Thermostat 3 Click Board™

The Click Board™ is equipped with all the necessary elements, required to provide a reliable operation: it has a varistor across the relay output contacts, preventing excessive voltage transients, it has a flyback diode for the backEMF generated within the relay coil, and a durable mechanical relay, that can withstand up to 20,000,000 mechanical cycles (no load connected). These features allow Thermostat 3 Click Board™ to be used for a wide range of applications that have to be thermally controlled: various home appliances, air conditioners, cooling fans, small heaters, etc.

How Does The Thermostat 3 Click Board™ Work?

The Thermostat 3 Click Board™ is designed around the MAX31855, as the main IC - a sophisticated thermocouple-to-digital converter with a built-in 14-bit analog-to-digital converter (ADC), from Maxim Integrated. The device also contains cold-junction compensation sensing and correction, a digital controller, a SPI compatible interface, and associated control logic. The Thermostat 3 Click Board™ is designed to be used with externally connected K-type thermocouple sensor. The communication with the host MCU is performed over the SPI interface, using the dedicated pins of the mikroBUS.

Depending on the temperature information obtained over the SPI interface, the host MCU can take the necessary action: it can either open or close contacts of the relay. The Click Board™ uses the G6D series PCB power relay, from Omron. This quality relay can withstand an amazingly large number of mechanical cycles, with no load connected. However, when there is a significant load connected at its output, micro-electric arcs cause the contacts to wear over time. With the maximum load current of 5A, it can sustain up to 70,000 cycles. Its contacts are made of silver alloy, yielding exceptional ON resistance of only 100mΩ (max).

The relay is activated by the host MCU. The voltage for the coil activation is 5V, while the current through the coil is 40mA. The MCU is not able to drive the coil directly, therefore an NPN transistor had to be added. Its base is controlled by the host MCU, allowing the coil to drain enough current from the 5V mikroBUS power rail. The base of the transistor is routed to the CS pin of the Click Board™. The transistor packs two biasing resistors in the same casing, so it can be directly used on the MCU pin, without external biasing resistors. A red color LED, labelled as ACTIVE is used to indicate that the transistor is in an open state and that the current is running through the relay coil.

When the current through a coil (or any other inductor) is suddenly changed, the backEMF will be generated, opposing the changes of the current. This can sometimes lead to damage to the control circuit: in this case, the transistor will become inversely polarized. To prevent this from happening, a flyback diode is added across the coil. During the normal operation, this diode does not conduct any current. However, when the coil is switched OFF, the inverse polarization will cause the current to pass through this diode with minimum resistance. This prevents inverse (flyback) voltage from building up, so the transistor remains safe.

Contacts at the output may be connected to a higher voltage and larger current may run through. To prevent high voltage transients in this case, a flyback diode is not a viable option. Therefore, Thermostat 3 Click Board™ uses a varistor (VDR). This component rapidly drops its resistance as the voltage rises above its rated clamping voltage. The excessive voltage transient will pass through the VDR since it will become a current path with the least resistance. During the normal operation, while the voltage stays below the rated clamping voltage, VDR has a very high resistance, so the current runs through the electrical circuit, instead.

The operating voltage of the Click Board™ can be selected by the VCC SEL jumper. This jumper allows selecting either 3.3V or 5V from the mikroBUS. The selected voltage will be applied to the VCC pin of the connected MAX31855 sensor.

SPECIFICATIONS

PINOUT DIAGRAM

This table shows how the pinout on Thermostat 3 Click Board™ corresponds to the pinout on the mikroBUS socket (the latter shown in the two middle columns).

Notes	Pin					Pin	Notes
	NC	1	AN	PWM	16	RLY	Relay Control
	NC	2	RST	INT	15	NC
Chip Select	CS	3	CS	RX	14	NC
SPI Clock	SCK	4	SCK	TX	13	NC
SPI Data Out	SDO	5	MISO	SCL	12	NC
	NC	6	MOSI	SDA	11	NC
Power Supply	3.3V	7	3.3V	5V	10	5V	Power Supply
Ground	GND	8	GND	GND	9	GND	Ground

ONBOARD SETTINGS AND INDICATORS