Issue
144 July 2002
An
80C31-Controlled Power Supply
by
Noel Rios
Construction
and Assembly
You
can construct the digital part of the power supply by
using point-to-point wiring. I built a PCB for the analog
component, however, the one I made was not that great
because its only purpose was to verify if the circuit
works (see Photo 2). For the digital part, I used ordinary
IC sockets and wrapping wire to connect the circuit.
You should also use IC sockets so you can remove the
IC if it’s defective.
|
(Click
here to enlarge)
|
Photo
2—Here’s a view from above the 80C31-controlled
power supply. The digital part is constructed using
point-to-point wiring. I used a ready-made PCB for
the analog part. |
Before
inserting the ICs, be sure to look for the presence
of 5 V at the VCC pins. Also, check if the ground pins
are indeed grounded. It’s a good idea to use different
colored wires so you can trace the signals of different
pins.
If
you’re going to make a PCB for the analog part, try
to be sure the sampling network is close to the output
connector or pads. Make the traces wide for nets carrying
power from pulsating AC to DC. To calibrate the power
supply, burn the calibrate program into to the 27C64
EPROM and then power up the circuit. Adjust variable
resistors R2 and R3 until you read 2.55 V on pins 1
and 7 of U9 LM358. Also, adjust variable resistor R27
until you get a reading of 2.55 V going to pin 12 of
U4 ADC0820. Key in 5.0 for voltage and adjust variable
resistor R11 of the analog portion until you get a 5-V
reading.
Troubleshooting
The
only way you’ll know if the circuit is wired correctly
is if you can see messages coming out of the LCD module.
The program will work even if the ADC or DACs are absent
from the circuit. However, this is not the case with
the LCD module. The program will hang if the LCD module
is not present with dps.bin or dps.hex loaded in the
27C64 EPROM. This is because the BUSY pin of the LCD
module is polled to check if the LCD is busy or not.
If
the circuit doesn’t work, then check for the presence
of 5 V at the VCC pins. You should also look to see
if the ground pins are connected to ground, and if the
reset circuit works. Use a logic probe or an oscilloscope
to verify this. Additionally, always make sure the crystal
and pins X1 or X2 of the microcontroller are in good
shape. If there’s no output on the LCD, then reassess
your wiring.
You
should also watch for the activity of enable E, RS,
and R/W pins. Adjust variable resistor R1 for good contrast
so you can see the characters. If Enable E is not pulsating,
inspect the NAND gate (U10). Check if the *RD and *WR
signals are pulsating at the pins of the NAND gate.
Another good idea is to verify whether or not the LCD
is being selected. To do this, probe pin 12 of the U8
address decoder U8.
Key
in 5.0 for voltage. If the reading is zero, look to
see if the VRDG from the analog portion is connected
to the digital portion, which can be confirmed by inspecting
pin 12 of the U7 CD4053. You should read 0.5 on a multimeter.
You
should always make sure that you wired the U4 ADC0820
correctly. Look for the *RD signal, *WR signal, and
chip select pulse at the pins of U4. Key in 0.05 for
current and connect a 4.7-W, 5-W resistor across the
output terminals. The reading for current should be
0.05 A on the LCD. If it isn’t, check for a 0.05-V reading
on a multimeter on pin 13 of the U7 CD4053. If that
doesn’t work, check if the IRDG signal from the analog
part is connected to the digital part.
The
U7 MC4741 of the analog part should be wired correctly
and the IC itself should be in good shape. If both voltage
and current readings are the same, see if the SELECT
signal at pin 11 of U7 CD4053 of the digital portion
is pulsing. If during the calibration phase there is
not 2.55 V at the U9 LM358, even if you adjust variable
resistors R2 and R3, then inspect your DAC wiring.
Scan
for the presence of *WR pulse at the pins of the U5
and U6 AD7524. In addition, you should check to see
if the chip select signals are pulsating at pin 12 of
U5 and U6 of the AD7524. If they aren’t, take a peek
at the 74HC138 address decoder U8.
And
don’t forget to verify the condition of the LM358 U9.
If you don’t find 2.55 V at pin 12 of ADC0820 U4, then
make sure the LM336-2.5 D4 is wired correctly. Finally,
if you don’t have –2.5 V at pin 15 of U5 and U6 of the
AD7524, check the wiring of LM336-2.5 D14. If the programmed
voltage and current are not correct, even if the voltage
DAC and current DAC have correct outputs, take a look
at the LM358 U5 of the analog part.
