Question on calculating resistance needed

[SkeptiDad]

I have a 24v 250ma wall adapter and a 12v 250ma fan, I'm trying to calculate the resistor needed to put in so I don't burn the fan out. I came up with 48ohm resister using ohms law 12/0.25. It seems low to me, am I missing something?

I realize I'm going to lose some current through the resistor. Am I even going down the right road for working this out?

 

[davenn]

you really need a voltage regulator, else the fan will never see 250mA
say a LM7812


Dave

 

[SkeptiDad]

Thanks

 

[davenn]

do a google search on the LM7812 datasheet and you will see info and suggested circuits for its use

Dave

 

[SkeptiDad]

Thanks for the advice, I will do that. I have a buddy with some extra ones too

 

[Arouse1973]

You will probably also need a heat sink. It's going to get pretty warm with 250mA through it.

Dave can you explain what you mean by it will never see 250mA, I am curious. Do you mean with the 48R or any value of resistor.

Thanks
Adam