■ PHOTO 5. The trick is
to keep the AD594’s
source voltage clean.
It’s also a good idea not
to waste any space
between the thermocouple wires and the
AD594 thermocouple
input pins. Notice
the different AD594
packages in this shot.
voltages to the UDK2559 pin 2 K input and the other bulk
device source voltage to the pin 7 K input. UDK2559 outputs
OUT3 and OUT4 follow the pin 2 K bulk device source
voltage and outputs OUT1 and OUT2 march to the bulk
device source voltage applied at the pin 7 K input.
The UDK2559’s output behaves like an NPN transistor
and each bipolar output module is fronted by an AND logic
gate. The UDK2559 ENABLE line is common to all of the
■ PHOTO 4. The idea here was to lay down just enough copper
to allow me to make point-to-point connections between
the UDK2559 and the five-volt regulator circuit. Everything
else is wired in as necessary. The ring of copper surrounding
the UDK2559 is tied to the UDK2559’s ground pins and is
intended to act as a heatsink for the UDK2559.
UDK2559’s AND gates as it is tied to one of the two inputs
on all four of the UDK2559’s AND logic gates. Each of the
UDK2559’s four INx inputs are independently tied to the
remaining input of the four AND logic gates. A logical low
output from the respective bipolar output’s AND logic gate
is needed to turn on the associated NPN transistor output
logic. That means we have to supply a logical high to the
UDK2559’s ENABLE pin in order to enable any sinking
action through any of the UDK2559 output pins.
To obtain a sink condition on a particular UDK2559
output after enabling the outputs with a logical high level on
the ENABLE pin, we must place a logically high level on the
input representing the output we want to take to the sink
state. Following standard AND logic, a logical high on the
ENABLE pin coupled with a logical low on an INx pin results
in a logical low on the AND gate output, which turns off the
associated NPN-based output. Both the ENABLE pin and the
selected INx pin must be logically high to sink current at the
selected UDK2559 output pin. It’s that simple. All we really
have to worry about is keeping the UDK2559 cool. That is
easily done by providing some ground plane area for the
UDK2559 ground pins, which also act as thermal dissipation
tabs. If we manage to get the UDK2559 too hot, its internal
thermal limiting circuitry will shut the device down.
+5VDC
THERMOCOUPLE WHITE (+)
INPUT -> RED(-)
1 +IN
14 -IN
3 +T
5 -T
2 +C
6 -C
4 13 COM 7 ALM- V-
AD594
V+ 11
.1uF
+
10uF
ALM+ 12
2K
VO 9
FB 8
COMP 10
1 MCLR/Vpp
23 RA0/AN0 4 RA1 5 RA2 6 RA3 7 RA4 RA5
11 12 RC0 13 RC1 4 RC2 15 RC3 16 RC4 17 RC5 18 RC6/TX RC7/RX
VCC 20
PGD/RB7 28 PGC/RB6 27 RB5 26 RB4 25 RB1 22 RB0 21
OSC1 9
■ SCHEMATIC 4. Don’t let the
word “thermocouple” scare you
off. Getting good temperature
data from a thermocouple is as
easy as getting temperature
data from an LM35C.
23 CAN_TX
24 CAN_RX
OSC2 10
GND 8 GND 19
PIC18F2685
86
January 2008