To get -30 from +30V supply

Discussion in 'General Electronics Chat' started by sharmilla, Dec 8, 2008.

  1. sharmilla

    Thread Starter Member

    Nov 3, 2008
    17
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    Hi all,

    anyone could suggest me, on how to get - 30V from a +30V supply.

    Many Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    If you only need a single supply, simply reverse the leads. That could make a ground reference interesting, depending on the power supply.
     
  3. sharmilla

    Thread Starter Member

    Nov 3, 2008
    17
    0
    Is it attached....
     
  4. Dominick

    Member

    Dec 6, 2008
    28
    0
    What are you powering?
    Anything Iv ever seen that had a -volt had a ref ground
    +30 -30 about 60 volts + to - correct?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Reversing the leads will reverse the voltage polarity relative to ground..
     
  6. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    Yes this can sometimes be a concept difficult for a beginner to get their head around. An isolated (transformer coupled) fixed DC power supply just supplies current at a specific potential (voltage) difference. The concept that it is a + or - DC supply all depends on which terminal is going to the circuit common of the application circuit.

    So for say a general purpose analog op-amp type circuit requiring both positive and negative voltages and a circuit common, one can use two identical isolated (transformer coupled) power supplies. One supply would have it's positive terminal wired to the breadboards common (ground) connection and the other supply would have it's negative terminal wired to breadboard common. The opposite terminal on each power supply would become the circuit negative and positive voltages respectively.

    This does not apply to non-isolated DC power supplies where one terminal is hardwired to a power common, (or neutral) connection and you are not free to swap polarities simply reversing connections to your application.
     
  7. sharmilla

    Thread Starter Member

    Nov 3, 2008
    17
    0
    Please find as attached.. I have attached two circuits in it.
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    It looks like you are drawing a P-channel Mosfet. That has an intrinsic diode from drain (anode) to source (cathode). The top circuit should not have worked, becasue the diode would be forward biased and always on.

    The bottom circuit is the correct orientation, but you may have problems turning off the gate with that configuration.

    John
     
  9. sharmilla

    Thread Starter Member

    Nov 3, 2008
    17
    0
    Im sorry error in the diagram...both the mosfets are Nchannel mosfets...IRFP460....ok so for the bottom circuit i have to run it using a P-channel mosfet if i provide a - voltage ...

    and for the top circuit i have to use N-channel mosfet if i provide a + voltage

    am i rite ...

    many thanks
     
  10. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,699
    907
    If the bottom circuit's mosfet is N-channel, then its diode is forward biased. To avoid getting a lot of confusion going here, I think you need to redraw the schematic you intend to use, including the gate drive, then proceed with trying to make it work.

    John
     
  11. sharmilla

    Thread Starter Member

    Nov 3, 2008
    17
    0
    dear all,

    I redrew the circuit as attached... is this correct ...
     
  12. thingmaker3

    Retired Moderator

    May 16, 2005
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    The gate signal MUST be of the correct polarity and magnitude with respect to the source. Please show your gate drive circuit.
     
  13. sharmilla

    Thread Starter Member

    Nov 3, 2008
    17
    0
    Hi the gate drive signal is similar to as attached ..please advise
     
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