# Interpolate variable 0-24 V to power a 12 V fan and control its speed

I want to replace the two noisy 24 V brushless fans on my printer with high performance silent 12v fans, more specifically Noctua A4x10 FLX

The 3D printer hot-end fan is on continuously with 24 volts, so I can use a DC regulator such as LM2596 to achieve a constant 12v to power the fan. The part cooling fan on the other hand uses variable voltage to adjust the speed of the fan during the printing process (probably between 14-24 V). I would like the 12 V fan to be able to vary its speed just like the 24 V fan does. From what I understand using a LM2596 will not allow the voltage to vary and will just continue to put out solid 12 V.

Is there a way to allow this variable fan speed functionality to translate to a small 12 V fan? Is using a simple voltage divider circuit the answer?

Thanks to Dampmaskin's comment it looks like the variable speed part cooling fan is in-fact PWM. In that case is it possible to convert 24 V PWM to 12 V PWM?

• Plenty of people make 24V fans that size, are none of them suitable? Apr 4, 2018 at 13:16
• There are other 24v fans available however I'm unable to find high quality silent fans from a reputable brand. I already have two spare silent Noctua 12v fans to hand. Apr 4, 2018 at 13:20
• Do you know that the fan speed is regulated by varying voltage and not PWM? I think you should find out for sure, either by consulting the documentation or measure it yourself. Otherwise you risk wasting a lot of time. Apr 4, 2018 at 14:00
• PWM fans are usually 4-wire so you have constant 24V input in one wire and duty cycle adjustment PWM signal in another wire. Nov 13, 2018 at 16:56

You could theoretically use a simple series resistor as a voltage divider to produce 7-12 volts over the fan but in practice it may get problematic. You should test the fan behavior versus 14-24V input voltage range.

As a more robust solution you could use a mosfet as a variable resistor. Use a green LED to bias the mosfet base and basically use the mosfet as a source follower. Replace the LED with diodes and/or another LED depending what you have available and what's your gate threshold voltage. You could also use the same idea with a transistor and a single diode, in fact a transistor would work better with the more predictable base voltage.

The transistor/mosfet will run hot so it should be a chunky TO-220 style, not petite TO-92 package one.

Edit: I guess a simple 220R series would do the same job but you need at least 3/4W resistor and it'll get piping hot.

• So, to clarify, the 24V label on the left is actually the PWM (0-24) input signal, yes? Nov 12, 2018 at 17:27
• Well, no, OP was asking for 24->14V voltage range adjusted for a 12V fan. So if the fan voltage is variable, this simple circuit will halve (approximately) the voltage so you get 12V to 7V instead which should work fine for the 12V fan. PWM fans work quite differently, they're constant voltage and use the 4th PWM wire to pulse the motor. You could also put a potentiometer there if you want to adjust the fan manually. Nov 13, 2018 at 16:50
• Yes, but if the input voltage on top of the 4K7s IS a PWM signal, then the output would be a 12v PWM signal, yes? In other words, a 24V PWM signal is equivalent to a variable 0-24 input signal. Dec 10, 2018 at 20:11
• @svenyonson If you mean equivalent with regards to driving 12V fan with this circuit, then more or less I guess. If the PWM signal is too low frequency the fan might not work as expected thought. I think usually the fan PWM control is synchronizing the fan rotation speed, which is not a great choice for pulsing the power. If it's something reasonably fast like 100Hz it should work fine. Dec 12, 2018 at 10:25

I took your idea of using a voltage divider and ran it through the simulator.

simulate this circuit – Schematic created using CircuitLab

I have modelled the fan as an inductor and here is the plot of a V1 voltage sweep (12 to 24V) and how the voltage across L1 and R2 looks like

• It's a DC fan so it looks like a 12V / 50mA = 240R resistance. So drop (short) R2 to have approximately correct voltage division. Apr 4, 2018 at 13:19
• You are correct, I did not consider that it would effectively act as a 240 ohm impedance Apr 4, 2018 at 13:31
• Looking back at this I'm not sure where "drop R2" came from. If you've got 240R in series and the fan, that is. L1 doesn't really belong there but doesn't change DC voltage division. Dec 12, 2018 at 10:28