Why does voltage sign matter for electrical devices and components

Why does the sign matter?

It matters because many electronic components only work with current in one direction.

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Figure 1. Polarity sensitive components.

• D1, a diode, has a symbol which shows the direction of current flow (in the direction of the arrow). If the polarity is reversed current cannot flow.
• Q1 and Q2 are transistors. Again, the arrows in the symbols show the direction of current flow, top to bottom as drawn. A circuit's design will take this into consideration and it will not function if polarity is reversed.
• Integrated circuits such as op-amps and microcontrollers contain several or thousands of transistors of one type or another and the whole circuit is polarity sensitive.

Circuits using passive components only, resistors, capacitors and inductors, are not polarity sensitive.

^{simulate this circuit}

Figure 2. A bridge rectifier accepts input of either polarity or alternating and converts it to DC of the required polarity.

Circuits can be designed in certain applications which are not polarity sensitive and can even work on AC power. Typically these use a rectifier to "rectify" the input voltage to the correct polarity.

Using your example of a hard drive, the ground reference is typically connected to the chassis of the computer, which is connected to the mains earth, which finds its way to earth potential in a building. So the chassis is at 0V. The supply voltages are something like +3.3, +5 and/or +12V.

If you put a reference voltage somewhere else, perhaps the +3.3V terminal, the voltages you measure might be -3.3, 0, +1.7 etc. but the main thing from the hard drive's point of view is that the terminal +3.3 on the hard drive sees 3.3V higher than the terminal for 0V or ground. If you connect -3.3V to the +3.3V terminal relative to the ground terminal the drive will be destroyed.

That's because virtually all electronics requires a certain polarity of electricity to work, and is often damaged with reverse polarity. Sometimes it is protected (for example, when the user can insert batteries backwards it might be wise) sometimes not. In some cases, I've designed products with a bridge rectifier so that if the user reverses the power it will still function perfectly, but that's fairly rare, and not always practical.

Even a circuit as simple as an LED + resistor will light up if the polarity is correct, and remain dark if the polarity is reversed (usually without damage in this case).

That is because there are devices in a computer that need both +12V and -12V, referenced to a GND in the middle. RS232 transceivers are the classic example.