Table of Contents

About

iMacs contain drives that are monitored for temperature by the underlying SMC. The temperature is monitored via a sensor that is coupled with the iMac hard-drive and will make the iMac drive the fans in order to achieve an optimal amount of cooling. In the event that the hard-drive is replaced by an SSD, then the temperature sensor will float and may have the side-effect of the iMac throttling the fan up to the maximum level. There are various third-party solutions that sell temperature sensors but SSD drives do not need cooling such that the temperature sensor is not needed. This guide will measure the temperature sensor and will then use just a single resistor to make the iMac believe that the temperature is nominal.

Searching for Compatible Sensors

A solution is offered by cmizapper based on what seems to be an SMD SOT-23 LM50 temperature sensor.

The sensor is meant to be placed on top of the drive and report the temperature to the motherboard and the SMC.

Looking up the LM50, it seems that the sensor input current varies with the temperature ranging from $100mV$ at $-40^{\circ}C$ to $1.75V$ at $125^{\circ}C$ such that the task reduces to placing an in-line resistor to bring the current down to about $425mV$ in order to obtain a perceived temperature of $0^{\circ}C$.

Probing the Sensor Nominal Voltage

Opening the iMac reveals an extra cable parallel to the SATA connector that connects between pins 2 and 7 on the harddrive extra port.

Since only two pins are connected to the hard-drive, the two pins can be measured to obtain the input current.

Using two pins connected to alligator clips, the voltage across the two pins with the hard-drive disconnected is measured to be about $2.5V$.

Voltage Divider

Now that we know that the input voltage is $2.5V$ and that the voltage should be dropped to about $425mV$, a voltage divider can be used comprised of two resistors.

For common resistor values $20k\Ohm$ and $5k\Ohm$ with an input voltage $2.5V$, the resulting current will be dropped to $0.5V$ or $500mV$ which is sufficient to make the iMac believe that the temperature is about $0^{\circ}C$.

Finishing Touches

The solution is probably the best choice when it comes to replacing an older iMac hard-drive with an SSD since the SSDs remain cool regardless of their usage. Furthermore, the hard-drive temperature sensor is not the only sensor to be found in an iMac such that the other sensors will manage the temperature and allow the mac to adjust the fan speed appropriately.