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Simple Voltage Regulator and Wiring Question

Discussion in 'General Electronics Discussion' started by geratheg, Jul 12, 2014.

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  1. geratheg


    Jul 12, 2014
    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. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Last question first, the ground pin MUST be connected.

    First question, will it heat up with just the input and ground connected:
    1. Practically, no
    2. theoretically, yes.
    A small current is drawn by the regulator to operate itself, so that will cause a very small amount of heating. In most cases this is insignificant.

    Also, if you exceed the absolute maximum input voltage, the device can be destroyed. In getting destroyed it will likely draw a large current, get very hot, and finally vent the magic smoke.

    In most cases you are also advised to place small capacitances between the input and ground, and the output and ground to ensure stability. In many cases you will want larger capacitances for filtering.
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    As Steve says, you NEED to connect the ground pin. The regulator regulates the voltage between the OUT pin and the GND pin. With the GND pin not connected, it couldn't tell what its output voltage was! (There are other reasons why the GND pin must be connected to 0V, but that's an obvious one.)
    Generally, power switching is done by intercepting the positive supply, because in most cases, the other side (the negative side) of the incoming power source (battery, DC adapter, bench power supply, etc) becomes the 0V rail, which is connected to the shields of connectors, to metalwork, and/or to mains earth at one or more places.

    The 0V ("zero volt") rail, also called the "common" rail, or "ground" or "earth" is the reference rail; other voltages are measured relative to this rail. Supply voltage rails, such as +15V, +12V, +9V, +5V, +3.3V, -5V, -15V, or whatever the circuit uses, are measured relative to the 0V rail. The 0V rail is a bit like the electronic equivalent of sea level. It is assumed that the 0V rail is not interrupted at any point.

    Power supply rails, like the voltage rails listed above, can be switched ON and OFF, but the 0V common rail is not normally switched. This is not necessarily a requirement for operation, but it is a widely used convention with several practical advantages.
  4. geratheg


    Jul 12, 2014
    I appreciate the replies.

    I was just curious whether the ground pin needs to be connected or does the ground pin need to be disconnected while the regulator is not in use (meaning the output pin is not connected)?

    Let's assuming 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?
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    If you want to turn off the 5V supply, the most common way way is to disconnect the unregulated positive voltage that flows into the input pin of the regulator. Disconnecting the ground will also work, but in more complex circuits there may be a common ground, so it is entirely possible that this will be ineffective.
  6. geratheg


    Jul 12, 2014
    Thank you for all the replies.
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