What value resistor?

[Palmiro]

Hello, I'll start with an apology - I'm sure this is really simple but I cannot work it out so please bare with me, I've just started on this road of learning!

I have a fan I'd like to run from a power adaptor:

the adaptor supplies 12v @ 350mA

the fan is rated 12v @ 18mA

therefore what value resistor should I use to bring the current down to required level?

Thanks for *any* help you can provide

 

[akg]

since they both works on same volt , you dont require a resistor, the 18mA denotes that , it will draw only 18mA from the power supply and 350mA on adaptor means , it is designed to supply upto 350mA .

 

[TrevorP]

As far as I know you wouldn't need one. See the adapter CAN supply 350mA. But it doesn't have to. It depends of the resistance of the fan. Seeing as the fan has a resistance of 666.6 Ohms (12/.018).

That's my guess at least.

Have you tried running it?

Also seeing as it's an engine, it will draw more current as it gets hard for the engine to move. Ex: you stick your hand in the way of the fan.

Others be sure to approve or disprove what I am saying.

 

[Palmiro]

Many thanks for your helpful responses akg and Trevor!

OK, so that's the situation in the case of a 12v power supply, now - just out of curiosity - what would be the case if I were to be faced with the following scenario:

PS: 19v @ 500mA

Load: 12v @ 18mA (same as before)

What value resistor?

As I mentioned this is just for my interest and learning purposes!

Thanks again

ps My "calculation" gives me 389 ohms...

 

[eng1]

R = ( 19 - 12 ) / 0.018 = 388.8 ohms. The closest commercial value is 390.

You can evaluate the the power dissipated by the resistor with the following formula:

P = R*I^2 = 390 * 0.018^2 = 126 mW.

 

[TrevorP]

A thing to remember with resistors is that because of how they are coloured you are only going to have 2 significant digits. So if you NEED something with lets say 3876 ohms. you'd have to go to 3900 because if you went below that not enough power would be dissipated.

 

[audioguru]

If you connect a resistor in series with an electric motor then its torque will be reduced so low that the motor might not start.
You need a voltage regulator circuit to reduce the voltage to feed the motor, not a resistor.

 

[philba]

just a guess here but is your power supply just a wall wart and you measured the voltage in an unloaded state? Unregulated supplies will suffer some voltage drop though your 18 mA load won't do much.

Basically, you need to match the voltage to the specifications of the device you are powering. Just as audioguru said. Check out a 7812 linear regulator.