Resistor Calculator

D

debaucher

[H]ard|Gawd
Joined
Jan 13, 2002
Messages
1,636
Hello everyone.
I went through all the calculations to figure out the resistors I needed to run some fans for a project at 7V and 5.5V when I stumbled on this website

It does all the calculations for you (resistor ohms needed and watt dissipation)

Just thougth I would give the website some props and the people at the [H] some help.

D.
 
I would never use a voltage divider to supply power to a fan, as soon as you attach a load, the voltage will change, resistors cannot regulate voltage..

when you calculate for 7V and 5.5V, as soon as you attach tha fan it won't be 7V and 5.5V anymore.. it all depends on the resistance of the fan though.. i'm not sure what are typical fan resistance values..

i woul recommend using a zener diode instead...

or s linear voltage regulator

but whatever works i guess...
 
Yea, I know it resistors are not perfect when it comes to voltage, but close enough for government work (as they say).

I did a quick test on my fans and found them to be within +/- 0.1V of my desired voltage after I had the resisters hooked up so that was good enough for me.

Mind you, I did the calculations myself (dug out the old physics book) before I found the website, but the website had the same calculations and results as me so all is good there.

This is just a handy tool for those who want to lower the voltage/speed of their fans to achive less noise etc.. not for any "mission critical" applications

D.
 
debaucher said:
Yea, I know it resistors are not perfect when it comes to voltage, but close enough for government work (as they say).

I did a quick test on my fans and found them to be within +/- 0.1V of my desired voltage after I had the resisters hooked up so that was good enough for me.

Mind you, I did the calculations myself (dug out the old physics book) before I found the website, but the website had the same calculations and results as me so all is good there.

This is just a handy tool for those who want to lower the voltage/speed of their fans to achive less noise etc.. not for any "mission critical" applications

D.
Click to expand...

+/-.1V ?? wow.. that's not bad.. i honestly would expect much worse... the only way i could see tha hapenning is if the fans have very high resistance..
 
Yea, I did these on a pair of 120mm panaflo's which I 'believe' have a high resistance.
In the past I did it on a cheap pair of no-name 80mm fans and found the variance to be higher (+/- 0.5V).
What I would normally do is have a variety of resistors and would temporarily hook up a resistor to the fan (wrap the bare wire around the resistor) and then power up the fan with my old cheap PSU to test the voltage of the fan.
If it was too high or low, then I would adjust the resistor accordingly....this is where the website I mentioned in my first post is great because you can just change numbers based on the voltage reading you got after adding the resistor to fine tune the resistor needed.

I know resistors are not perfect in this application, but for the everyday person who doesn't want a rheostat and doesn't need to be perfectly accurate, this is quite handy and quick to do.

edit.. oh and thanks for the replys... I had totally forgotten about this thread..lol
Plus, the real thanks should go to the person who made the website, not me... I am just passing the information along for those who can use it.
 
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