? Parrallel Resistance and wattage rating

[Hybrid fourdoor]

I want to use a 2.5K pot on something (a fan) and the value is too high (fan only spins on the very last 90% of dial). However if I put a 200ohm fixed resistor (3W) and a 2.5K pot (2W) in parrallel I can get a value range thats more usable.

Now my question is, does using the two in parrallel help raise the equivalent wattage rating, or am I limited to the rating of the component that has the lowest resistance at any given time.

I tried a smaller pot before and although it worked similar to one above, it lit up everytime I adjusted the dial ( ), so I want to make sure I'm doing the safe thing. Also this will not be a constantly adjusted fan, just something I can have lower most of the time and then turn up should the temperature ever get out of hand.


BTW: Application is a 5 1/4" fan on a computer case.

[Sch3mat1c]

With parallel resistors, voltage across the terminals is equal. Current is distributed according to I = V/R. P = VI so dissipation will be distributed in a similar fashion, with the lowest resistance hogging the most power and current.

If you want an expression for it, the algebra is easy.

Tim

[timpac167]

Better to get a variable resistor with high current ratings.

If you have two resistors in parallel with equal resistance, then you double the power ratings. IE: Two 100 ohm in parallel with 10W rating would give a 50 ohm resistance capable of dissipating 20W because the power is split evenly between the two.

However if the resistance are not of equal value, then you will need to find the voltage across it and bust out the equations.

If you parallel a 200ohm resistor across a pot that maxes out a 2.5k, the pot isn't really effective until about 1kohm, because at all other times, most of the current will flow through the 200ohm resistor because of the lower impedance.

From 1k ohm down to 200 ohm, the power dissipation will begin to balance out, and the total impedance will tend towards 100ohm when the pot is at 200ohms.

From 200ohm and below the pot will have the lower resistance, so the power dissipation starts to go towards the pot's favor and when the pot reaches it's lowest reading, the 200ohm resistance would almost be neglected.