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  • MCP9501 PCB Temperature Monitor

  • Created: Jun 22, 2016

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The lifespan of an electronic component shortens as it continues to operate in an environment wherein the temperature is high. It is important that in any electronic systems, the temperature must be monitored and the system must be provided with cooling devices in case an overtemperature condition occurs. In electronic devices, heat usually originates from power supplies, power amplifiers, drivers, etc. Though these sources of heat are designed to operate in high temperature, some passive and active electronic components around it are not designed to operate in high temperature for a long period of time. In order to prevent damages to these non heat-tolerant components, some deploy temperature switches on the PCB, near to the heat source.

This reference design features the MCP9501 temperature switch from Microchip Technology. The MCP9501 is a hot-option, open-drain, active low temperature switch that outputs whenever the temperature rises above its factory set threshold. The available temperature thresholds for the MCP9501 are +5°C, +15°C, +25°C, +35°C, +45°C, +55°C, +65°C, +75°C, +85°C, +95°C, +105°C, +115°C, and +125°. Inside of the MCP9501 device are resistors for setting the threshold, thermal diode to monitor the temperature, and a comparator to determine if the voltage level across the thermal diode is greater than the voltage drop across the resistor that sets the threshold. The device measures the PCB temperature by monitoring the voltage drop across the thermal diode located in the die. With the help of the low-impedance thermal path provided by the device pins, the device can effectively monitor the PCB temperature. In order to get a reliable result, it is recommended to place the MCP9501 device as near as possible to the heat source.

In this design, only the +45°C, +55°C, +65°C, +75°C, +85°C, and +95°C thresholds are used to provide reference to the ATmega328P-PU microcontroller. The MCP9501 devices initially output a HIGH signal. When the temperature rises above its factory set threshold, it outputs a LOW signal and only returns to its HIGH state if the temperature falls below the threshold minus the hysteresis set based on the hysteresis select pin. The device has two selectable switch hysteresis, 2°C and 10°C. If the Hyst pin is connected to GND, the hysteresis is set to 2°C, and if the pin is connected to VDD, the hysteresis is 10°C. In this circuit, the Hyst pins are connected to VDD so the hysteresis is set to 10°C. For example, if the PCB temperature is above 45°C, the Output pin of U1 will pull the PD0 pin of ATmega328P-PU to ground. This Output pin will only return to HIGH state if the temperature falls below 35°C. The ATmega328P-PU monitors the output pins of the MCP9501 devices and controls the speed of the intake and exhaust fans. The DC fans provide ventilation to the system. The higher the PCB temperature, the faster these fans will rotate. Since the MCP9501 temperature switches are designed to be placed near to the heat source, the ATmega328P-PU can’t be placed to the same board where the MCP9501 devices are placed because the heat might affect the performance of the microcontroller. In this case, the outputs of the MCP9501 devices can only be connected to the ATmega328P-PU through J1 and J2. The MCP9501 devices and the ATmega328P-PU operate with a 5V power supply, while the optocoupler, transistors, indicators, and the DC fans operate with a 12V power supply.