Boards

Ready-made MAX31855 module/breakout boards

The only boards I have and have tested with are the "Maxim MAX31855PMB1 Peripheral Module"(brand new, TODO: test it) and the "Unbranded MAX31855K Breakout Board". On all other boards, I have only reviewed schematics and online data.

Maxim MAX31855PMB1 Peripheral Module, Module Datasheet, Module Datasheet - Schematic page

This appears to be positioned as an evaluation board, peripheral module, and reference design. This appears to be the reference design used by other breakout boards. It goes for about $21. This is a good value when you consider it has the Type-K socket, Electrostatic protection circuit, pads for additional noise suppression CAPs, 4 layer board, and series resistors on the SPI Bus signals (these appear to help with noise).

Usually, when I see a Manufactures's evaluation or break-out board, the price is not within reason for a hobbyist's budget. This appears to be positioned as a regular product, not just an evaluation board.

Sparkfun Thermocouple Breakout - MAX31855K, Schematic

Compared to the Maxim board, this board is missing the electrostatic protection circuit, comprised of a duel TVS diode and the mini thermocouple socket is an optional addition you solder on. The pads for two optional shunt capacitors in the filter circuit are also missing. From photo's this board looks like it is two layers. The backside is a ground plane. The top side has ground surrounding traces. It looks like the layout has excellent separation of the analog measurement side from the digital side. They did a good job of keeping the Analog traces away from the Digital.

The Sparkfun has a 10K pull-up on CS. For HSPI pins GPIO15 is the hardware SS (Subordinate Select). The required pull-down resistor for booting on GPIO15 and the 10K pull-up on the breakout board will leave GPIO15 at an undefined logic level at boot time. This will most likely result in a boot failure or unreliable booting. You will need to use another GPIO pin for ChipSelect. Or you could easily remove the resistor with a hot soldering iron.

Digilent Pmod TC1 (Revision A), Pmod TC1 Reference Manual, Schematic of the Pmod TC1

TBA.

The "T+/T-" terminals only have a single capacitor for filtering the thermocouple connection.

It has a π filter on the 3.3V supply line. I think this is a good addition when you consider that the MAX31855 is really an analog and digital device. Isolating the digital supply noise from the rest of your system is a good idea when you are trying to measure microvolts.

Adafruit Thermocouple Amplifier MAX31855 breakout board, Schematic

The Adafruit breakout board has an onboard LDO regulator and level conversion components for both the SCLK and CS SPI bus signals. Compared to the Maxim board, this board is missing the electrostatic protection circuit, comprised of a duel TVS diode. The pads for two optional shunt capacitors in the filter circuit are also missing. It has screw-down terminals in place of a mini Type-K socket.

Regarding the resistor/diode network for performing level conversion for both SCLK and CS. I had concerns about the use of a 1N4148 instead of a Schottky Diode, this Electronics StackExchange Q/A supports my concerns. This also, will not work with the HSPI option on the ESP8266. The pull-down resistor on GPIO15 and the pull-up in the level converter create an undefined logic level. You will need to use another GPIO pin for ChipSelect.

Unbranded breakout board

There are breakout boards that appear to be clones based on Adafruit's open-source design. The boards I have seen, do not adhere to their parts list values.

My unbranded MAX31855K breakout board uses the Torex XC6204, Mark 4B2X for the LDO regulator. This part appears on a lot of and breakout boards and ESP8266 development boards (tangent - bad choice for ESP8266). This unbranded board uses the Torex Semiconductor's XC6204 with ~0.89μF output capacitor. Per most datasheets of similar CMOS LDOs, this value must be >= 1.0μF. The input capacitor is also missing. The datasheet calls for a minimum of 0.1μF; however, most of the test data presented was done with 1.0μF. Also, most datasheets recommend when using a ceramic to use an X7R or X5R dielectric capacitor. These are more temperature-stable.

Unbranded MAX31855K Breakout Board Change Summary

1. The two ~0.5μF ceramic CAPs are almost okay. The net value falls just under 1μF. This may be due to part tolerances. To guarantee stability, add an additional CAP to make the net value, a temperature stable value over 1.0μF.
2. The breakout board is missing a CAP for CIN. Add a CIN to the backside of the board where the header is connected.
3. The MAX31855 is missing a 0.1μF bypass CAP. This value would have a different resonant frequency than the ~0.5μF CAP. I don't know if it matters. No harm in paring one on to the existing CAP near the chip.
4. Added 150 Ohm resistors to the off-board SPI Bus connector: SS, SCK, and SDO.