MAX6675/MAX31855 Digital Thermocouple SPI Adapter with UEXT Interface

Contents

1 Overview
2 Features
3 Pin Configuration
4 Technical Details
5 Resources
6 Dimensions

Overview

Compared to semiconductor temperature sensors, thermocouples have a wide working range, and do not need any external excitation power. They are cost effective and interchangeable. In some high temperature application, they may be the only practical choice with a reasonable price. Signal conditioning is the most tricky part for thermocouple temperature sensors, it deals with micro volts amplification and the cold junction compensation. Thanks to the MAX6675/MAX31855, all the signal conditioning had already been done inside the chip, and the amplified analog voltage is digitized and accessible to external MCU via SPI interface.

Features

3.3V/5V Powered.
UEXT Interface
SPI Interface

Pin Configuration

5-pin 2.54mm Connector:

MISO: Sensor SPI output
SS: Sensor SPI slave select input
SCK: Sensor SPI clock input
5V0: 3.3V/5V power supply (VIN)
GND: Ground

UEXT Connecor:

Per UEXT definition and pin 1, 2, 9,10 are marked on the PCB, 3.3V only.

Technical Details

To make the sensor work out of box with 5V power supply, there is a LDO regulator serves as a 5V(3.3V) to 3.3V converter, which is as shown in the picture below.

TC_LDO

The 5V to 3.3V converting is necessary because unlike the MAX6675, the new MAX31855 could only be powered by 3.0V-3.6V. R1 should be removed to prevent flowing of reverse current, if UEXT interfaced is used.

Below is the schematics for the UEXT and 5-pin 2.54mm connector

TC_CONNECTORS

J1, J2 and J3 are used to enable single row layout of the UEXT interface.

Two voltage dividers are used to reduce the 5V level signal to safe 3.3V voltage, as shown in the picture below.

TC_LEVEL_SHIFTING

If the module is powered by 3.3V via 5-pin connector, R4 and R5 should be removed to make sure the MAX6675/MAX31855 gets the correct level of voltage. A 74HCT1G125GV buffer, which is powered by VIN to convert MISO to proper level.

MAX6675/MAX31855 and its peripheral components are shown in the picture below. TC_MAIN