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Old 25th August 2013, 04:35 AM   #1
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Default Newbie Issues with 12V PS

I am building my first power supply. I started with a very small one intended on powering a small opamp-preamp. I am building the power supply separate from the preamp board as this is a huge learning excercise for me. This is actually V1.3 as I have had two "issues" where it was just easier to start over.

Anyhow, I got mine pieced together. I am using a 1.6A rectifier bridge, then two 470uF/35V caps (one per side), then a 7812/7912, then another two 470uF/35V caps to the output of the power supply.

When I tested it, I got 11.64Vdc and -12.06Vdc.... I assume the output on my transformer is equal, or would almost have to be. Voltage coming off of the rectifier is equal (+/- 14.6Vdc).

My first question, shouldn't the voltage be the same? Second question, what is acceptable variance/tolerance between the two sides?

Thirdly, if you read something that strikes you as ignorant or outright wrong, that is a very distinct possibility. However, please feel free to point out the error of my ways. I am very interested in learning as much as I can.

Thanks!
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Old 25th August 2013, 05:39 AM   #2
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If you read the Datasheet for the 7812/7912 you will find that there must be a minimum voltage across the regulator for it to work. This is known as the "dropout voltage".

IIRC this is about 5V for the 7812. In which case you are feeding the regulator too low a voltage for it to work correctly.

In other words it must have a pre-regulator supply of at least 17V.

Last edited by KatieandDad; 25th August 2013 at 06:09 AM.
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Old 25th August 2013, 06:29 AM   #3
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Reading up on the newer 7812's the dropout voltage is 2V min. Regulation is +/- 2%.

So ideally you want about +/- 15V to drive them. Output should be 11.76 - 12.24V.

Usually one would use a 12-0-12V transformer with them.

The other issue commonly ignored is minimum load. The 7812 works better with a minimum load. An LED at 10mA ie with a 1K resistor across each of the 7812 and 7912 is ideal.

Last edited by KatieandDad; 25th August 2013 at 06:32 AM.
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Old 25th August 2013, 06:42 AM   #4
nezbleu is online now nezbleu  Canada
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I have always found the 79xx regulators fussier about having a few mA through them to make them regulate. Either way, you need to draw some current through the regulators. As K&D said, you are borderline in terms of dropout voltage. Motorman, read the datasheet for the regulator IC's. They usually have some nice sample schematics, which include things like protection diodes (so turn-off transients don't destroy the regulator or downstream circuits), and minimum requirements to make everything stable.
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Old 25th August 2013, 08:23 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by nezbleu View Post
I have always found the 79xx regulators fussier about having a few mA through them to make them regulate. .......
this difference is usually referred to in the datasheets.
Most 79xx require a minimum of 10mA.
Most 78xx require a minimum of 5mA.
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Old 25th August 2013, 05:58 PM   #6
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Thank you, all, for the advice. It looks like the diodes are a really good idea too. Sad that I thought I found the perfect chip amp transformer that had dual secondaries of 20-0-20 and 12-0-12. Will now probably just buy a transformer and move on. Thanks again!
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Old 25th August 2013, 06:46 PM   #7
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I don't see cause to give up on the transformer. Since it has center-taps, you could replace the bridge with a full wave rectifier and gain a diode drop on each rail. That should put the input to the regulators >15V. Then make sure the preamp at idle exceeds the minimum load for the regulators, and incorporate other recommendations.
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