Nuts and Volts - January 2008

■ 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 ⁴¹³^COM₇ALM- _V-

AD594

_V+11

.1uF

10uF

ALM+ ¹²

2K

VO ⁹FB ⁸

COMP ¹⁰

¹MCLR/Vpp ²³RA0/AN0 ₄_{RA1 5 RA2 6 RA3 7 RA4 RA5}¹¹¹²^{RC0 13 RC}¹⁴^{RC2 15 RC3 16 RC4 17 RC5}₁₈RC6/TX _RC7/RX

VCC ²⁰PGD/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.