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±5V from up to +12v

Discussion in 'Electronic Basics' started by Daniel Pitts, Nov 4, 2013.

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  1. Daniel Pitts

    Daniel Pitts Guest

    I'm trying to design a remote controlled volume controller, and the chip
    I've chosen to control the volume needs ±5v for the analog side of the
    circuit, and +5v for the digital.

    I have several options for input voltage, mostly based on old AC
    adapters I have laying around. In particular I have a "selectable"
    adapter which can select up to 12v, but I may have others with higher
    voltages (I haven't looked through them recently).

    One idea I had was to chain 7805s together:


    +12v connected to VinA & VinB

    GndIn connected to GndA

    VoutA connected to GndB

    If I understand correctly, I could then use VoutA as my reference point,
    making GndA -5v and VoutB +5v.

    If I do this, am I going to let the magic smoke out? Is there an easier
    way to do what I'm trying to do? It seems like most ±5v chips I've found
    require a bit more complicated external components, and are far more
    expensive.

    Thanks,
    Daniel.
     
  2. Tom Biasi

    Tom Biasi Guest

    It won't work like that. The 7805 needs the ground connected to the
    minus with respect to the positive input.
    You may be able to rig up something with a 7805 and a 7905.
     
  3. Daniel Pitts

    Daniel Pitts Guest

    The VoutA would be -7 with respect to the +12v, so the VinB would be at
    a at the right place respective the +12v.
     
  4. Tom Biasi

    Tom Biasi Guest

    Hook it up and get back to us.
     
  5. Daniel Pitts

    Daniel Pitts Guest

    I could probably calculate it, but I don't yet have that, no.

    If I don't have the appropriate "splitter", what will happen? Will I be
    drawing more current across the regulators, or will they just not
    regulate reliably?

    Is there a better approach than this? One or more DC to DC converters
    perhaps? I'm open to alternative suggestions as well.
     
  6. Daniel Pitts

    Daniel Pitts Guest

    I was planning on using only one +12VDC adapter.

    Here's the diagram of what I was thinking:

    7805(A)
    +------+
    | +5|------>+5
    +--|Vin |
    WW | | 0v|--+
    +-----+ | +------+ |
    |~ +12|--+ 7805(B) +--->GND
    | | | +------+ |
    |~ 0v| | | +5|--+
    +---|-+ +--|Vin |
    | | 0v|-+---->-5
    | +------+ |
    | |
    +----------------+


    So, looking at this, I think I saw your earlier point about needing the
    splitter...

    Well, I think I do, but every time I try to write it out, I can't figure
    out how to phrase it.
     
  7. Tom Biasi

    Tom Biasi Guest

    Yes, I thought of that but the OP said he had higher voltages available.
     
  8. Jasen Betts

    Jasen Betts Guest

    b
    +12 in -+--------[7805]-- +5
    | |
    | a |
    `---[7805]-+--- 0V
    |
    |
    gnd in -------+-------- -5

    that will work, but only if the -5 half has a larger load than the +5
    half, you might need to add something there (a resistor?) to make sure.
    if you have at-least one of the DC-DC convertsers with an isolated output you
    can stack their outputs
     
  9. Jasen Betts

    Jasen Betts Guest

    What advantages would that give over the stacked 7805 topology,
    It seems to me you'd have twice the hassle keeping the 0V where you
    want it than you have with the stacked 7805 topology,



    With two 7805s where you can do something like this.

    ---------+-----+----------[7805]---------- +5
    ^ | | |
    | |e [100] |
    | _\| | |
    Q1 |---+-[7805]---+-+---+----+---- 0V
    + /| | | | |
    12V |c | [1K] | |
    - | | | |/ |
    | | +---| |
    | | | | |\| |/
    | | | |/ e+--|
    | `--[1K]--------| | |\|
    | | |\| [1K] e|
    v | e| | |
    -------------------+------+-----+----+----- -5
    Q2 Q3 Q4

    Q1 eg: TIP42
    Q2,Q3 eg: BC547, PN2222, or 2N2903 etc
    Q4 eg: TIP41
     
  10. As others have said you've got issues with the current flow.
    I did something similar, splitting 48V to +30, GND, -5.
    But I still needed to know the direction of current flow into the 'Gnd' terminal... in my case it was always a current sink.

