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If I were to have an IC (Just some standard 5v microcontroller), and for some reason I only had a -5v supply, could I technically connect -5v of the supply to the GND pin of the micro and the ground of the supply to the 5v pin on the micro? I've always thought this would work, but everybody I've talked to has said it's a bad idea. I assume they say it's a bad idea because if you had a device on the same system that was spitting out voltage spikes down the central ground, it could overvolt your micro.

So, more of a 2 part question

  1. Is powering a 5v micro with a negative voltage on the GND pin possible?
  2. If so, what are the downsides of doing it this way?
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    \$\begingroup\$ Of course it will work, the micro only needs 5V across it in the right direction. It can't know or care which side of the 5V you call "ground". The issues arise if there's other circuitry connected that is positive with respect to your "ground". (For example interfacing with the other circuitry could be more complex.) \$\endgroup\$ – John D Aug 27 '14 at 5:18
  • \$\begingroup\$ Use a inverting power supply to convert -5V to +5V and power up the MCU:) \$\endgroup\$ – AKR Aug 27 '14 at 7:06
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It is totally possible to power the MCU off a -5V rail as you describe.

The person who told you about spikes in the "GND" which becomes the MCU VCC pin should be aware that spikes could just as well show up on the -5V rail. The MCU can be over voltaged simply by applying too big of voltage difference between the MCU VCC and GND.

The main downside of using the -5V supply is the implication that it comes into a system that also has a +voltage rail or two. Any circuitry in the system that uses such +V and the "GND" as its supply will not be able to interface directly with the MCU. Special level shifting circuitry would have to be applied to translate the negative domain voltages of the MCU to the positive voltage domain of the other circuitry.

If the system power supply is just the single voltage rail that you describe as -5V then just switch the leads around and consider it a +5V supply.

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Voltage is a potential between two points; therefore, a -5 volt supply would work when connected as you mentioned (PSU -5v to IC ground and PSU gnd to IC +5v input). If this were your only connection, there would be no issues (assuming that the PSU was supplying stable power). The problem would be if you have other components in that circuit that are treating the power supply ground as reference (zero) voltage, since their ground potential is now 5v different from the IC's ground potential, leading to potential (sorry, I couldn't resist) undesired current flow or voltages between various points in the circuit. In the worst case stuff would start burning up. There is nothing special about ground; it is just a reference for other voltages throughout the circuit. Ground in a given circuit could be at 10kv relative to earth ground and you could still be supplying only 5v to your IC if the other rail were at 9995v relative to earth ground.

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  • \$\begingroup\$ lol on the "leading to potential"! (I'm easily amused) \$\endgroup\$ – Enemy Of the State Machine Aug 27 '14 at 5:23
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VCC is still going to be +5vDC above ground. If you are using a regulator, like a 7905 (-5v regulator) you can do plenty with your uC. BUT if in the circuit, you use something that uses a 0v GND reference, (for an ADC for instance) you are then in for a mess. If you choose -5v as GND, and stick with it for the entire circuit, there shouldn't be an issue. Just do not connect anything back into the circuit that uses 0v as GND that the uC could see. If you are getting voltage spikes on your "ground", it wouldn't matter how it was connected as your uC would see it if your transient polluted 0v was your ground reference or your VCC. Make sure the VCC pin is +5v more than the GND pin, regulate and stay consistent for the entirety of the circuit.

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If the IC and the peripheral components it connects to (exclusing the power supply) are connected to nothing, the connection you propose is fine. That would be the case if the IC is a microcontroller and its peripheral components are leds, and isolated buttons.

However, as soon as you connect some external devices, your negative power supply is likely to become a problem. That's especially true if your power supply is connected to the electrical grid (mains): perhaps the ground of your supply is connected to earth, and the ground of the other supply of that external device too. Now when you connect the GND of your IC (the -5V output of your PSU), and the GND of that external device (also the ground of its power supply), you actually short-circuit your power supply, thru the earth connections of both power supplies. Even if the ground output of one or both supplies is not earthed, you might run into problems due to capacitive leakage from mains to ground output, and power supply noise, which turns into a signal; e.g. in an audio or video context.

So in summary: for an overall system with at most one mains-connected power supply, what you propose is fine (and very justifiable if a negative regulator has better characteristics than a positive one). Problem starts when your power supply is mains-connected and you connect the GND of your IC (the -5V output of your PSU) to the GND of another system that has a mains-connected power supply.

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