quick Rheostat Question.... (delta fan)

[javertiz]

I am looking at a rheostat which is 100Ohms and 5W.... but I needed for my Delta 38 Black Label. It seems that the Delta consumes 6W+. Do I need a different Rheostat or does the one above suffice....

Can't stand the noise !! Please help !! :-)

-Alex

[donny_paycheck]

6 watts is a bit for a rheostat to handle. I'd either go with a voltage regulator or do the 7 volt mod to it to quiet it down.

[riprock]

There are rheostats that can handle more wattage than 5w. Just goto an electronics website and look at their rheostats. You can get a 50watt 100ohm rheostat (although it's gonna be pretty big and give off alot of heat).

A rheostat is rated in Ohms and watts.

Digikey carries many, but they can be expensive:
http://www.digikey.com/scripts/us/dksus.dll?Criteria?Ref=39417&Cat=22414184

Also, rheostats often are referred to as potentiometers.

[donny_paycheck]

Originally posted by riprock
There are rheostats that can handle more wattage than 5w. Just goto an electronics website and look at their rheostats. You can get a 50watt 100ohm rheostat (although it's gonna be pretty big and give off alot of heat).

Oh yeah, that thing'd be HUGE...but if it's low voltage then it won't necessarily get hot. The more volts go through it the hotter it will get regardless of the amperage of the load. Rheostats are funny like that....they vary voltage cuz they're an inductive load while potentiometers vary the current through a load cuz they're a resistive load. A rheostat absorbs all the power that DOESN'T go to the device and dissipates it as heat while the pot in series with the load just makes the load seem to have a higher resistance to the flow of electricity thus draw less current. This is why pots don't get nearly as hot as rheostats but are crappy for speed controls in series with fans. I hope this makes sense. Actually, I hope I'm right on this one too!:p

[Rkjob]

I am using 5 watt 100 ohm rheostats ordered from radio shack to contrl my delta 80mms (68 cfm). They work fine. On my old heatsink, i had the same type rheostat controlling a ystech 60mm at 40cfm and it worked fine.

[ButcherUK]

Originally posted by donny_paycheck


Oh yeah, that thing'd be HUGE...but if it's low voltage then it won't necessarily get hot. The more volts go through it the hotter it will get regardless of the amperage of the load.

No, the heat is directly proportional to the current (amperage), current is also directly proportional to voltage via ohms law as rheostats are ohmic resistors.


Rheostats are funny like that....they vary voltage cuz they're an inductive load while potentiometers vary the current through a load cuz they're a resistive load.

No, rheostats and potentiometers are the same basic thing. A rheostat usually refers to a two contact variable resistor whereas a pot has three terminals, however the terms are often used interchangably. Neither of them is inductive.


A rheostat absorbs all the power that DOESN'T go to the device and dissipates it as heat while the pot in series with the load just makes the load seem to have a higher resistance to the flow of electricity thus draw less current. This is why pots don't get nearly as hot as rheostats but are crappy for speed controls in series with fans. I hope this makes sense. Actually, I hope I'm right on this one too!:p
A rheostat works by forming a potential divider with the load. If you have a load with resistance Rload and a rheostat then the voltage in the load is Vload = Vsupply * Rload / (Rload + Rrheo). The result is less current drawn by both the rheostat and the load due to a higher overall resistance.
A quick example using the 6W fan (24 ohms) and the 100 ohm rheostat. At minimum speed the rheostat is set to 100 ohms. This gives a fan voltage of Vfan = 12*24/124 = 2.3V. The fan is stopped. Current flow is I = V / R = 12 / 124 = 0.097A. The fan dissipates Pfan = 2.3*0.097 = 0.22W. The rheostat dissipates Prheo = 9.7*0.097 = 0.94W.
The maximum power dissipated in the rheostat with the 6W fan is actually about 1.5W. This is at 25 ohms. So that 5W rheostat is plenty powerful enough for the fan.

A pot on the other hand is used to vary the voltage to a load, the wiper moves between the two end contacts of the pot one of which is connected to the supply and the other to ground. This means the pot forms a potential divider. This uses more power though as there is always a current flow through the pot (unless the power is maxed). This means pots get very hot when running fans at low speed, by comparison a rheostat gets hotter at higher speed due to more current flow overall.

[donny_paycheck]

lol man I knew it! Been a long time since basic electronics school I guess...anyway, there ya go javeritz...