7809 after a 7812 ???

Discussion in 'General Electronics Chat' started by DUFFER, Aug 14, 2013.

  1. DUFFER

    Thread Starter New Member

    May 3, 2013
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    Can you sucessfully operate voltage regulators in series?
    I want regulated 12v and regulated 9v. I have a regulated 12v supply (plug in adapter).
    If subject has been covered, I'm sorry but I can't find it
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Should be fine.
     
  3. WBahn

    Moderator

    Mar 31, 2012
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    Sure, as long as the 9V regulator will operate from 12V.

    On advantage to doing it this way is that the noise on the 9V output will almost certainly be considerably less than it would have been had you not done it this way because it is starting out only having to deal with the noise that made it through the 12V regulator.

    I routinely did this on test systems where I might use a commercial bench supply to give me ±18VDC to my board. Then regulate that to ±15V and filter the bejesus out it before and after, and finally regulate that to ±12V. The end result was significantly less noise in my analog circuits than if I used the bench supply directly (and it was a pretty clean supply) and I was surprised how much difference using the intermediate ±15V regulators were compared to just coming into the board with ±15V from the bench supply.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Another reason in favor of this approach is that voltage is dropped over two devices instead of one, distributing the heat.
     
  5. WBahn

    Moderator

    Mar 31, 2012
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    Another trick I'll do is to put a resistor in series with the input and size it so that at whatever current I decide is my max current I want to support and whatever the lowest value I want to allow for as the nominal input voltage that the resistor will drop enough voltage to place the regulator just above its drop-out voltage.

    This means that as I pull more current from the regulator, the resistor will automatically shift a good portion of the heat to itself and it serves as a decent fail-safe short-circuit protection. But it does add noise to the regulator's Vin. Can't have everything.
     
  6. DUFFER

    Thread Starter New Member

    May 3, 2013
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    Thanks to all. I have been out of the game for 40 odd years and the learning curve is kinda steep but it's like being back in the 60's
     
  7. WBahn

    Moderator

    Mar 31, 2012
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    But not the good part, I wager! :rolleyes:
     
  8. DUFFER

    Thread Starter New Member

    May 3, 2013
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    My hands feel like I am wearing baseball gloves and my eyes think I am trying to read something a mile away. I'm no longer up to wild sex and drugs but damn it's fun to be back messing around again and I'm glad I got back in before everything is SMD. (When I'm dead bugging, it's kinda hairy and I still can't see the need for low voltage soldering irons if the tip is grounded). Frustrations abound, but everyday you wake up breathing is a good day. Thanks again all.
     
  9. ScottWang

    Moderator

    Aug 23, 2012
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    The similar thread like this.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    The problem with that approach, if you are going to use the different voltages for things, is that noise on the lower voltages feeds up to the higher voltage supplies, though with some filtering. Since your lowest supply is probably your digital supply (i.e., your noisiest circuit) and your analog supplies (i.e., your most noise-sensitive circuits) are higher up, you have a good likelihood of coupling digital noise into your analog circuits.

    I would take a tree approach. Come in with, say, 19V and regulate that to 15V. Then power two regulators from there, one of which works down to your analog supply voltage and the other that works down to your digital supply. Of course, it would be better to come in with the 19V and tree off from that point with two 15V regulators. Classic cost/performance tradeoff.
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If you are no longer up for wild sex you need to switch to different drugs.

    I actually prefer SMDs when prototyping. WTF to do with those long leads all over my pretty breadboard?
     
  12. LDC3

    Active Member

    Apr 27, 2013
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    Do you use a soldering iron or wave soldering?
     
  13. DUFFER

    Thread Starter New Member

    May 3, 2013
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    If you think drugs can make up for lack of muscle elasticity and joint stiffness then you obviously didn't LIVE through the sixties :D

    Are you useing adapters for the SMD's to breadboard them ? As for messy wires, if they don't destablize or degrade the circuit so what.
    FUNCTION OVER FORM. If you need pretty, save it for the case.
    With my only slightly shakey hands, I still need interlead isolators for soldering SMD's but ADVERSITY IS GOOD FOR THE SOUL.
     
  14. ScottWang

    Moderator

    Aug 23, 2012
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    The home experiments did not like that bad, but if I have some situation as you said, I will using different power sources, as (1)±5V,(2)±9V,±12V,±15,(3)50V,did you see the linked page and the power what I made.
     
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