Need to use same power supply if powering different voltages devices?

[hurricanehenry]

Hi - if I am powering different voltage levels such as 5V, 3.3V, etc. devices or just even multiple 5V devices that are to be connected, I should use the same power source with a common Ground right?

e.g., Arduino at 5V + Pi at 3.3v voltage levels, using a level converter.

If I am connecting such devices to a laptop, should I have a separate USB 5V power out (just the ground and 5V+ lines) drawn from the laptop USB ports (and multiple ports in parallel if current is insufficient)?

[golden_labels]

The answer depends on the situation. In general: no, they only need to share common ground.

If they are different voltages, then already in the question you say they are different power sources.
If they are at single voltage, it depends on whether the outputs are directly tied together or not. If they are not, it doesn’t matter.

The thing you shouldn’t do is directly linking outputs together.⁽¹⁾ Each power supply provides slightly different voltage, so they will interfere with each other. Each of them is trying to maintain a stable output of its own, affecting feedback loops of all other power supplies, which then react and try to adjust. The effect is either a huge imbalance between what current each power supply delivers or some of them falling into oscillation, trying to follow each other.

As for mixing multiple USB ports, I would rather consider using a separate supply instead of such mixing, if power from a single USB port is insufficient. But even before that, you should be aware, that you are limiting power if you use only the supply lines.⁽²⁾ That is because data lines are required for power negotiation and without it a port will deliver only 500mW, compared to 2.5W even the oldest USB could provide, and 15W available over USB3. In other words: by merely paralelling power lines together you use two ports to obtain 200mA, while a single line with negotiated power over could give 500mA even on USB1.

⁽¹⁾ If there is some length of wire or other noticeable resistance between them, modern regulators that already have very tight tolerances may in practice not do anything particularly crazy. But still, even if it will work, it’s a fast way to have your device reviewed by Big Clive.
⁽²⁾ Some hosts are not caring about power negotiation and will happily dump everything they have. Which is why such solutions may seem to work. But this is out-of-spec for USB: a poor choice for anything that has to be reliable in different scenarios, a dishonest move for commercial devices.

[ledtester]

should use the same power source with a common Ground right?

e.g., Arduino at 5V + Pi at 3.3v voltage levels, using a level converter.

Deriving all power from one source does simplify things as you only have to turn that one supply on to power everything.

Also, when using two distinct power supplies sequencing them can be an issue. Your circuit may not operate correctly or even become damaged if one supply is on when the other is off.

So basically it's safest and easiest to derive all your power from a single source.

If you need more than 100mA @ 5V then you shouldn't derive your power from USB unless you can negotiate it from the USB controller.

[hurricanehenry]

Many thanks to both for insightful and helpful replies.

I guess using the same power source will indeed simplify everything and make it less worrying for me.

My main worry really is just possible ground loop currents and unintended current flows (e.g., 5V higher to 3.3v).

I am not totally clear about this concept (how to power these multiple voltage devices) but it just seems there is the potential for problems when each supply's ground is floating, with respect to the other power supplies. The voltage is only guaranteed relative to each supply's own ground. I suppose for a Switched Mode PS it's not isolated and is linked to the wall supply ground.

I probably will end up taking power from the same 5V wall adapter (of sufficient amperage) and then with one of the 5V lines, put it through a DC-DC converter to 3.3.

[tooki]

Suppose you need +5V and +12V. You get two power supplies, one for +5V and one for +12V. By connecting each power supply’s negative (0V) terminal to your circuit’s ground, you’ve given everything the same 0V reference so they’re not floating with respect to each other.

Note that I didn’t say to connect each power supply’s ground, but rather its negative/0V terminal. That’s because 0V and ground aren’t necessarily the same thing. We generally use “ground” to mean the 0V reference in our circuits, even if they’re actually floating. But we need to distinguish that “ground” from actual connected-to-planet-earth ground (I’ll use the electricians’ term for this: protective earth, or PE). A typical single-output power supply module like the one shown in the attached image has 5 terminals: AC line and neutral, PE, V- and V+.

The PE terminal must be connected to the PE of your AC power cord, and in most cases, to the project enclosure if it’s metal. Whether you connect it to your circuit ground depends on the circuit’s requirements.

The V- terminal is what you connect to your circuit ground (i.e. 0V reference) if you need a positive output voltage (say, +12V). Then the V+ terminal is your +12V output. (The reason it’s not labeled as 0V is because someone else might need it as a -12V power supply, in which case the V+ connects to circuit ground and V- becomes the -12V output.)

[tooki]

I probably will end up taking power from the same 5V wall adapter (of sufficient amperage) and then with one of the 5V lines, put it through a DC-DC converter to 3.3.

FWIW, that’s a safe, practical approach that’s a good choice until you’ve got a more solid understanding of the subject matter.

I’m probably going to do the same thing on a project I’m working on now. I might use an internal 5V supply, but my MCU, which represents only a small share of the load, needs 3.3V so I’ll just derive that from the 5V using either a DC-DC step-down (buck) converter, or maybe even just a linear regulator.

[chinoy]

my circuit I need 12v, 5v, 3.3v
I take voltage from a 12 Volt bat. And feed it to a 12 volts regulator. The output from the 12 volt is then feed to the 5 volt. The output from the 5 volt is feed to a 3.3 volt regulator.
Earlier I would have one 12 volt line in which feed each regulator. But I found parts lasted longer by chaining the regulators.

[golden_labels]

What kind of regulators? A buck converter should not experience shortened life span, so I assume it was a linear regulator. If yes, it’s because the amount of power dissipated on it as heat while going directly from 12V to 3.3V (8.7 W/A). Chaining means that the power is distributed across two devices. Though, if 3.3V sucks a lot of current, that increases dissipation on the 5V regulator, leading to the same problem.

[hurricanehenry]

my circuit I need 12v, 5v, 3.3v
I take voltage from a 12 Volt bat. And feed it to a 12 volts regulator. The output from the 12 volt is then feed to the 5 volt. The output from the 5 volt is feed to a 3.3 volt regulator.
Earlier I would have one 12 volt line in which feed each regulator. But I found parts lasted longer by chaining the regulators.

I think the parts might last longer (the voltage regulators are quite durable, even over decades, but only if you don't overheat them) but you waste a lot of energy like that via heat, especially if you are using batteries.

I have not done it, but you might be able to just produce the same effect using some resistors and voltage dividers.