Negative P/S

Discussion in 'General Electronics Chat' started by DexterMccoy, Mar 20, 2014.

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  1. DexterMccoy

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    Feb 19, 2014
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    This Op amp is after a Power supply regulator that is a positive +10 volts

    The op amp inverts the +10 volts into a -10 volts to make a negative supply

    My question is what does Q7 do? what function does it do in the circuit?

    What kind of circuit is this called please, the op amp and Q7 combination to turn a positive voltage into a negative voltage?
     
  2. DexterMccoy

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    here is the schematics
     
  3. magnet18

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    Dec 22, 2010
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    Q7 is a buffer, it provides more current to whatever is down the line than the op amp could, the op amp would normally only be able to provide a couple mA, a transistor can provide much more

    the opamp is in a negative feedback inverting setup
    the transistor is a buffer

    I would call it a "negative feedback voltage inverter with a current buffer", I don't know if it has an official term
     
  4. DexterMccoy

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    Thanks for the help

    But the Q7 buffer has no biasing resistors, it's direct to supply voltage on the collect and direct on the emitter to the output

    So the Q7 is not biased? any reason why

    I thought OP amps outputted more current than a transistor did

    Plus the Negative supply rail is not regulated, only the Positive supply
    I have seen this type of stuff often when the negative supply is no regulated but the positive supply is, any reason why?

    I need supply doesn't source current? only the positive supply sourced current more?
     
  5. ErnieM

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    Apr 24, 2011
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    It is biased by the nature of the op amp feedback. The only thing missing is anything to limit current in case of a fault downline.

    Typically it is the other way around.

    Necessity. Cost.
    Positive sources, negative sinks. That's typical in most supplies. Offhand I do not know of a positive regulator that sinks or a negative regulator that sources.
     
  6. BillB3857

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  7. DexterMccoy

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    How is the transistor biased by the op amps feedback?

    How would you limit the current in cause of a fault?
     
  8. magnet18

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    transistors have a current gain
    Iout=Ibase*β (some multiplication factor, usually in the hundreds)


    usually a resistor or a fuse, depending on the scenario

    there are also much more advanced methods
     
  9. DexterMccoy

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    Yes they use milliohm resistors in series as a fuse , a series resistor on the output of the p/s. But these aren't current limiting circuits
     
  10. magnet18

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    resistors by nature will only allow some certain maximum current for a set voltage range
     
  11. GRNDPNDR

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    Mar 1, 2012
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    So I was playing with this circuit in multisim because it looked like something I actually needed.

    Using an LM148FK, it shows as having a -18V output, with an 18V input.

    If I use an LM148J then it only shows as being 157mV.

    What gives?
     
  12. atferrari

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    Dexter, is it possible that you post the whole schematic? If you do not like to disclose it, it's OK for me.
     
  13. kubeek

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    Sep 20, 2005
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    Spice opamp models have different levels of complexity, and IIRC model 2 doesn't include the supply voltage limits, while level 3 does.
     
  14. GRNDPNDR

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    Mar 1, 2012
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    I don't quite understand.

    does that mean if I buy any LM148, the result would be -18V out, with +18V in?
     
  15. kubeek

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    With what power supply? Please show your schematic.
     
  16. DexterMccoy

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    Feb 19, 2014
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    Why are the diodes in the middle , it's separates the filter caps

    Mostly I see the zener diodes at the END of the power supply, not placed in the middle

    Why are the zener diodes placed in the middle of the filter caps?
     
  17. kubeek

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    Why do you think it matters where they are drawn on the schematic?
     
  18. DexterMccoy

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    Because the zener diodes are before 10 filter caps and after 4 filter caps

    That must be a reason for this
     
  19. MrChips

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    Oct 2, 2009
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    Nope.

    You have to learn the purpose of decoupling capacitors and why the designer drew them that way.
     
  20. DexterMccoy

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    Yes they are decoupling caps but are drawn like filter caps right?
     
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