Silent and on Demand Extruder Cooling Fan Circuit for Anet A8

In this instructable, I show you how to build a simple circuit to improve the extruder fan behavior of an ANET A8 3d printer. It's a kind of thermistor based controller to drive the fan.

I'm am unhappy with that little noise piece of plastic/metal, called a fan, attached to my Anet A8 extruder! You turn the printer on, and it's already at its loudest noise as if everything is burning up and you almost need a fire extinguisher. How loud it is! Although many of us remove/disable it when printing at low temperatures, there is a purpose on that fan. You may need it when printing at high temperatures, using all-metal heatbreakers or increasing retraction distance to reduce stringing on some filament types.

The principal component of this project is a thermistor. A thermistor is a type of resistor whose resistance is dependent on temperature. Its resistance increases (NTC) or decreases (PTC) when the temperature rises. NTC is commonly used to measure temperature (there is one such thermistor on the aluminum block near the nozzle). PTC thermistors, instead, subtle increases the resistance at certain temperature level and are commonly used as overcurrent protection. For example, you should find a PTC thermistor attached to rechargeable lithium batteries. If the battery heats up, the temperature increase causes the PTC resistance to increase as well, and so the current is limited to prevent batteries from exploding.

The advantage of a PTC thermistor for this project is that we don't need to interpret the temperature, given we choose the PTC adequately. However, we need to invert its working logic, that is, when the temperature rises, we desire to power on the fan. Another question is the trigger temperature of the thermistor. I found a 45ºC PTC thermistor around my desk and believe it is a good temperature tradeoff. You can try it with a PTC designed for other temperature as well. It will not change the circuit given below.

The complete list of components:

• 1 PTC thermistor Normally Closed (NC) (I used a JRMA45 found on a battery protection circuit. See the description here. You can buy a similar one at ebay, at amazon, or search for TLRS-9700 45C NC.)
• 1 BC549 NPN transistor (a standard NPN transistor should work. Just check the correct pinout.)
• 1 1/4W 10kΩ resistor
• 1 1/4W 330Ω resistor
• 2 1/2W 270Ω resistors (or an ~100Ω resistor 1W). These resistors are used to limit the current to the fan. A large resistance implies less current (less velocity and noise), but of course, reduces the heat dissipation.

The working principle of the circuit is described below (I design electronic circuits as a hobby, so don't expect a precise explanation. Kindly tell me any error you find).

The JRMA45 PTC thermistor has a zero resistance while the temperature is below 45ºC. So, current can flow through it. In this case, the voltage divider formed by R1 e R2 is "enabled" and outputs only 0.38V to the base of BC549. This voltage is not sufficient to activate the transistor and current can't flow between the transistor collector and emitter. Thus, the fan is off.

To the contrary, when the temperature rises above 45ºC, the resistance of the thermistor subtle becomes very high, disabling current flow through it and also through R2. The voltage divider thus is "disabled," that is, the current flow through R1 directly to the base of BC549. This time, the voltage is sufficient to enable the transistor and current flows between collector and emitter enabling the fan.

The two 270Ω resistors limit the current flowing through the fan. As the fan is rated .12A, a resistor of 100Ω should be used to let the fan run in its designed maximum speed at 12V. I've limited it a bit further to reduce noise, using two 270Ω resistors in parallel (135Ω). The resulting fan velocity proved to be sufficient as sometimes the fan is disabled while the printer is under operation.

You can use the schematic and board files attached (designed in Eagle) to make a PCB for the circuit. As there are plenty of instructables on how to build PCBs, I will not formally describe this step and let you choose your favorite method.

Of course, you can solder the components together without a board (as there are only five) and isolate their legs with heat shrink wire wrap. A good idea may be to hide the set of components in the winding pipe.

To install the PCB to the printer, cut the original fan cable near the extruder (10 cm or more distant from the fan) and attach it to the board. You can use female pin heads to make it look more clean and professional. The leg coming from the printer board should be plugged into the INPUT port, observing the polarity: red wire on INPUT pin 1 and black wire on INPUT pin 2. The other leg (to the fan) should be attached to FAN port. Again, red wire on FAN pin 1 and black wire on FAN pin 2.

The thermistor should be soldered to THERM_PTC port. The pinout doesn't matter. Let sufficient wire to position the thermistor near the aluminum heatsink where the heatbreak is attached to (throat pipe). I've placed mine as you can see in the picture. Do not put it in the front of the fan airflow, as it will reduce the temperature of the thermistor and not the temperature of the heatbreak.

You can see the fan in operation in the attached video. The fan is off while the hot end is warming up to the target temperature (190º) and suddenly powers on when the thermistor temperature is reached.

During printer operation, I've observed that the fan becomes on and off in small intervals. When the print job finishes, the fan continues to operate for three or four intervals, and them powers off completely.

And that is it! Of course, there is room to improve the project. A relevant complement is to design a support to fix the small PCB near the extruder. I appreciate your suggestions and comments, thank you!