If you look at the specifications for certain electrical devices, computer hard drives for instance, the voltage requirements would be something like +6.6v or +12v. Why does the sign matter? Why couldn't -6.6v or -12v work? I thought when it came to voltage, only the magnitude of the difference of voltages matter?
Why does the sign matter?
It matters because many electronic components only work with current in one direction.
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.
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).
Why couldn't -6.6v or -12v work? I thought when it came to voltage, only the magnitude of the difference of voltages matter?
The square of the magnitude of the difference is proportional to power, essentially you are asking why isn't it enough to just make sure each device gets the right amount of power. For something like a heating element that just takes electrical power and produces heat, that is all that matters.
However for most interesting things the direction of current flows is important. Consider a permanent magnet DC motor that spins clockwise for positive voltage and counterclockwise for negative voltage. If your hard drive used one of those, and you gave it the reverse voltage, clearly it is not going to work right because the disk would be spinning the wrong way (among many, many other reasons). Direction of current flows is very important in this case.
Yeah, don't think so much about the actual numbers of the voltage (magnitude), but more so how the voltage is referenced to ground and how current flows in loops. Your power supply is looking for a high voltage at 6.6V and a lower voltage at the GND pin, and current flows from high to low potential. If you switch the two potentials then current would flow from the ground into the positive terminal, which isn't good if your component has polarity, and most things beyond resistors do and care about current flow. That's just how I think of it.
The simplest answer is that some components are polarized, that is to say they are only designed to work in one direction. If the flow of current is moving in the opposite direction, they will not work as expected.
For instance if you were to add a diode ( a component that only allows current flow in one direction) to a simple circuit that connects the two poles of a battery in reverse, it would heat up as it prevented current flow in the wrong direction until it melted. These are frequently used to provide reverse polarity protection, and can be quite useful when protecting integrated circuits (ICs) which are susceptible to reverse current.