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Old 25th August 2004, 03:14 AM   #1
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Default 12vac to 12vdc what all do i need

this subject has been troubleing me for weeks i had just bought a new transformer that converts 115 volts ac to 12 volts ac , so my question really is do i just need a rectifier or do i also need other things

if you could help that would be awesome thanks
you can aso e mail me at Big_Tires@hotmail.com
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Old 25th August 2004, 07:12 AM   #2
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A bridge rectifier and an output smoothing capacitor may be all you need. How much current do you need to supply?
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Old 25th August 2004, 08:37 AM   #3
hjelm is offline hjelm  Sweden
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If you have 12VAC RMS from the transformer you will have some 12*1.41 -1.4v after the rectifier and caps. ~15.5 volts depending on how much current you consume.
If you want to have 12V from this i recommend to use a three terminal voltage regulator (like a 7812) that takes the 15.5 down to 12 V, the regulator might need a minimum current draw so check the datasheet of the regulator and the thing you want to supply power to. You need another cap after the regulator as well.

78XX datasheet Contains a typical application, see top of page 8.

If you have 12V peak then a bridge rectifier and some caps will give you less than 12 volts due to the voltage drop over the diodes.

It all depends on what you want to do with it. There are more advanced solutions as well.

Edit: Just noted that this thread is placed in a strange location shouldn't it be moved to some other location???
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Old 25th August 2004, 05:48 PM   #4
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Thanks for the help im going to power 2 computer fans to cool my home amplifier, but i tested my transformer and it puts out 14.7 volts.
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Old 1st September 2004, 11:32 PM   #5
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Computer fans will not last long with overvoltage due to the way it is constructed. The coils will burn up. A 12VAC transformer will output 18V peak, this is what you will see on the filter caps with no load. Slightly less depending on current drain. You should make a regulator for 12V. If you want to keep it really simple, use a 7812 Voltage regulator IC. Or you could use a 13V zener diode as a reference and amplify this reference with a darlington BJT, emitter follower, Ve will be at 12V. Either will work just fine.
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Old 1st September 2004, 11:33 PM   #6
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oooopss my bad....7812 was already mentioned. BTW LM317 adjustable regulator will work also.
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