Best way to reduce fan voltage

Felix Randel

I've an 80mm ys tech fan, 12volt and i want to half the voltage, would the best way be to add a resister to the cables going into the fan, from the power output.

koneko

AFAIK http://www.7volts.com has some mods/info on it, (like getting ur fans on 7v).

Inspector Gadget

As far as these things go, a small percentage of fans will refuse to start (but run happily enough once started) at 6v. If you just want a fixed lower voltage, then by all means 7v them as referenced above.

Your alternatives are:

• Single resistor (actual fan voltage will be current draw dependent)
• Two resistors in voltage divider configuration (output voltage will be a known fraction of the input voltage)
• Linear voltage regulator (output voltage will be a fixed, or variable, depending on the type of regulator, voltage, but will be stabilised a bit by the device). Regulator itself will get hotter as gap between input and output voltage increases - may require small heatsink.
• Pulse Width Modulation or PWM - "chop" up the incoming DC voltage and smooth it out to provide variable DC-ish voltage. Fewer heat problems than any of the other solutions, but trickier to implement and tends to screw up the fan's speed sensor output)

Just my 2c...
Gadget

PPC

I'd make a switch like


The switches work a charm and you can buy the parts from Peats, Maplin or Farnell.

You need to get a toggle switch that has 2 on positions only and locks on either one.

Gerry

I'm currently running 2 of my fans at 5v, my 120mm runs perfectly at this speed, it only has trouble starting at around 4 volts. The 80mm doesn't seem to be perfectly happy at 5 volts, so I'm gonna jack it up to 6.

IMHO the best way to run at 6v is: get a rheostat (potentiometer which can handle a few watts). Peats and maplin have them. Then run the fan at 7v, but connect up the rheostat. You can then drop the fan to 6v, and due to the small difference, the rheostat will not produce much heat.

Eventually I'm going to go for a voltage regulator like the LM317, interfaced to my serial port, so I can control it with software. Not very difficult really.

Inspector Gadget

The other problem with undervolting fans is that the minimum start voltage increases as the fan gets older; bearing wear (especially in fans with sleeve, rather than ball, bearings) amongst other, less significant things, will mean that the motor may eventually refuse to start at a voltage it would happily start at a few months before.

Personally, I'd keep the fans at a little above half their rated voltage to be relatively sure they'll start every time, but I could be overstating the importance of this stuff.

Gadget

Gerry

No, you are absolutely correct. Mine are a bit above that critical voltage, but I reckon my 120 needs more voltage to start than it used to. Just thinking about the software control of fan speed, I will need to get a voltage regulator which can take a binary input of say 4 or 8 bits.

Inspector Gadget

Originally posted by Gerry
No, you are absolutely correct. Mine are a bit above that critical voltage, but I reckon my 120 needs more voltage to start than it used to. Just thinking about the software control of fan speed, I will need to get a voltage regulator which can take a binary input of say 4 or 8 bits.

Considered a DAC, or, better again, a digital potentiometer? (Maxim do the latter - have a gander at http://dbserv.maxim-ic.com/DigitalPotentiometers.cfm). I presume you're familiar with MAX232s and that sort of mullarkey?

Gadget

Felix Randel

Originally posted by Gerry
IMHO the best way to run at 6v is: get a rheostat (potentiometer which can handle a few watts). Peats and maplin have them. Then run the fan at 7v, but connect up the rheostat. You can then drop the fan to 6v, and due to the small difference, the rheostat will not produce much heat.

So how would I go about connecting one, solder it to bear wire?
From reading your post, i take it that i would use the rheostat in conjunction with the SPDT switch mentioned by PPC.

what i really want is the ability to change voltage, more then having a fixed lower voltage, as you said a time may come when I may be forced to increase voltage.

Inspector Gadget all nice suggestions, but how would I go about doing them?

Inspector Gadget

Found a nice little page that seems to cover many bases:

http://www.procooling.com/articles/html/quiet_fans_-_power_control_met.shtml

Gadget