You need to calculate the current draw of the 5 fans, then refer to the datasheet for the 7812. 7812's are designed to deliver 1 amp. I just checked one example of a 12vdc 120mm fan that uses about 5 watts, or about .4 amps. You would be doing well to get two of these fans in your circuit with the 7812. And if you have any other devices or circuits depending on that regulator you will run into problems.
The same considerations need to be accounted for with the 5 volt regulator. What are you planning on powering with it? Is this for a CD ROM? What is the power rating for the drive?
If I may cut in for a sec with my limited experience and ask a question...
Why are you regulating the voltage coming from the 12V taps on the transformer that will then go into a 12V fan? Seems to me all you would need is a high-current full-wave bridge rectifier and maybe a capacitor.
Like Audioguru said, I don't think the fans will care if the power they see is not pure
im using a 15v transformer not 12v that a circuit i found on the net
i just seen some 120mm fans 12v 0.25amps **broken link removed**
what power supply will i need to run 5
Something you should consider. It is great and all to purchase lower power fans to accomodate a simple regulator circuit, if the cooling capacity you require isn't critical. I recieved a PM that seemed to indicate that you have a traget temperature and a defined area that you wish to cool. Lower power fans will in turn move less air. Fans are rated by how many cubic feet,meters, etc of air they move per minute. This is intregal to the rate of heat transfer and exchange in this type of system. Have you calculated all of this or are you guessing?
When you rectify a transformer with a bridge rectifier then the output is humps of voltage at double the mains frequency. The peak voltage is 1.414 (the root of 2) times the RMS voltage of the AC. The voltage swings from 0V to +17V minus about 2V for the rectifiers for a 12V transformer. If you add a large enough filter capacitor then you will have +15V that is fairly smooth.hi
i was thinking of using a 12v 50va transformer and put it into a bridge rectifier will it need smoothing out with any capacitors?