After adding a second fan to my camper van's fridge+freezer cabinet to help decrease the compressors' run time, a custom fan controller was needed. So I whipped one up with the following functions:
- automatic turn-on when either the fridge or the freezer starts
- adjustable voltage for each fan to dial fan speeds independently
- delayed turn-off after fridge & freezer stop, to fully cool down the coils
The cabinet setup before this upgrade was described in this post
The circuit only uses basic components. Could have used an Arduino micro-controller for simplicity & flexibility, but I wanted to make sure that in case of issues future owners could easily diagnose and repair things instead of depending on source code files that might be long misplaced.
The Fritzing schematic & breadboard file is on Github
Happy quieter van nights and fresh beer to all !
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Circuit
The circuit comprises 3 sections in series:
- OR-ing of the fan signals from the fridge & freezer
- Retriggerable Monostable 555 circuit
- Voltage regulators powering the fans
Theory Of Operation
The Fridge and the Freezer fan signals come from splices into the wires from these appliances' own fans.
The D3 and D4 diodes OR these 2 signals to create the trigger signal for the 555 chip.
When either fan is active, Q4 grounds the 555's monostable inputs, which triggers the 555's output and starts up the cabinet fans.
The 555 is implemented as a retriggerable monostable: as long as either the fridge or freezer fan is active Q4 keeps the monostable inputs discharged, thus keeping the fans on. When the appliance fans turn off, Q4 releases the inputs, which lets the RC circuit charge and starts the countdown to turn off the 555's output. The countdown duration can be set via the R15 potentiometer, up to ~2 minutes.
The 555's output toggles the Q5 transistor that provides power to the LM317 voltage regulators. At first glance the use of a whimpy BF393 NPN transistor seems ill-suited for this 'power' application. But in this context it is sufficient, for the following reasons:
- although a MOSFET would present a lower voltage drop, my fans run at a low ~7V voltage to lower the noise so Vdrop doesn't matter
- the Noctua fans used are spec'd at 100mA max. So Q5 should see no more than ~200mA, which is well within its 500mA rating. And in reality, since the fans are run at less than 7V, Q5 only has to supply less than 100mA
Finally, the LM317s are adjustable so each fan's voltage can be set independently. The pushing fan (bottom of cabinet) is run at a slightly higher speed than the exhaust (upper) fan.
It Works !
The whole thingamachip was soldered onto a prototyping board:
Test results:
Parts
BF393 NPN transistors: this is an obsolete part I had in my drawers, but any bog-standard NPN transistor that can drive ~500mA should do. Both transistors are used in saturation mode so their analog characteristics don't matter. Can be replaced with a MOSFET.
Note
that if the Noctua fans are replaced with a more powerful model, the BF393 feeding
the LM317s might need to be replaced by a beefier transistor.
555 timer: used the LMC 555CN version. There is a myriad of 555 versions out there, but I suppose any would do as long as its max Vsupply is at least 15V.
LM317s in TO220: equivalent regulators should work as well, like the 7805 family, as long as they can handle a ~15V supply. Truth be told, I actually used LM338s on this board 😏
Fans: the Noctua NF-P12 Redux 1.08W 1700rpm is exceptionally quiet and low power. It has 4 wires but still works with only the VCC (yellow) and GND (black) lines. And it gracefully handles low VCC values down to ~6.3v, allowing for broad speed control without having to use the PWM & TACH signals.
Of course, using an Arduino micro-controller would allow for an even broader speed range and guarantee precise rpm values. But in this application it wouldn't make a difference.
Terminals: used this series of right angle, 5.08mm pitch, pluggable screw terminal block that has a strong latch that is reliable for mobile applications, and that works well with both bare and ferrule-protected wire ends
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