Would you do this to a 7812/7912 - diyAudio
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Old 29th February 2012, 06:30 PM   #1
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Default Would you do this to a 7812/7912

This is being sold as a replacement for the "WallWart" by a famous manufacturer:
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Old 29th February 2012, 06:52 PM   #2
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The regs just wont work as the input will fall below Vin(min) causing regulator dropout on every half cycle.

At very least the 1000uFs should be on the other (input) side of the regs.
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Old 29th February 2012, 07:02 PM   #3
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No, I would not. In addition to the previous post, the common mode rejection transformer should be on the input side of the regulators followed by a capacitor, and the inductors to pos common and neg common are just (wrong) asking for oscillations, just tie them to ground.

Last edited by RJM1; 29th February 2012 at 07:20 PM.
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Old 29th February 2012, 07:16 PM   #4
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Quote:
Originally Posted by KatieandDad View Post
At very least the 1000uFs should be on the other (input) side of the regs.
The common mode choke should be on the other side of the regulator as well. (I see you said before I could add a picture of output impedance). This takes a 7812/7912 and turns it into a worse device.
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Old 29th February 2012, 09:36 PM   #5
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:envy:
Lots of postings with readings of an AP...
Is this becoming a standard equipment for DIY?
...must start saving money....
:envy:

Erhm, back to topic:
1. Pretty strange, it also does not appear to be escpecially cheap.
2. Are you sure that the cap on the 79 output is 1uf?
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Old 29th February 2012, 10:40 PM   #6
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Quote:
Originally Posted by ChocoHolic View Post
Lots of postings with readings of an AP...
Is this becoming a standard equipment for DIY?
...must start saving money....
2. Are you sure that the cap on the 79 output is 1uf?
it's 1mF, my bad.

The Z-Out of an LM78xx regulator would be about 8 milli-Ohms @1kHz.

I bought the AP pretty in-expensively, but had it upgraded by selling my 1987 MB. :wink:

I like the line of Inspector Clousseau ("Pink Panther") when he was asked how his wife afforded such expensive clothes on a police inspector's salary: "She saves a little bit out of the household budget."
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Old 1st March 2012, 08:45 PM   #7
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I have to review our household budget.
...confident that this will cover my financial gap for AP-purchase....
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Old 2nd March 2012, 03:51 PM   #8
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More problems -- the 7912 died! -- can't take the stress of all that unfiltered ripple. The choke values are specified as 22uH and 2x22uH, but the common mode choke measures 36uH.

Here's a simulation of what the output is supposed to look like on a 7812 without (green) and with the chokes:
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Old 2nd March 2012, 04:01 PM   #9
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The original Wall-Wart was most probably safety tested and labeled. Often equipment manufactures use Wall-Warts so that they don't have to safety test their units.
Has this commercial replacement been safety tested?
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Old 2nd March 2012, 04:12 PM   #10
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There's not even a manufacturer listed on the device. It says, however: "Designed in the United States and Made in the PRC".

Does not give me a lot of confidence in the state of engineering education in the US.

...and as I am in the "coatings business" in the US, I can smell the organic solvent based coating which would probably be illegal in the US.
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