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Power question - seems simple.

  1. Sep 23, 2010 #1
    Hi All,
    New to the forum. I am a Ham operator and attorney by trade, not an EE, hence my question. I have a bench PSU for my radios +13.8V 15-20A. I want to use this as my source for a project that has been mulling around in my head. But first I need to convert/invert my power source into +/- voltages with 1-1.5A per side (more Amperage would be nice but not necessary). I have looked at the 34063 but the power constraints are too low.

    What are my options?

    Would like to clean the power up as much as possible, but this is not for RF or audio. Also, each rail will be channeled through a voltage regulator circuit (e.g. LM317 or similar). Will this clean it up?

  2. jcsd
  3. Sep 23, 2010 #2


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    Staff: Mentor

    Welcome to the PF!

    You can invert the voltage using an inverting topology DC-DC converter, and then do as you say to use a linear voltage regulator to smooth out the final negative rail.

    National Semiconductor makes the Simple Switcher series of DC-DC converter controller ICs. They should have something that can provide the current that you need. Have a look at their website -- let us know if you aren't seeing what you need.

  4. Sep 23, 2010 #3
    If your +13.8 volt supply is already mains powered you can readily change this to +/- 7volts max. This can even be variable.

    However the zero will not be compatible (at the same zero as your original supply)

    If the supply is mains powered is there any point using an inverter, compared to a second supply? The latter option would be far cheaper and less complicated.
  5. Sep 23, 2010 #4
  6. Sep 23, 2010 #5
    Thanks for the quick replies. My project, whether more complicated or expensive than it should be, is this: to make a portable/detachable variable power supply, with independently variable +/- outputs; using my existing bench psu or even a automobile power socket as the source. It can be expanded upon but that is the gist of it.
    The point here is no so much economics but seeing the idea come to life. Very similar to hobby furniture makers with shops in the garage. The homemade furniture is unique, but always more expensive than something ready made that merely offers the same functionality. Of course, that ideal does have limits.
    Last edited: Sep 23, 2010
  7. Sep 24, 2010 #6


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    Staff: Mentor

    Okay, then you do need an inverter unit to give you the negative rail. Then follow the positive and negative rails with variable linear regulators for simplicity and low output noise. You can look at the LM317 for the positive adjustable regulator (in a big package with heat sink), and the LM337 for the negative adjustable linear regulator.
  8. Sep 24, 2010 #7
    So allowing for inefficiencies you will be drawing around 25 watts for each of the + and - supplies.

    This is certainly achievable with portable inverters.
    You can easily get 12 volt DC to 120 or 240 v AC invertors at 50 watts.

    Since you haven't yet specified, your next decision is the voltage range required, since you have specified variable supplies.

    To use a 317/337 you will have to step such an invertor down as 120 volts+ is too high for them. Lower voltage inverters are less common.

    You should also realise that the 317 / 337 will not adjust all the way to zero without extra circuitry.
  9. Sep 24, 2010 #8


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    Staff: Mentor

    Sorry, by Inverting DC-DC, I meant just invert the + voltage into a - voltage rail:

    http://www.national.com/cat/index.cgi?i=i//310 [Broken]

    Not the type of inverter that takes DC and makes mains-level AC.
    Last edited by a moderator: May 4, 2017
  10. Sep 24, 2010 #9
    Yes I am aware that you can also get DC - DC static inverters.
    It's just that such devices to supply 4 amps+ can be quite costly and offer less flexible voltage options.

    Despite the 'Mr Chequebook' approach from the OP I am still considering cost.

    Added to which such design considerations are part of the fun of the project aren't they?
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