    Can you use LM317's and LM337's? Then I used the power supply negative lead as -5V, used the 337 to make a 'ground' 5 volts above that, and the 317 for the +30. You may be able to do something similar.. or if bi-directionalcurrent in the 'ground' then some sort of load resistor to the appropriaterail... (as others suggested.)

    George H.
     
  11. Or make the 12 V into 10 Volts, and then a power opamp as rail splitter.
    (I've got these TCA0372's in my parts box waiting for a project.)
    I guess it depends on what sorts of currents are involved.

    George H.
     
  12. Phil Hobbs

    Phil Hobbs Guest

    Careful with those--they work great, except that the heat sinking is
    quite a bit less great. The best package runs 22 K/W junction to
    "case", whatever that means in a plastic-encapsulated package with no
    power tab. That's quite a bit considering that the chip can dissipate
    over 40W before thermal shutdown kicks in. (Steady-state thermal
    transfer calculations would put the die at about 900C with an infinite
    room-temperature heat sink--a nice bright red-orange glow.)

    They run warm to the touch just with their quiescent current.

    As a rail splitter, you have to watch out for the chip hitting thermal
    shutdown, which will probably make one rail collapse. A couple of beefy
    Zeners on the output would help prevent Joergish noises.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  13. Grin, Hey, the TCA0372's are burning a hole in my parts bin :^)
    (maybe the OP only needs ~100mA)
    I must admit when you (or someone else?) pointed out these 1 amp IC's in a dip8 package I was a bit 'concerned'.
    (I'd really like a nice to-220 style metal tab.. or a surface mount with 'power pads' on the bottom.)

    George H.
     
  14. Ian Malcolm

    Ian Malcolm Guest

    The other replies have covered in some detail the disadvantages of
    starting with +12V DC in.

    Do you have any adapters with AC output between 8V and 12V (RMS)?

    If so, a much better approach would be to half-wave rectify the AC with
    a diode and electrolytic reservoir capacitor for each supply to get
    positive and negative unregulated supplies with a common 0V, then using
    7805 and 7905 regulators respectively to get your regulated +/-5V
    output.

    A 8 or 9V AC supply is ideal for +/-5V output, but higer voltages can be
    used although the regulators will run hotter. Component choices depend
    on input voltage, load current on each output and mains supply
    frequency, so tell us what AC output supplies you have available
    (nominal voltage and current + measured output voltage unloaded) and the
    required output currents so we can help you pick the best one.

    If you dont have any AC output adapters, the use of a DC-DC converter
    module to get a negative rail has already been mentioned. There are a
    few issues with ripple on the output, and potentially with power supply
    sequencing so you had better tell us which volume control chip you
    intend to use.
     
  15. Daniel Pitts

    Daniel Pitts Guest

    [snip]

    Is there a program to interpret that script? Hopefully one that runs on
    Mac, DOSBOX, or the web?

    Thanks,
    Daniel.
     
  16. LTspice IV download from www.linear.com/designtools.

    petrus bitbyter
     
  17. Daniel Pitts

    Daniel Pitts Guest

  18. Daniel Pitts

    Daniel Pitts Guest

    Thanks for all the suggestions everyone. I see how my original thoughts
    were flawed.

    I also realized I should probably get a better idea of what my amperage
    will be. I'm guessing <100ma, but I'm not entirely sure.

    I'm also considering using just a pair of digital POTs instead of this
    fancy schmancy volume control IC. That way I only need +5v, which is a
    much easier problem for me to solve ;-)

    Thanks again,
    Daniel.
     
  19. Daniel Pitts

    Daniel Pitts Guest

  20. Kaz Kylheku

    Kaz Kylheku Guest

    Idea:

    * Use a 5V (unregulated) adapter. Regulate to 5V with a 7805.

    * Use a charge pump IC to generate a -5V rail from the regulated +5V rail.
    You can get a center-tapped transformer for a couple of dollars, or rip it out
    of something. You can build your own dual-voltage AC adapter easily with a
    center tapped transformer and bridge rectifier or discrete diodes. Your device
    then just has to provide the reservoir capacitors and regulators.

    Why not find some discarded old device (e.g. piece of audio gear) which has
    a dual supply in it already and re-use its power entry and perhaps more.
    Throw out the circuit board and build your own device in its place.

    You can also use two identical AC adapters to get the equivalent of a
    center-tapped transformer, if you can live with the clunkiness.
     
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