calculating resistance

there isnt really a section for electronics so…

i have a 12v fan inside a network switch which i want to stfu cos its in my room. i want to drop the fan to 6v. since its in a switch there is only a 12v rail for fans. My calculations are:

P/V, where P=0.2 watts printed on the fan, and V=12v supply. That gives me 0.016A, so i can now work out the resistor for 12v. So i do V/I, 12/0.016=750 Ohms. now i think if i change V to my desired voltage of 6 and do 6/0.016= 375. I think i need a 375 Ohms resistor or close enough to drop it to 6 volts and run quietly. Am i right there that 375 ohms will drop 12v to 6v for a 0.2W load? Or have i probably gone wrong somwhere.

Hey thx for looking it over if anyone does :slight_smile:

One word :wink: … >

Plenty of on-line calculators for doing stuff like that…

Anyways pen to paper blahblah scribble scribble…

Purchase Resistance = 750 ohms should see it right :smiley: or even a 680 ohm resistor.

For a more accurate result :wink:

I did look at google, tho tbh i couldent find a calculator that related to my situation. Meh, anyway i tried it by hand cos id like to know wether i could work it out on my own or not.

EDIT: although saying that, the calculator you found is bang on my exact situation, aces!

My math was ok aswell, which im happy abut, it was just the bit at the end about putting the 6v in that was wrong…fan R of 700 is making the voltage 12v so another 700r will halve it again to 6. Aces.

0.2 W seems a very low power for a fan especially one which is generating an unacceptable noise level. Have you got a multimeter, can you check the DC resistance of the fan. Then fan power P = V^2/R just as a check.
If you want to halve the voltage across the fan then add a series resistance equal to the resistance of the fan. Check your selected resistors maximim power handling P=I^2 R(resistor) or you could end up with it getting a bit hot.

That’s assuming :wink: its not overvolted by the manufacturer on the switch :wink:
0.2w is not much for a fan tbh

/bah too slow again beaten by balrog :lol:

A whining little 60mm or 40mm I’d take a guess at, similiar to a nb fan, small but intensely annoying pitch. If that is the case and the fan is accessible, wallop an old Pentium Heatsink on it.

More to the point, if you’ve a switch that warrants a fan, why the hell is your DC output so LOW :Poke: Do I sniff network overkill ?

The only fan assisted switch I have is a monster of a nasty 10/100 hp 24port thingy.


Same here 24port 3com is the onlything I have with a fan, 40mm 5v one
everything else is passive.

But I’d expect a 40mm to be more like 1w, wonder if he’s reading the current
and its .2A @ 12v i.e. 2.4Watts ?

Yeah the switch is the size of a VCR, and i have it cos i can :stuck_out_tongue: Plus managed switches are cool and ive learned about consoling in with hyperterminal thru an RS232. But the noise had to go. Little 60mm jobs.

DC output is nil cos no DC project intrests me anymore. in seti classic a WU meant somthing to me, at the beginning around 130 hrs of work and towards the end around 2 hrs. havent recaputred that feeling of achivement with anything else.

PMM yes it could be the current i read, cos i recalled it from memory and i havent cracked the switch open again to confirm.

Hmm it probably was current actually.

oh well, I’ll get in here quick and then run off…


Come on you know you want to … … and


:dstar: & :vader: & :tiefight: VS :borg: = :dstar2:

My new calculation for resistance, to prove it to you that …

wait for it

ok, maybe you know what’s coming but




LOL I like that formula :chuckle:

Moving to hardware because well… its hardware :chuckle:

If its 0.2A then its a 2.4W fan with a DC resistance of 60 ohm. If you put a 60 ohm resistor in series to drop it to 6V, the current will drop to 0.1A. Be careful, you will need a resistor rated at 1W at least. The small resistors you get in Maplin or RS for example are normally rated at 1/4W. If you can’t get a 60 ohm 1 W resistor, you could use four 240 ohm 1/4W (you’ll only get 220 or 270 ohm btw) resistors in PARALLEL with each other, in SERIES with the fan.

Yeah. and get crunchin’ you slacker.

Hm so is it 60 O or 700 O that im wanting? whos calcs are right?


Both. If its 0.2W - you need 700 Ohm 1/4W resistor else if 0.2A you need 60 ohm 1W.

Oh yes. Indeed.