Reduce voltage with a diode

[ThomsCircuit]

Got a 5v fan (40mm) with the following specs.
I used these specs to calculate the required potentiometer needed to control the speed

Vf = minimum fan voltage (2.5)
Vs = (power supply), (5)
Rf = fan resistance in ohms (35.7)
Rr = potentiometer resistance in ohms
Vf1 = fan voltage, (5)
Va = fan amps, (0.14)
Calculate Pot size:
Rf = Vf1/Va (36)
Rr = ((Vs*Rf)/Vf) – Rf ((5*36)/2.5) - 36 = 36ohms IVE GOT A 50R

Ok thats done now the issue I have is the pot rating is .5 watts and the fan is .7 (amps x volts) or IxV (.14 x 5) =.7
Is this a problem? .2watts higher? if so can I lower the volts to the fan by 1.4 using a pair of diodes? I understand most silicone diodes use .7volts
here is my proposed schematic.
i have 1n4002, 1n4007, bat85, bat46

 

[Externet]

Hi. A potentiometer is meant to pass much smaller currents as to drive a power transistor base/gate to control speed properly. Why is the supply to the fan dropped from 5V to 3.6V with D1 + D2 ? Do you realize doing that again with more diodes in series can step down the voltage to 2.9V, 2.2V, 1.6V, 0.9V, 0,2V which with a multi position switch can provide stepped voltages to 0V instead of destroying potentiometers ?

 

[AnalogKid]

Much better to drop the voltage through diodes, as the reduction is much less dependent on the stability of the current drawn.

ak

 

[ThomsCircuit]

Do you realize doing that again with more diodes in series can step down the voltage to 2.9V, 2.2V, 1.6V, 0.9V, 0,2V which with a multi position switch can provide stepped voltages to 0V

Thats a pretty cool suggestion.

 

[ThomsCircuit]

drop the voltage through diodes

In this project the fan is used a few minutes at a time once per week. Is any silicone diode better than another? ie 2004,2007

 

[Pommie]

The 1N4004 is rated at 400V and 1A.
The 1N4007 is rated at 1000V and 1A.

So either will work fine.

Mike.

 

[crutschow]

silicone diode

Those are kind of rubbery.
Silicon diodes are better.