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Old 9th August 2007, 08:52 AM   #41
Nordic is offline Nordic  South Africa
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Hi andrew I was going to, but was told it would not work...

Still think it should be able to drop 12V to 7V and hold 600mA.
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Old 9th August 2007, 09:31 AM   #42
AndrewT is offline AndrewT  Scotland
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are you using series or shunt regulation?
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Old 9th August 2007, 09:34 AM   #43
Nordic is offline Nordic  South Africa
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Lol Andrew, I'm clever, but not edumicated...

Useing standard 7812 datasheet layout... cap from input to ground/cap from output to ground...
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Old 9th August 2007, 09:51 AM   #44
AndrewT is offline AndrewT  Scotland
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Hi
the standard 7812 with common connected to 0v is a series regulator.

Your PSU with a 12Vac transformer and 12Vdc regulated output should work.

For 600mA output you require at least 1200uF smoothing. But probably better with 4700uF.
Measure the Vac from the regulator input to ground and from the regulator output to ground. Most DMMs operate in this mode, but a few don't. All three of mine from different manufacturers pass this test.
Remove the load and measure the Vac and Vdc at the input and output of the regulator.

The 50VA transformer can supply upto 1Adc from it's 2Aac rated secondaries. If wired correctly this should be OK and should not overheat on 600+600mAdc.


BTW, post 41 should have said "what is series and shunt regulation"
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Old 9th August 2007, 11:28 AM   #45
djk is offline djk
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Drop from FWB=1.4V
Drop from regulator=2.5V
Drop from series pass transistor=.7V

Total drop=4.6V

To maintain 12V out the minimum input voltage needs to be on the order of 16.6V when the line drops to zero every 8.3mS/10mS, 60hz/50hz.

A fudge factor should probably be added for low line voltage.
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Old 9th August 2007, 12:37 PM   #46
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Ilimzn stay calm, I know there are very few knowledgeable persons on this forum who actually understand the Vertical phenomena............

So be patient for these rookies!!!!!!
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Old 9th August 2007, 12:42 PM   #47
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by Nordic
Sorry to bug you, would LM317 be a better candidate for this small drop?
I look forward to your informative post every day....
I was aware of everything you said about the 12v reg, I just didn't think I was that close to its threshold.....
AndrewT and djk gave you some excellent answers. The thing is that most DVMs will give you a false sense of what is going on if you only measure DC. AndrewT has also given youa way to measure the AC ripple, most DVM will work that way, and measure peak to peak ripple, but keep in mind they will show a value that is 1.41 times less, because they are calibrated to show effective voltages for sinewaves, yet measure peak voltages.

Regarding LM317, it has roughly the same dropout performance as the 7812 but yu have the added bonus that it's output voltage is adjustable. If you can't satisfy dropout requirements at 12V output, you can easily try 11V or 10V - not so easy with the 7812
LM317 has several low drop-out variants, most popular would be the LT108x series from linear technology. You can sometimes find these on old computer motherboards (486 and Pentium I). normally they are drop-in replacements for LM317.
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