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120mm fan on rheostat need help !

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fender123

Member
Joined
Aug 7, 2002
Ok here's the deal, i have a Sunon 108 cfm 120 mm fan rated at 6.8 watts, on a 5 watt 100 ohm rheostat, it works but i get from full speed to off in a quarter of the rheostat rotation possibilities leaving 3 quarters of the rest of the rheostat at off.

So, what's the problem ?
Thx for the help guys.
 

Diggrr

Underwater Senior Member
Joined
Nov 29, 2001
Since your fan takes 6.8 watts (.56 amps), 100 ohms is more resistence than you need.
If you can find one (I haven't looked), 30-50 ohms would be more like what you're looking for. The rheostat you have would work fine on another fan that drew less amperage.

On the other hand, you can continue using the one you have, just place an index mark by the knob so you know when to stop turning.
 
OP
fender123

fender123

Member
Joined
Aug 7, 2002
so since the rheostat is bigger, it takes less to go from 100% to off ?
 

Mr. $T$

Member
Joined
Sep 15, 2001
Radio Shack has 3watt ones that are 25ohms Which is perfict.

Just put a heatsink on one and that way it can handle all the wattage.
 

Since87

Member
Joined
Jul 30, 2002
Location
Indiana
A 15 Ohm rheostat like the one shown here will allow you to adjust that fan's voltage from 12V to 7V. This is nearly ideal, because the fan would probably never be off even at the lowest setting of the rheostat.
 
OP
fender123

fender123

Member
Joined
Aug 7, 2002
Yes the Radio Shack rheostats are perfect, except they don't ship to Canada..... and the ones they sell on the Canadian site are 1watt.....lol, that's why i couldn't get those.
 
OP
fender123

fender123

Member
Joined
Aug 7, 2002
I actually bought 3 of those 5 watts 100 ohm rheostats at once, here's my second question, how many 80 mm case fans could i hook up to one rheostat ?

I know the power consumption of 3 of the fans they are Antec, one is rated at .225 amps, the other 2 at .14 amps, the other one i don't know the amp rating, it came with my Chieftec full tower clear fan, i'll have to stop the computer and look. LEt's just say it's .225 amps for now.

I have zero knowdledgs on amps and their relation to power consumption so i am assuming one would just add up all the fans to get total power consumption ?

So this would mean all the fans on the same rheostat would be drawing : .730 amps ? so would the 5 watt 100 ohm rheostat be able to handle all the case fans ?

Thx guys.