voltage regualtor

Discussion in 'General Electronics Chat' started by george medina, Oct 21, 2014.

  1. george medina

    Thread Starter New Member

    Oct 21, 2014
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    I chose a 3.3 voltage regulator. please help

    a) What is the output of the voltage regulator if we set the power supply too low, i.e. 1 V? What about too high, i.e. 5 V? What about really high, i.e. 20 V?
    b) Will this also work for negative voltage? If not is there a way we can modify the connection of the voltage regulator to the circuit so that it will work for -VCC?
    c) How does this protect the circuit against reversed polarity?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    If the supply voltage falls below the rated regulated output voltage PLUS the drop-out voltage, the regulator no longer regulates. A linear regulator can only regulate downwards, not upwards.

    Any excess voltage means that the regulator has to dissipate power as heat. For example, suppose the load takes 3.3V@1A.
    If the input voltage is 20V, then 16.7V@1A = 16.7W must be dissipated as heat. If the regulator is not properly cooled with a very large heat sink it will overheat, burn out, or go into thermal shut-down.

    (b) It will not work for a negative voltage. For a negative output you need a negative supply and and negative voltage regulator.

    (c) It doesn't. You can end up damaging both the regulator and the circuit connected to the regulator.
     
  3. george medina

    Thread Starter New Member

    Oct 21, 2014
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    then I should use possibly diodes, instead to protect the circuit from reverse polarity. if so, which didoes? WE want to limit input voltage to the circuit to 3.3volts
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,009
    3,233
    Here's a reverse voltage limiter using a MOSFET that has a very low forward voltage drop, much less than a forward biased diode for a properly selected MOSFET. For voltages less then 10V you need a logic-level type MOSFET that fully turns on (look for the ON resistance measurement value, not the threshold voltage, in the data sheet) at the minimum voltage it will see.
     
  5. george medina

    Thread Starter New Member

    Oct 21, 2014
    10
    0
    k. thanks
     
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