How to make a 5v fan run on a 9v battery help

[Ethan]

Hi there. I have been having some difficulty with this computer fan I received a while ago. I'm using a 9v battery, but the fan can only handle 5v, so I wanted to use a resistor. Here are the fan specs:

DC5V
0.34A

I am pretty sure I need a 14 or 15 ohm resistor, but am not quite certain. Also, do I connect the resistor to the negative or positive wire?

Thanks, I know this is a dumb question...

 

[BeerBelly]

That's a lot of power to expect from a standard 9v battery.

 

[MikeMl]

Positive, negative, doesn't matter.

Putting a resistor between a fixed voltage supply and DC motor is problematic. The starting torque suffers when operating the motor on a DC supply with a resistor in series. Also a 9V battery will barely supply 0.34A, and not for very long...

 

[Vizier87]

Well it depends on your application... what are you planning to do with the fan?

 

[Mike odom]

who says it'll only tolerate 5V? A DC motor's speed is voltage controlled. Hook it straight to the 9V battery, and it'll run like a banshee until the battery voltage drops to 5V... which will probably only be a couple of minutes anyway...


Type IEC
name1 ANSI/NEDA name2 Typical capacity (mAh) Nominal voltage
Primary (disposable)
Alkaline 6LR61 1604A 565 9
Zinc–carbon 6F22 1604D 400
Lithium 1604LC 1200 9.6
Rechargeable
NiCd 6KR61 11604 120 7.2 8.4 (some)
NiMH 6HR61 7.2H5 175-300 7.2 some:3 8.4 9.6
Lithium-ion polymer 520 7.3

 

[dr pepper]

I've a power supply thats runs a 24v fan on 32v, and its ran for months.

 

[tcmtech]

If you have them just put 8 or so diodes on series.

A typical diode has about .5 volt forward drop so around 8 of them should get your 9 volts down to 5 volts without having current limiting issues like a resistor would when the fan starts up.

 

[Menticol]

who says it'll only tolerate 5V? A DC motor's speed is voltage controlled. Hook it straight to the 9V battery, and it'll run like a banshee until the battery voltage drops to 5V... which will probably only be a couple of minutes anyway...

I suspect that the OP is talking about a 5V computer bushless fan, in that case 9V will damage it. The main (and irreversible) symptom of overvoltage is the fan failing to reach full speed, even when the voltage is reduced back to 5V.

 

[Dragon Tamer]

If it is a brushless motor than an over voltage could possibly damage it, it's hard to tell if a 9V battery would be capable of doing this. I think that a 9V battery would lack the current to do any real damage to the motor if it was hooked right up.

If it is a huge concern, you can probably make a zener voltage regulator that uses a small transistor to boost the current output. Add a power switch and you should be set.

I am however going to have to agree with mike. If memory serves, a typical 9V battery is rated for 900mAh, so drawing 0.34A would only give about 2 hours of good run time.