I have a dc power supply and its output is 19volts dc 3.42 amps. I want to run 4 computer cooling fans each 2.16 watts 12 volts.
I need to know how I can reduce the voltage to 12 volts
Thanks paul.

A box, a resistor, a zener diode and a heat sink. And KISS can tell you how.

A simple circuit to do what you want would be made up of a zener diode and a resistor. If this is going to be a permanent (or long term supply) to the fans then I would suggest an electronic voltage regulator circuit. Both are easy to make, Please let me know if you would like these circuits and if the fan supply will be one that you plan on using for a long time.

A zenier diode is out of the range for this project.

You could use a 7812 with a heatsink. This is a 1 AMP regulator. You need to get the heatsink mounting kit as well. It consists of a thermal washer and an insulating washer. This is a linear regulator and this creates heat. A few (2) capacitors increase stability. Should be on www.national.com website. You should be able to purchase at DigiKey Corp. | Electronic Components Distributor | United States Home Page and/or mouser.com. Info should be in the datasheet. It needs 3 V minimum between input and output.

If your ambitious there is a switcher CAD program on National's site that will allow you to design a switching regulator for your application. This regulator is much more efficient.

Hi Newton1Law. This power supply will be used for a long period
Iwould like these circuits thanks woodguy

Using national's Webbench. You can create a design. I used 18 to 22 V input and 12 V 1A output. WEBENCH&reg - Choose a Part

National will even customize a kit for you. Some parts you may have to locate on your own.

This is abit overkill, but it can work too: 3-30V 3A Power Supply Kit (K7203)

You can use the 19 V input from your supply or you can just add a transformer. Hammond Industries has good info on their website for power supply transformer selection for unregulated voltages. You would need at least 5 V above the 12 V or you can move the input and only need 3 V. SO, you can shoot for about 17 V unregulated.