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Old 23rd October 2005, 07:16 PM   #1
hbarki is offline hbarki  Indonesia
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Default Power transformer voltage question

I think I chose the wrong power transformer for my power supply. It is 35VA, 2x22V parallel secondaries, which should give me about 1.4 x 44 = 61 volts of output going to the 7812 and 7912 regulators. My filter caps are rated at 35V, and I want to have a +/- 12V output from the regulator. I think 61V will blow up the regulators and caps.

Other than buying a new transformer, is there any way I can reduce the output AC voltage coming from the secondaries?

Thanks for your help!
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Old 23rd October 2005, 07:39 PM   #2
tool49 is offline tool49  Canada
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Don't forget that each regulator should only see half this voltage so each should only see 30.5V. It is still a bit high though. Technically the LM7812 is good for up to 35V so you should be alright, just make sure it has enough heatsinking.

What you could do is, if you are too worried, use a LM317/LM337 combo instead as these only care about the differential between your input and output. In your case, 30.5-12=18.5V which is way inside the range allowed by the LM3x7 regulators.

Hope this helps!
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Old 24th October 2005, 07:41 AM   #3
AndrewT is offline AndrewT  Scotland
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you could try two alternatives

1. Add a preregulator with it's output referenced to the final voltage not to ground. See Jung and Borbely (I think it is the super reg).

2. Build a CRCR PSU before the regulator. It will drop a significant voltage before the regulator and when current increases it drops it even further so reducing power dissipation on the regulator.

B.T.W. Tool49 is right, each 22Vac winding after rectification and smoothing will give about 30Vdc. For regulated 12Vdc you should use a 12Vac to 15Vac transformer. By using a higher voltage you are throwing away a large part of the VA rating that you are paying for (space and price). But if you use either or both of the two alternatives you do gain a bit back in return for the excess voltage.
regards Andrew T.
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Old 24th October 2005, 07:53 AM   #4
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If you have two 22VAC windings put them in series and ground the mid point. A full bridge rectifier would give you just what you need.

Jan Didden
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