Reducing fan speed / voltage issues

[Steve]

Hi all,

Hoping some electronics folk can give me some advice on this...

I'm trying to reduce the voltage to a large Noctua fan, here are the fan specs:
https://noctua.at/en/nf-f12-industrialp ... cification

I found this handy website for calculating the resistance required, depending on voltage wanted:
https://www.blackfiveservices.co.uk/fan ... 1&Target=9

But the website doesn't seem to take into account the energy needed for the fan to begin turning... I've tried reducing to 7v by using 85ohms resistor, but the fan just doesn't spin. I then tried about 48ohms but the fan was still crazy fast/loud. I then tried two 15 ohm's in series (30 ohms) but the fan didn't turn. I then tried two 18 ohms in series (36 ohms) and the fans were super fast/loud again! According to the website that's 9.2V

So basically the lowest speed I could achieve without the fan cutting out was 9.2V, using two 18 ohm resistors in series, but it's still too loud :/

Is there any way to get this fan quiet?

Cheers

[exxos]

It's not so simple as you found out. Most PC BIOSs "blip" the fan to 12v to get it spinning then lower the voltage.

Might just be easier to look for a lower rpm fan instead ?

[Icky]

@Steve - I thought Noctua fans came with a couple of reduction cables to reduce the speed?

If you don't have any I think I may have some spare ones.

[Steve]

I don't think this particular fan came with a voltage reduction cable. I think I'll just put the original fan back in because it wasn't so bad, I just thought the Noctua at 2000 rpm would be quieter than the 3600 rpm fan the NAS came with. But I realised that the 3600rpm fan was PWM and was actually running lower than the 2000 rpm non PWM Noctua.

This is how I spend my Sunday... Pity me

[Badwolf]

Put a big arsed cap in parallel with your resistors to allow a bit of early-doors in-rush current?

BW

[exxos]

Put a big arsed cap in parallel with your resistors to allow a bit of early-doors in-rush current?

Not a bad idea! Likely need something around 4,700 - 10,000uF to give it a kick long enough to get the thing spinning. Likely 4,700uF will give a kick from 12V to 9V in about half a second based on a 100R series resistor.

[Badwolf]

Put a big arsed cap in parallel with your resistors to allow a bit of early-doors in-rush current?

Not a bad idea! Likely need something around 4,700 - 10,000uF to give it a kick long enough to get the thing spinning. Likely 4,700uF will give a kick from 12V to 9V in about half a second based on a 100R series resistor.

Yeah, that's quite big-arsed!

I'm sure we can come up with something better involving more resistors, caps and mosfets, but I'm a hack-first-and-see-how-it-goes kind of a guy.

BW

[exxos]

Yeah, that's quite big-arsed!

I'm sure we can come up with something better involving more resistors, caps and mosfets, but I'm a hack-first-and-see-how-it-goes kind of a guy.

Hence my comment of it be easier to just change the fan

I would assume there is already cheap fan speed control is out there somewhere anyway which would do that.

[Badwolf]

I would assume there is already cheap fan speed control is out there somewhere anyway which would do that.

Bah. Where's your sense of adventure?

I reckon @sporniket's already designing a 555 circuit with a variable duty cycle based on an RC charge time constant as we speak!

Steve will have it assembled by sun-down.

BW

[exxos]

Bah. Where's your sense of adventure?

Already did that about 25 years ago If I find a picture at some point I will post it.