power supply advice

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mart_1986

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hi there

im planning to build a power supply 5v and 12v fixed whats the best circuit to use i found this one on google

**broken link removed**

how many 120mm computer fans will the the lm7812 run i need it to run about 5
 
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?
 

5v is for a fone charger

so what regulator will i need mate
 
Maybe an LM1085-12 would be sufficient for your needs.

But this opens a can of worms. You're going to need to make sure your rectifiers are adequate as well, and your transformer.

Also consider that if the 7812 is more convenient, you could parallel a couple of extra regulators, but you need to understand that extra circuit components will be needed to make it work. One regulator will try to "take over" and carry the load without components to help spread it.

Refer to this tutorial:
High Current Voltage Regulation - Electric Circuit
 
LM340T5(5v) or T12(12v) positive regulators are in TO-3 packages. Case is the ground lead so a heat sink needn't be insulated. Most times rated at 5 amps. Some 78xx in TO-220 packages are rated as low as 500mA. Be sure to check the data sheets for the IC in hand.
 
use a transistor as shown with the regulator to get more amp as you want.
 

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mbarazeen said:
use a transistor as shown with the regulator to get more amp as you want.
Click to expand...
An emitter-follower at the output of a voltage regulator ruins the voltage regulation.
But I guess the fans will not care.
 
Last edited:
i just seen some 120mm fans 12v 0.25amps **broken link removed**

what power supply will i need to run 5
 
Last edited:
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
 
mart_1986 said:
im using a 15v transformer not 12v that a circuit i found on the net
Click to expand...

Then in this case I would recommend that you follow the advice above and try using two or three 78XX regulators.
Doing a quick search Jameco.com offers a couple regulators that can handle more than 1 amp.

Using the 12V @ 1.5A regulator should allow you to power 2-3 fans per regulator. The reason I only suggest 2-3 is that from my understanding you want to allow some capacity for a slight amp spike at fan start up.
 
mart_1986 said:
i just seen some 120mm fans 12v 0.25amps **broken link removed**

what power supply will i need to run 5
Click to expand...


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?
 

im just guessing the reason i was going to use these sort of fans is because i need to get fresh air in to my pigeon loft with out cause air drafts but allows me to get fresh air into the loft and not beening to noisey as these will be mounted out side which wont have access to rain

im thinking of having them on a therostats so i can try and maintain 21c but as long as im getting the fresh air thats the main thing
 
use more than 2A diodes in series with 15V to drop it to 12V. i hope 4 or 5 such doides can solve your problem. it may give you 2.5- 3.5 V voltage drop depends on the slection and can easily work for 5fans of 0.25A.
 
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?
 
mart_1986 said:
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?
Click to expand...
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.
 

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