Quick Hack: Converting a computer fan from thermostatic to PWM control

As seen on Hackaday!

About a week ago I needed to replace the CPU fan in my home server as it was running slower than it used to. The Cooler Master Vortex Plus that I chose for my home server uses a standard 92mm fan, and uses the 4-pin connector standard to provide tachometer (speed) readout and PWM speed control.

The Vortex Plus fan’s sleeve bearing was proving to be the weak point of the cooler, and after many, many years of continuous operation, the bearings had lost lubrication and worn themselves down. I had another 92mm fan in my scrap bin, the Nidec TA350DC, but this would prove to be a challenge to adapt it for use in a normal computer system. This fan came from an old Dell Optiplex desktop and used a proprietary 3-pin connector (therefore there was no PWM control), and it was thermostatically-controlled. The fan used a 10kOhm NTC thermistor to measure the airflow temperature, and would increase its speed as the temperature increased (and therefore the thermistor’s resistance decreased). This would prove to be a challenge with implementing that fan as a CPU cooler, as the motherboard uses a PWM (pulse-width modulation) signal to control the fan speed. My proposed solution was to take advantage of the low-current thermostatic control circuitry and effectively override the fan’s own autonomous control system, as opposed to forcing the fan to run at full speed and using high-current MOSFETs to PWM the fan’s power supply, as I felt that doing so could disrupt the fan’s tachometer signal to the motherboard.

PWM Mod Circuit

I used the existing thermostatic control circuit to my advantage, since the thermistor forms the low side of a voltage divider. All I needed to do was use an N-channel MOSFET (specifically, the 2N7002) to short the thermistor pins when the FET’s gate terminal is pulled high, and I swapped the thermistor with a plain 10 kOhm resistor to effectively disable the fan’s autonomous control. I presumed the tachometer signal should be compatible with existing motherboards, and therefore not require any modifications.

As per the PWM fan control specifications, the speed control signal is a 5-volt digital signal, with a frequency of approximately 25 kHz and a variable duty cycle of 30-100%, and is a non-inverting signal. This is especially convenient as this means I don’t need to invert the logic signal before feeding it to the N-channel MOSFET controlling the thermistor input circuit. I did need to protect the gate from ESD (electrostatic discharge) damage, as the gate can only handle 20-30 volts before the gate’s microscopically thin insulation breaks down, rendering it useless. I used a BZX84 5.1-volt Zener diode to act as ESD and overvoltage protection. In the end, my assembled circuit board was actually slightly shorter than the thermistor it replaced!


After all this, I had a fan that would accept a PWM control signal and had at least some control over its fan speed. However, I later realized that the tachometer signal was not working, causing my motherboard to report that the fan had failed. At this point I didn’t really want to come up with another circuit (perhaps a Hall effect sensor) to sense the fan’s speed, so I simply took the easy way out and just disabled the warning in Intel Desktop Utilities 🙂 . I might revisit this mod sometime in the future if I need to do this again.




Atomic Pi Adventures, Episode 1: Adding external PCI Express expansion by removing onboard Ethernet

As seen on Hackaday!

TL;DR: The Atomic Pi single-board computer CAN be expanded through PCIe. It’s just a massive pain to do so, even if you have steady hands. Let’s just say it’s a long story…

DISCLAIMER: The modification performed in this blog post can, and has, caused permanent hardware damage to my Atomic Pi, albeit repairable with much skill and effort. Reenacting what I’ve done requires significant experience with SMT (surface-mount technology) components, some barely larger than a grain of sand (I consider 0402-size components to be “oversize” in this instance). I accept no responsibility for damages arising from attempting this modification.


Single-board computers (SBCs) are all the rage nowadays, with the Raspberry Pi being the most well-known in this category. SBCs are compact computers, carrying their own CPU and memory, and usually some on-board storage and various I/O connections (e.g. USB, HDMI, Ethernet). Most of these computers use the ARM architecture, found on almost all mobile devices today. However, some use the x86 architecture, which is used in higher-end tablets, laptops and desktop computers.

Recently, the Atomic Pi made waves in the electronics hobbyist space, boasting an Intel Atom Z8350 quad-core CPU with 2 GB of RAM, 16GB of eMMC storage, Gigabit Ethernet, Wi-Fi, USB 3.0, built-in speaker amplifiers, and lots of general-purpose I/O (GPIO) pins – all for less than $40 USD!

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eMMC Adventures, Episode 4: Recovering data from physically damaged BGA eMMC Flash storage chips

As seen on Hackaday!

The ball grid array (BGA) chip package has been instrumental in getting modern electronics to fit in smaller and smaller spaces, as it uses tiny balls of solder on the bottom of the package to make electrical connections, instead of copper leads on the edge of the chip package. This allows for hundreds of connections to be made in a small amount of PCB area, but their size also makes them very vulnerable to damage as well.

One common way for BGA chips to become damaged is called “pad cratering“, where the copper pad on the package’s substrate (basically a wafer-thin circuit board) separates and leaves behind a crater.

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Performing safer AC line voltage measurements using isolated amplifiers

DISCLAIMER: AC line (mains) voltage is not something to be taken lightly! Attempting to safely handle line voltages while minimizing the risk of harmful or fatal electric shock is the main motivator for me to design and build this circuit. However, I am no electronics engineer and I definitely have no formal training on international standards pertaining to high-voltage safety. I accept no responsibility, direct or indirect, for any damages that may occur if you attempt to make this circuit yourself, including personal harm or property damage. Additionally, there is no warranty or guarantee, express or implied, on any content pertaining to this blog post (or any other posts).

UPDATE (November 19, 2018): Added isolation voltage ratings for the amplifier and DC-DC converter.

As seen on Hackaday!

Back in mid-2017 I won a Keysight DSOX1102G digital storage oscilloscope (DSO), a piece of equipment long on my wish list but never acquired until then. One thing I’ve wanted to be able to measure with an oscilloscope for a long time was the waveform of the AC utility (in other words, the wall outlet). However, doing so presents a very real risk of blowing equipment up or shocking yourself (and possibly other people). In order to prevent this, I needed a way to perform measurements on the AC line without being directly connected to it; in other words, I need galvanic isolation.
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eMMC Adventures, Episode 3: Building a custom adapter to use cheap eMMC-based 32GB SSD modules

As seen on Hackaday!

While on my quest for more eMMC-based storage devices, I stumbled upon a few devices that piqued my interest: eMMC-based SATA SSDs! I found two models of particular interest: Dell had M.2 modules with a 2.5″ adapter, and HP had custom boards intended for use in cheap laptops (for example, the HP 14-an012nr). Although the former was easier for me to use (but not acquire), I will be focusing on the latter in this blog post.
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Upgrading a passive Power over Ethernet splitter with 802.3af compatibility

As seen on Hackaday!

If you haven’t heard of Power over Ethernet, chances are you’ve experienced its usefulness without even knowing about it. Power over Ethernet (PoE for short) does exactly as the name implies: power is sent over the same Ethernet cable normally used for data transfer. This is often used for devices like IP phones and wireless access points (often you see these APs in restaurants and other establishments mounted to the ceiling to provide Wi-Fi access), as it is far easier, cheaper and safer to provide low-voltage power instead of wiring in AC power which requires the help of a licenced electrician.


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