please anyone help engineering question??

Posted on 2015-09-01 06:16:00 in Engineering by DOLLY

I'm making a magnetic stirrer. I m getting confused about how to apply Ohm s law in some calculations. The AC/DC adapter i m using as a power supply gives me 1A of current at 12V. This is going to power two 0.25A fans. They are connected in parralel circuit.

1.) Does this mean the fans have a resistance of 48Ohms each (R=V/I So 12 x 0.25 = 48. OR do I calculate the load resistance in the circuit by using the current left after feeding the fans (1-0.25=0.75. R = V/I, So 12/0.75 = 16Ohms resistance per fan.

2.) Secondly, and separately. Say I have a simple 12V circuit with nothing in it apart from a single 20 Ohm resistor. 12/20 = 0.6A. Is this the total current in the circuit or is it how much the current is reduced after passing through the resistor, meaning it would be 0.6A more before the resistor?

3.) All the potentiometers I can find are like 1kOhms-500kOhms. Is this a rating of (A) their range, (B) their maximum resitance, or (C) their minimum resistance.

4.) I might need to slow the fans down using a potentiometer. Someone told me that if the fans have 48Ohms resistance then adding a 20Ohm pot on top of that would take me down to 5v, how has he calculated this?

If anyone could help me I d be so grateful. It s all doing my head in.

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