Issue
162 January 2004
Remote
Observation Station
PVCC
CONTROL BOARD
A
prototype of the PVCC board is shown in Photo 3. Figure
3 depicts the functional block diagram.
|
(Click
here to enlarge)
|
Photo
3—The power MOSFET Q1 has a 20-mW RDS, which would
dissipate about 2 W at a full 10-A charging current.
Two watts is about all the power you would want
to dissipate from a surface-mount part. |
|
(Click
here to enlarge)
|
Figure
3—The MC68HC908QY4 16-pin SOIC has just the right
complement of I/O for this project. |
The
board, which has a 4500 mils × 3000 mils form factor,
was made on FR4 1-oz. double-sided copper PCB. The MC68HC908QY4CDW
(U3) is a member of the Motorola Nitron family. This
MCU features a 16-pin SOIC, 4 KB of flash memory, 128
bytes of RAM, and 14 I/O pins. It can be powered at
3 or 5 V. I chose the 5-V supply to allow the MC68HC908QY4CDW
to reach its maximum clock speed of 32 MHz. Table 1
breaks down the microcontroller’s resources and their
uses.
| Table
1—Every I/O signal is used here. Port PTA0/AD0
pulls double duty, serving as the serial link in
Configuration mode and the light sensor input in
Charge mode. |
A
serial communications connection to the PVCC is provided
via J1 and U5 (MAX232). Both transmit and receive lines
on the logic side of U5 connect to PTA0 in a half-duplex
arrangement, enabling communication with a PC. You can
configure the PVCC using the ConfigPVCC Win32 configuration
program. The single-pin arrangement is accomplished
via diode (D8) and pull-up resistor (R19).
Programming
access to the microcontroller’s flash memory is provided
via JP6, which is wired in accordance with the Motorola
MON08 specification. JP6 is accompanied by DIP switch
S3 (MON08 isolate), which is used to disconnect four
specific microcontroller pins that are needed by the
MON08 interface.
Two
DC-DC converter-type power regulators are built on the
PVCC board. Both regulators are based on Linear Technology’s
LT1766IGN chips. These devices are monolithic buck 200-kHz
switching regulators that accept a wide input voltage
range (5.5 to 60 V). U2 is the 5-V version of the LT1766IGN;
therefore, it provides VCC (5 V) for all of the ICs
on the board. U1 provides auxiliary power for the video
camera, the 433-MHz ATV transmitter, and any other off-board
devices. The voltage regulation point is set via resistors
R7 and R8. Note that resistors R7 and R8 are fixed for
an output of 10 V.
The
LM1881 video sync separator provides timing signals
to the HC08 microcontroller that then allow the microcontroller
to generate a stable video overlay signal synchronized
to the original video signal. This overlay signal is
added back into the original video signal to create
the overlay effect. Two signals are generated by the
LM1881: a vertical sync signal that is connected to
port PTB0, and a composite horizontal sync signal that
is connected via the microcontroller’s IRQ input. I
studied a variety of hardware and software schemes before
settling on this arrangement, which seemed to allow
the MCU response to the video signal to be the most
repeatable.
Two
outputs from the microcontroller are used in the charge
control circuit. Output PTA4 is used to control charging
via a load switch consisting of a low-power N-channel
MOSFET and a high-power P-channel MOSFET (see Figure
4). As you can see in Figure 2, output PTB2 is used
to turn on and off the loads (camera and transmitter).
This is important because the ability to turn on and
off the loads enables a low-voltage disconnect (LVD)
feature, which prevents the loads from over-discharging
the battery during long periods without sunshine.
|
(Click
here to enlarge)
|
Figure
4—On the station’s control board (the photovoltaic
charge controller, or PVCC), the MOSFET load switch
arrangements control charging and 10-V auxiliary
power. |
