Floating Power Supply?

Discussion in 'The Projects Forum' started by nigelmercier, Oct 9, 2010.

  1. nigelmercier

    Thread Starter New Member

    Oct 9, 2010
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    I need a floating 6V supply, fed from a nominal 12V (10.8V -14.4V). Not sure what the current drain is, but the device normally uses 4x AA batteries.

    Has anyone got a simple circuit to do this?
     
  2. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    I think by using a LM7806 voltage regulator you can achieve this, but be aware that the ground will be common and not independent (since you want the power supply to be floating). No problem if your input supply is floating.

    You can build the circuit yourself if you read the datasheet. There are lots of datasheets on google.
     
  3. nigelmercier

    Thread Starter New Member

    Oct 9, 2010
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    So not floating then.

    Next!
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Actually it is. The vast majority of wall warts are not grounded, so by definition they are floating. The regulator at this point becomes unimportant, whether it is positive or negative. At the output pins you can not tell the difference.

    Any power supply using a transformer and not feeding ground through is floating. This applies for the majority of variable power supplies.
     
  5. nigelmercier

    Thread Starter New Member

    Oct 9, 2010
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    There is no "wall wart" involved. I want a floating 6V from 12V, as per my OP.
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When the 12 Volts and the 6 Volts have the - or + connection in common, the voltage converter does not need to be floating.

    Can you draw a set-up how things need to be connected?

    Bertus
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    You are making a distinction that doesn't exist. Ground in the case of an electronic circuit is not the kind of ground you are referring too. You didn't say, but if the 12VDC you are feeding is grounded the job gets a lot harder, and probably over your technical abilities. My assumption (since you did not specify where or what the source voltage is coming from) is it is floating.

    A wall wart is floating, it meets your requirements, unless it is one of the rare types that bring the wall ground to the DC side. These are extremely rare, and easy to tell because it means there is continuity between the ground lead on the AC outlet.

    Since you didn't specify, and expect us to know without telling us the details, we are going to have to wait for those details before anyone can definitively help.

    The ground referred to on chips have nothing to do with the ground you are thinking about. They are two different things. Cumsoftware gave you the straight scoop, if you bothered to listen. The chip he named would work, a simple 3 pin device with some extra capacitors.

    Any power supply that plugs into a AC wall socket is probably floating. Looking at the internal schematic there will be a ground labeled, which has to with convention in electronic schematics and not whether it is floating.

    A positive or negative regulator will work, and one of the 3 pins will be labeled ground. This doesn't mean squat for your application, either type will work, the output will still be a positive and negative output and no ground.

    Exactly what do you think ground is?
     
    Last edited: Oct 10, 2010
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,669
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    Bill, this
    looks like the power source is a 12V battery, not a wall wart.
    nigelmercier: if you really need it isolated, you need a flyback SMPS.
     
  9. joaquim

    New Member

    Apr 12, 2010
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    Why make such an easy question into a science fair? Buy a transformer with a push switch, they come with different currents, but if it's a battery operated PC-Speaker or whatever, 500mA will do... you can buy those almost everywhere, "Bilteam" or "Elfa" if you lived in Sweden or "Scandinavia" - Norway, Denmark, Sweden, well let's count Finland too as part of Scandinavia, it's been apart of Sweden since the 13:th hundred until 1809... they still learn and speak Swedish in north of Soumi(SF), I just had to mention that... :D - settings are often 3, 6, 9, 12 and 15V. The cord is marked with "+" and " - ", connect right pole to right cord, set the right voltage and you're good to go! :D
    and all for only 6 - 8 Euro...
     
    Last edited: Oct 10, 2010
  10. Wendy

    Moderator

    Mar 24, 2008
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    Where's the fun in that? Besides, a LM7806 is around a $1, and will handle up to an amp. The type of power supply your talking about is neither regulated nor has much current, both bad things.
     
  11. nigelmercier

    Thread Starter New Member

    Oct 9, 2010
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    Yes, and yes. I thought that my orginal question was clear enough, but it seems others think they know what I need better than I do! :D

    So, has anyone got a schematic for a flyback SMPS?

    Input: nominal 12V (10.8V -14.4V)
    Output: nominal 6V (5V - 8V)
    Current: around 100mA
     
  12. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Actually you didn't provide all the details at the start and those hunting guesses are why the first responses would have been better to ask you for more details. So no you you could have been a little better with your clarity.

    Don't gripe about the answers because it all starts with your question. Better questions get better answers.

    A small switching power supply is not going to be that tricky, since efficiency is not going to critical.

    100mA is way over what you were likely to get from the 4 AA batteries unless they were draining within 15 minutes.
     
  13. nigelmercier

    Thread Starter New Member

    Oct 9, 2010
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    I have learned over the years that if you give the full details of a requirement, then most of the replies will be "You don't want to do it that way ...". Well, yes I do!

    I gave the exact requirement without complicating it: a floating power supply for 6V from 12V. I assumed that readers of this forum would understand what I wanted: a 6V feed in which neither output is connected to either input.

    It was those who assumed I didn't know what I wanted who went off on a tangent :rolleyes:

    As with most current measurements, different meters give different values. The 100mA comes from my bench PSU, which registered 0.10A, but measuring from the existing wall wart (which is no longer needed, hence the original question) shows around 10mA. I went for the higher value.
     
  14. tom66

    Senior Member

    May 9, 2009
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  15. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Here's a good resource for you:
    http://www.dos4ever.com/flyback/flyback.html

    [eta]
    Look at the flyback converter, Figure 16, towards the bottom of the page.

    The transformer will need to be 1:1 instead of 1:10; 100uH on both sides should do it.

    You'll also need to use an optocoupler/optoisolator to provide the feedback.
     
    Last edited: Oct 10, 2010
  16. David Bridgen

    Senior Member

    Feb 10, 2005
    278
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    A good question.

    Most people who use this and the other electronics forums should be asked.

    They might learn from this http://www.davidbridgen.com/earth.htm

    .
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    Problem is, it is already convention. I learned that way in college, and most people know it that way.

    I tend to agree, but convention rules.

    When speaking of electronics, ground is the common point by which all other voltages and signals are measured. It doesn't have anything to do with floating, until a second power supply is introduced.

    Having tried to help the OP, there really isn't anything I can offer. Even SMPS have a common point that is usually labeled ground.
     
  18. David Bridgen

    Senior Member

    Feb 10, 2005
    278
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    Exactly Bill.

    Most people are taught incompletely and inexactly. Even the instructors are confused.


    .
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    No, convention is what it is. They are not confused, they are teaching an already established language to their students so they can talk with others. Languages are usually imprecise, which is why we are always asking for schematics and other info (which noobs rarely understand why it is needed).

    But enough of this hijack, for this is what it is. If you want to pursue the conversation you need to start another thread. Maybe we can pick up on the one we had several years ago? I do remember you and your arguments then. Neither of us have changed our stances.
     
  20. nigelmercier

    Thread Starter New Member

    Oct 9, 2010
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    I know exactly what ground is, which is why I did not refer to it in my OP :D

    As explained elsewhere, my requirement is a 12V/6V DC-DC converter with a floating output: in other words there are no common connections between inputs and outputs.
     
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