Determinine resistor needed for circuit...?

Question

Hello,

I have a 12v 0.28mA fan that I need to run off a 12v 500mA power supply. What resistor (or other component) should I use to get the 500mA down to the required 0.28mA to power the fan properly.

I tried to use it straight from the 12v 500mA supply but it didn't work so I assume that's what I need to do.

BTW: I would really appreciate it if you could supply the forumla or equation used to find the value but it's not necessary ;)

Thanks in advance!

Jeffrey

Answer 1

It should work fine as it is. The current is dependent on the load. When you connect the fan it will draw 280 ma. When nothing is connected, no current is drawn.

Look at the 500 ma number as the max current available.

If you tried it and it didn't work, something else is wrong. either the supply won't actually deliver the 280 ma needed, or your numbers are wrong.

What happened when you connected it?

Are you sure the fan needs DC power? (It probably does). Are you sure the supply delivers DC and not AC? Because I think the problem is in the supply. Either it's an AC supply or it's a very cheap supply that doesn't deliver.

Did you test it with a meter? Reasonable multimeters can be bought for less then $20. If you are going to be fooling around in this area, you need a meter.

You can test the motor by connecting it to a 12 volt battery. Even a 6 volt battery should make it move.

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Answer 2

I never saw a fan using 0.28ma that small current. Perhap this is 0.28 ampere.........280ma.

Assume 0.28ma is true,and you have  a supper micro-motor.

You do not need any current limitter to limit the current into 0.28ma to make the fan running. Because both the power supply and motor voltage are the same 12 volts. Motor shall draw 0.28ma from the 12 volts power supply. When the load of motor increases,the current draw shall be more than 0.28ma.  May be you need some protection device to protect this micro-motor (0.28ma motor is very expensive) being burnt by the power supply. The cheapest way is connect a 50k ohms 0.5 watt potential meter in series. Of course,to build a power supply that has a current limitter is the best way. Or you can buy one from market........specification: 0 to 30 volts,0 to 1 amperes power supply is very common and not that expensive.

If your motor is rate at 0.28 ampere, then change the potential meter into 50 ohms 5 watts.

By adjust the resistance value of the potential meter,the current shall be decreased. Because the voltage drop acrosses the potential meter increasing,the voltage supply to your motor is no more 12 volts.

Answer 3

The fan should work fine. It doesn't, because it's broken. Double check the wires to make sure they are OK. If not, get a new fan.

Answer 4

You don't need anythings. 500 mg is the maximun current . The actral current should be 0.28 mg