Slowing fan with resistors

[necronom]

I have replaced the fan on my Apollo 1240 card, but even though I went for one that I thought would be okay, it still sounds like a jet engine. I was thinking it would be easy to slow it down with a resistor. I know almost nothing about electronics.

I looked at http://www.blackfiveservices.co.uk/fanspeed.shtml, and by entering the details of the fan (12v 0.04 Amps), I though I'd reduce it to 7 Volts. It gives me values of:

Target resistance: 214.286 Ohms
Power dissipated by resistor: 5V * 0.023A = 0.117W

I see there are 220 ohm resistors, so I though I'd get one.

Once I have the correct one, do I just insert it into the red wire (two wire fan)? Does it have to be any way round? Should I get a white ceramic one, or one with coloured lines round it, or are they all the same thing?

I'm thinking a variable one might be better, then I can twist the knob until it's how I want it. Would one of these do?:
http://www.ebay.co.uk/itm/10X-10-Pac...eRj1ju3faVtF-w. I guess choosing the 500 ohm would cover all options. Would I just connect the red wire to the middle pin and either of the other two? Could I do any damage by setting it to the wrong position with the knob?

Any help would be appreciated.

The fan is a Nidec D03X-12TL 12V DC 0.4A
http://www.nidec.com/en-EU/product/fan/category/F010/G020/P2000020/

[ajk]

You have it pretty much right. Resistors don't have polarity so you can connect them either way round. The physical size of a resistor basically determines how much power it can dissipate without overheating. The most common leaded resistors (the ones with the coloured bands) are quarter watt, or 0.25W rated, so at 0.117W you'd be safe with one of those.

A variable resistor, or potentiometer, is handy for trying out different voltages in practice but I wouldn't necessarily leave it in the final build. You could get e.g. a 0-1000 ohm potentiometer, use that to determine what kind of fan speed you want to have, and measure the resistance. Then get the closest matching regular resistor and build that into the cable.

One potential caveat is that fans are designed for a certain voltage and may not start spinning at a lower voltage. You'll want to be sure that you are not too close to that threshold, because obviously having the fan fail to start would lead to overheating of the CPU. If you have a potentiometer, it's easy to test what that minimum voltage is for a specific fan.

[necronom]

Thanks. I have it working

I tried the fan with one of those universal mains adapters that let you change the voltage. It was working at 4.5v, so 7 should be okay for it.

I've just pulled off a resistor from a circuit board that was due to be thrown out (an old TV STB), that looks like it's Red, Red, Brown, Gold (220 Ohm, I believe). I tried it on a multimeter, and it was showing 215, so that looked good to me.

It's now soldered onto the red wire, and it's so quiet now than the noisy thing is the PC PSU that's powering the Amiga!

[zipper]

Good to have old electronic scrap - I turned down a noisy external USB disk fan with something found from my pile of junk.

[Turran]

Cant you just connect it to 5 and 12 volts? That gives you 7 volt. Or perhaps there is no 5 volt on the apollo, what do I know =)

[hooverphonique]

Originally Posted by Turran

Cant you just connect it to 5 and 12 volts? That gives you 7 volt. Or perhaps there is no 5 volt on the apollo, what do I know =)

yes, normally this works well, but only in cases where they share the same ground, which will be the case for 99% of all computer equipment.

[xArtx]

A single resistor in series is for limiting current.
To make a voltage divider requires two resistors,
which is why a pot has three leads and is how it's normally used.
For a voltage divider, a pair of resistors are connected in series,
Then each end of that assembly is connected across the power supply.
If the resistors are the same value, the voltage at the mid point where
The resistors are connected together will be half that of the supply voltage.
A pot is acting as two resistors with a variable mid point.

As per my knowledge potential caveat is that fans are designed for a certain voltage and may not start spinning at a lower voltage. You'll want to be sure that you are not too close to that threshold, because obviously having the fan fail to start would lead to overheating of the CPU. If you have a potentiometer, it's easy to test what that minimum voltage is for a specific fan.

IC programming

[Daedalus]

Originally Posted by RafZwicke

As per my knowledge potential caveat is that fans are designed for a certain voltage and may not start spinning at a lower voltage.

True, and also bear in mind that that as a fan ages, friction in the bearings increases so the starting current (and thus the voltage) increases steadily with age. A 12V fan that starts consistently well on 5V now might not do the same after a year or two of use.

[ajk]

He has copied my reply from earlier word for word... spammer?

[Daedalus]

@ajk

Ooh, good spot! I did think it sounded funny at the start of the 1st sentence...

[nogginthenog]

Originally Posted by hooverphonique

yes, normally this works well, but only in cases where they share the same ground, which will be the case for 99% of all computer equipment.

Is there any downside to this? Will it affect the 5v / 12v levels in the system?

[bebek]

Not at all ...