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Let's say I wanted to power two circuits: a 12V circuit and a 3V circuit. Could I use a 15V power supply and a 12V voltage regulator with three pins: Input, Output, and Ground. Input to Ground would be 15V, Output to Ground would be 12V, and presumably Input to Output would be 3V. Can I use the Input to Output voltage difference to drive a circuit?

EDIT:
It sames the answerers agree that this would not be advisable. I'm trying to think of a good alternative. I could use a 12V power supply to power the 12V circuit and in parallel power a 3V LDO. However, that seems like it would waste a lot of power. Am I forced to using two separate power supplies?

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  • \$\begingroup\$ Yes, two supplies is the way to go. Please see this answer. It is to an unrelated question, but it talks about deriving multiple voltages from a single supply. \$\endgroup\$
    – bitsmack
    Commented May 2, 2017 at 16:11

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Most positive regulators can only source current, not sink it, so what you suggest would only work if the current being drawn from the 12V to ground was more than the 3.3V circuit. If it was not then the 12V output would rise towards the 15V rail and your 12V circuit would experience overvoltage, and the 3V circuit would not get enough voltage.

The same thing would happen in reverse if you tried to use a -3V negative regulator hung off the 15V input. It would work provided the 3V load was ALWAYS greater than the 12V load, otherwise the 3V load would see overvoltage and the 12V load would not get enough.

Usually when multiple voltages are required you want the grounds to be common so this seldom comes up.

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  • \$\begingroup\$ so what would you suggest I do? \$\endgroup\$
    – honi
    Commented May 2, 2017 at 15:37
  • \$\begingroup\$ I will guess that your 3V current requirement is substantial and primarily digital, and your 12V requirement is also for substantal current (in the tens or low hundreds of mA in each case). Based on my wild-a**-guess I would suggest a switching regulator 15V->3V for the 3V and probably a 7812 for the 12V. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 2, 2017 at 16:43
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This will not work. A voltage regulator will use some manner of letting energy from the input go to the output. It will measure the output constantly to verify that the output at the right level - if it is too low, it will increase the amount of energy let through, if it is too high, it will decrease it (this process is called feedback). However, it cannot take energy from the output and send it to the input. In fact, trying to do this will break most linear regulators, and will just be inpossible in switching types.

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  • \$\begingroup\$ do you have a suggestion for what i should do in that case? \$\endgroup\$
    – honi
    Commented May 2, 2017 at 14:31
  • \$\begingroup\$ If what you want to do is simply increase the voltage: there are many circuits that will do this. They are a specific type of voltage regulator called "boost converters". However, instead of decreasing the voltage, they "pump" the energy up to a higher voltage. If you want to do something else, you need to first more accuratly discribe what this is exactly before we can help you. \$\endgroup\$
    – Joren Vaes
    Commented May 2, 2017 at 14:34
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    \$\begingroup\$ I think you mean "it cannot take energy from the output and send it to the input". \$\endgroup\$
    – Adam Haun
    Commented May 2, 2017 at 14:36
  • \$\begingroup\$ @AdamHaun good catch, let me fix it. \$\endgroup\$
    – Joren Vaes
    Commented May 2, 2017 at 14:36
  • \$\begingroup\$ I don't want to increase the voltage. I want to have two separate circuits with different voltages, but sharing a power source. \$\endgroup\$
    – honi
    Commented May 2, 2017 at 15:43
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This might work if:

  • the 15V input voltage is stable enough (e.g. if you have 15±1V on the input, you'll have 3±1V between the input and the output of your 12V regulator)
  • the load on the output of your 12V regulator consumes more current than your 3V load, in all circumstances.

Note that if you don't have a 12V load, supplying a dummy load will result in essentially the same efficiency as you would get with a 3V linear regulator (that is, around 20%), without any actual regulation. So, if you need to provide your electronics with 3V, consider using a 3V regulator.

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