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LM 7805 and LM 7905 Can I create +/-5V 1A supply using these regulators with single floating 12 V input?

George Rutherford
Prodigy 10 points

I need to create a +/- 5V 1A supply from a single floating 12 V source (the output of a DC-DC converter).  Can I do that with the LM 7805 for the +5V and the LM7905 for the -5V, or will there be a problem defining a "common" for the +/-5V supply, since the input is not referenced to that common?  I found a similar circuit for a variable +/-V output supply that used rectified AC as the input, but that used the center tap of the transformer as the common all the way through.  The circuit is found at http://www.eleccircuit.com/dual-variable-regulator-power-supply-5-25v-by-lm7805lm7905/  , and I can't vouch for its accuracy.

Thanks for your help.

  • 0
    TI__Mastermind 22610 points

    You are correct that you will need to create an artificial ground for the +/- regulators. This would typically be at 1/2 of the input DC voltage. In this case your input voltage is 12V, so the artificial ground will be at 6V.

    This gives the LM7905 a -6V input and the LM7805 a +6V input, with respect to the artificial ground.

    Two issues to deal with ...

    1) These specifications for the regs call for a minimum of 2V from Vin to Vout for proper operation. A floating 15V supply would be better.

    2) The artificial ground will need some active drive to deal with the possible +/-1A that it may need to handle. No simple resistor divider.

    Q: Is this an application that must have linear? There are SMPS solutions that can easily create +/-5V from single positive input.

  • 0 in reply to Donald Jones
    Prodigy 10 points

    Donald Jones said:

    You are correct that you will need to create an artificial ground for the +/- regulators. This would typically be at 1/2 of the input DC voltage. In this case your input voltage is 12V, so the artificial ground will be at 6V.

    This gives the LM7905 a -6V input and the LM7805 a +6V input, with respect to the artificial ground.

    Two issues to deal with ...

    1) These specifications for the regs call for a minimum of 2V from Vin to Vout for proper operation. A floating 15V supply would be better.

    2) The artificial ground will need some active drive to deal with the possible +/-1A that it may need to handle. No simple resistor divider.

    Q: Is this an application that must have linear? There are SMPS solutions that can easily create +/-5V from single positive input.

    Dear Mr. Jones,

    would you be so kind and specify the SMPS that can do that and how.

    Thak you and best regards.

  • 0 in reply to Tilen Thaler
    TI__Mastermind 22610 points

    These are generic constructs for BUCK type SMPS. Details of circuit operation for these circuits is well outside the scope of the Linear Regulator forum.

    Generating the second voltage requires the use of 'coupled inductors' or 'fly-back transformers'.

    One basic format looks like this ...

    The other looks like this...

    If the the output voltages are set to >+/-7V then linear regulators could be used for post-regulation.

    Again, these are generic images. If you have specific circuit questions I do suggest you start a new post over in the Simple Switchers forum where the experts can see it and they will respond.