computer-type fan
was mounted on the top of the outer case for cooling
purposes, because the pass-transistor heatsinks that
I used were not too large.
Lastly,
Figure 3 shows you how the ’8535 MCU would typically
be connected to the rest of the circuit. It doesn’t
show the exact wiring of the SIMM100 including the bus
connections, because this detail isn’t needed when constructing
the circuit from scratch (i.e., if you’re not using
the SIMM100 module). The SIMM100 documentation will
give you all of the necessary information regarding
the header and bus connections on the module.
|
(Click
here to enlarge)
|
Figure
3—Take a look at the MCU, IsoLoop isolators, and
the user interface. Some of this circuitry is actually
contained on the SIMM100 module. |
