driving two pc fans off a transformer/ ampege calculation

[Orbilius]

I am always trying to learn about electronics but two things always bog me down 1 the math im terrible at it, 2 i have dificulty understanding the relationship between amps and volts and watts.

So please bear with me and my possibly dumb questions.

I have several fans scavanged froman old pc they have different info on them as to their power characteristics which i will list below,

Fan 1 DC 12 V 1.4W
Fan 2 DC 12V 0.15 A
Fan 3 DC 12V 0.32 A This is just what is written on the fans Am i rightin thinking that i could work out the amp rating of the fan marked 1.4 w by testing its resistance and then doing V/R = I

Now my problem is i now have a laptop and wish to use the fans to make a laptop cooler, i have a spare mains transformer which can out put from 1.5 V to 12 V at 1200Ma (max). What i need to know is this, can this transformer be used to run one or more of these fans. i have a hunch that i can if i wire the fans in either series or paralel (to divide the amperage?) but im not sure, one problem is im not sure of the units involved I.E is 0.15 A the same as 1500Ma.

I do have some experiance making circuits but mostly from just following circuit diagrams, and i have read a number of books on electronics forrest m mims etc. but ive never been able to really discuss what ive read with anybody to see if ive understood it correctly.

If a device (in the above case a fan), says 12v 0.15A dous that mean, It needs 12volts available to run at its top speed,(maximum power watts?) and that it draws 0.15 of an Ampere (15% of an Ampere?). What implications does this have for the device supplying the power, say a transformer rated as the above or say, a nine volt battery. So if 0.15A is more than my transformer is rated for what is happening in the transformer, is the fan basically over pulling juice from the transformer or battery for instance, is this even the right way to think about it. I am really really bad at math so please take this into acount if answering my question, like if you were explaining to a very small child

Also im a sculptor and i want o make a peace which has a slowly pulsing light in it , An LED light would be perfect. I know there are ways to do this using the way a capacitor discharges and im sure i ussed to have abook somewhere with acircuit diagram in it i could use, i would really apreciate it if somone could help me with this as well.

Cheers.

[Simon (Webmaster)]

Hi

Fan 1 DC 12 V 1.4W
Fan 2 DC 12V 0.15 A
Fan 3 DC 12V 0.32 A This is just what is written on the fans Am i rightin thinking that i could work out the amp rating of the fan marked 1.4 w by testing its resistance and then doing V/R = I

You can work out the current of the first fan using the formula I = P / V (see http://www.electronics2000.co.uk/data/i ... cpower.php). From this, the current of the fan will be 1.4 / 12 = 0.12A, or 120mA.

one problem is im not sure of the units involved I.E is 0.15 A the same as 1500Ma

1 amp (A) = 1000 millamps (mA), so 0.15A = 150mA. 1500mA would be 1.5A

If a device (in the above case a fan), says 12v 0.15A dous that mean, It needs 12volts available to run at its top speed,(maximum power watts?) and that it draws 0.15 of an Ampere (15% of an Ampere?).

Correct. The device will draw the stated current at the stated voltage. For a resistive load, reducing the voltage will also reduce the current drawn.

To run your 3 fans you need to connect them in parallel to the power supply. To work out the current required you need to add up the current drawn by each fan, so the total is 0.12 + 0.15 + 0.32 = 0.59A (or 590mA).

What implications does this have for the device supplying the power, say a transformer rated as the above or say, a nine volt battery. So if 0.15A is more than my transformer is rated for what is happening in the transformer, is the fan basically over pulling juice from the transformer or battery for instance, is this even the right way to think about it.

As long as the power available from the power supply is equal to or greater than that required by the load (fans in this case) there is no problem. The fans will not draw too much current (provided the voltage is as specified). However if the rating of the power supply is less than that of the load it will be overloaded. In short you can run your 3 fans in parallel from the power supply you have.

Finally check that the 'transformer' has a DC output, not AC. This is actually normally referred to as a power supply unit, as it contains extra components to convert the AC to DC and regulate the voltage. A transformer on its own simply changes the voltage.