Inverter With Silenced Fan

This is a DC to AC inverter upgrade project.

I like to use solar energy in my household for lighting, feeding USB chargers and more. I regurarly drive 230V tools with solar energy through an inverter, also using tools around my car powering them from the battery of the car. All these scenarios require a 12V-230V inverter.

However one drawback of using inverters is the constant noise made by the integrated cooling fan.

My inverter is rather small with 300W maximum output power. I run moderate loads from it (e.g. my soldering iron, rotary tool, spot lights etc), and the inverter usually does not need a constantly forced air flow through its casing.

So let's save ourself from that terrible noise of a fan angrily splitting the air with its full power, and control the fan by a temperature sensor!

I dreamed of a fan-control circuit with 3 states:

1. The inverter is cool and the fan is running silently on low RPM (rounds per minute). The custom LED indicator glows green.
2. The inverter getting warmer. Fan is switched to its full speed, and the LED turns yellow.
3. The inverter raises its temperature even higher. A noise maker buzzer cries out, indicating that the level of the heat would harm the inverter, and the fan can not compensate the amount of heat dissipation.

As soon as the increased fan activity is able to cool down the inverter, the circuit automatically steps back to state 2 and later to the calming state 1.

No manual intervention ever required. No switches, no buttons, no maintainance.

You need at least the following components to smart-drive the fan of the inverter:

• an operation amplifier chip (I used an LM258 dual op-amp)
• a thermistor (6.8 KΩ) with a fixed value resistor (4.7 KΩ)
• a variable resistor (500 KΩ)
• a PNP transistor to drive the fan, and a 1 KΩ resistor to preserve the transistor
• optionally a semiconductor diode (1N4148)

With these components you can build a temperature driven fan controller. However if you want to add LED indicators, you need more:

• two LEDs with two resistors, or one bi-color LED with one resistor
• you also need a NPN transistor to drive the LED

If you also want the overheat warning feature you will need:

• a buzzer and one more variable resistor (500 KΩ)
• optionally another PNP transistor
• optionally two fixed value resistors (470 Ω for the buzzer and 1 KΩ for the transistor)

The main reason of I implemented this circuit is to mute the fan. The original fan was surprisingly loud, so I replaced it with a low power and much more silent version. This fan eats just 0.78 Watt, so a small PNP transistor can handle it without overheating, while also feeding the LED. The 2N4403 PNP transistor is rated to 600 mA maximum current on its collector. The fan consumes 60 mA while running (0.78 W / 14 V = 0,06 A), and the LED consumes an additional 10 mA. So the transistor can safely handle them without a relay or a MOSFET switch.

The buzzer can operate directly without a resistor, but I found its noise too loud and annoying, so I applied a 470 Ω resistor to get the sound more friendly. The second PNP transistor can be omitted as the op-amp can directly drive the small buzzer. The transistor is there to switch the buzzer on/off more seamlessly, eliminating a fading sound.

I placed the LED on the top of the inverter's housing. This way it can easily be seen from any viewing angle.

Inside the inverter I placed the extra circuit such a way that it does not block the route of the air flow. Also, the thermistor should not be in the air flow, but in a not so well ventilated corner. This way it mainly measures the temperature of the internal components and not the temperature of the air flow. The main heat source in an inverter is not the MOSTFETs (which temperature is measured by my thermistor) but the transformator. If you want your fan to response quickly to load changes on the inverter you should sit the head of the thermistor to the transformator.

To keep it simple I fixed the circuit to the housing with double sided adhesive tape.

The circuit is powered from the inverter's cooling fan connector. Actually the only modification I made on the inverter's internal components is cutting the wires of the fan, and inserted my circuit between the fan connector and the fan itself. (The other modification is a hole drilled in the casing top for the LED.)

Variable potentiometers can be any type, however helical trimmers are preferable because they can be fine tuned and much smaller than knobbed potentiometers. I initially tuned the helical trimmer which turns the fan on to 220 KΩ, measured on the positive side. The other trimmer has been preset to 280 KΩ.

Semiconductor diode is there to avoid inductive current flowing backward when the electromotor of the fan is just switched off but the rotor is still rotated by its momentum. However applying the diode here is optional as with such a tiny fan motor the induction is so small that it can causes no harm to the circuit.

LM258 is a dual op-amp chip consisting two independent operation amplifiers. We can share the thermistor's output resistance among the two op-amps input pins. This way we are able to turn on the fan at a lower temperature and the buzzer at a higher temperature with using only one thermistor.

I would use a stabilized voltage to drive my circuit and get constant on/off temperature points that are independent from the voltage level of the battery the inverter running on, but I also want to keep the circuit design as simple as it can be, so I gave up the idea of using a voltage regulator and an opto-coupler switch to drive the fan with the unregulated voltage for maximum RPM.

Note: The circuit presented on this schematic covers all the prementioned features. If you would less or other features than the circuit must be modified accordingly. For example leaving out the LED and not modifying anything else will lead to disfunction. Also note that the values of the resistors and the thermistor may be different, however if you use a fan with different parameters than mine you also must modify the resistor values. Finally, if your fan is bigger and requires more power, than you will need to include a relay or a MOSFET switch into the circuit - a small transistor will burn out by the current your fan drains. Always test on a prototype!

WARNING! Life-endangering!

Inverters having high voltage inside them. If you are unfamiliar with the safety principals of handling high voltage components YOU SHOULD NOT OPEN AN INVERTER!

With the two variable resistors (potentiometers, or helical trimmers in my case) the levels of temperature where the fan and the buzzer goes on can be customized. This is a trial and error procedure: you have to find the proper settings by several trying cycles.

First let the thermistor to cool down. Then set the first potentiometer to the point where it switches the LED from green to yellow and the fan from low to high RPM. Now touch the thermistor and let it to warming up by your fingertips, while you tuning the potentiometer until it switches the fan off again. This way you set the temperature level to about 30 Celsius. You probably want slightly higher temperature (maybe above 40 Celsius) to switch the fan on, so turn the trimmer and test the new on/off level by giving some heat to the thermistor.

The second potentiometer which controls the buzzer can be set (for a higher temperature level, of course) with the same method.

I use my fan controlled inverter with great satisfaction - and in silence. ;-)