Simple Voltage Regulator and Wiring Question

Discussion in 'General Electronics Chat' started by geratheg, Jul 11, 2014.

  1. geratheg

    Thread Starter Member

    Jul 11, 2014
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    I've never used a voltage regulator before and am somewhat new to circuits.
    I'm curious about an LM78xx voltage regulator, if you connect the input and ground pins to the voltage source, without connecting the output pin will the regulator heat up?

    The reason I'm asking is I'm curious whether the ground pin is necessary on it if the circuit will be close (if the output pin was connected of course) with just the input and output voltage?

    Also an unrelated question about wiring a switch. Do you usually wire it to the positive (red) wire or to the black wire and why if it will work both ways?
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    The ground pin is necessary because it provides the point of reference.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I assume you are referring to DC supply system.
    It is probably a precautionary measure in case the common or negative is connected to a ground or chassis etc, if the common contacts earth ground either accidentally or by design, there is still a potential from the +rail to any bare chassis points.
    In low voltage systems it is not really a concern, mostly done just as normal practice.
    Max.
     
  4. geratheg

    Thread Starter Member

    Jul 11, 2014
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    In an LM78xx voltage regulator, if you connect the input and ground pins to the voltage source, without connecting the output pin will the regulator heat up?
    I'm just curious, can the ground pin be connected or does the ground pin need to be disconnected while the regulator is not in use (the output pin is not connected)?
     
    Last edited: Jul 11, 2014
  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    If the LM78XX has the proper bypass capacitors around it, it will not heat up if the output pin is disconnected. See the data sheet for information on the bypass capacitors.

    The ground pin must always be connected even when the output is not connected.

    Mark
     
  6. geratheg

    Thread Starter Member

    Jul 11, 2014
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    I appreciate the replies.

    Let's assume I'm using a regulator with a 5V output.
    If I wanted to open the circuit where the output (5V) connects, would I just need to open that? Would it be okay to keep the input and ground connected and the 5V disconnected?

    If so, would it be more efficient if ground was disconnected since the voltage regulator wouldn't draw any extra "small" current while it is not in use?
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Yes, and yes.
    If you disconnect the output and the ground that would be more efficient. However, if you disconnect only the ground then the output voltage will be equal to the input voltage minus 5V.
     
  8. geratheg

    Thread Starter Member

    Jul 11, 2014
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    Thank you for all the responses.

    Thank you for answering my question.

    As for this, so instead of setting it to 5V, it would subtract 5V from the input.

    For example, if you connect 15V input, without connecting ground, the output will be 10V? But if you do connect ground the output will be 5V?
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That's what LTspice tells me when I run a simulation.
     
  10. geratheg

    Thread Starter Member

    Jul 11, 2014
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    Interesting, I'm glad I could learn something.

    Thank you once again.
     
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