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Many power supplies for desktop computers or PCs have negative voltage, including AT (advanced technology) and ATX (advanced technology extended) standards. Like most users, you may wonder why power supplies have negative voltage for motherboard configurations.
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Understanding Positive and Negative Voltages
Current flows through a circuit when two terminals have a substantial electrical potential difference. This difference in potential electrical energy depends on the number of electrons in the two terminals, which brings about the concept of polarity, i.e., positive and negative.
A negative terminal implies that the particular point in the circuit has more electrons, as they are negatively charged, compared to a positive terminal. So, electrons flow from a negative terminal to a positive terminal if there is enough potential difference, calculated as voltage.
Current or electricity, measured in amps, flows in the opposite direction of the flow of electrons, i.e., from the positive terminal to the negative terminal. This polarity isn’t the same as negative and positive voltages. A negative terminal doesn’t imply that the voltage is negative.
A ground can be negative compared to a positive terminal with a sufficient potential voltage difference, which is indicated by the polarity signs on a typical battery. Negative voltage means that it has more electrons than the ground.
Thus, the ground becomes the positive terminal for the negative voltage. And any positive terminal relative to this ground and negative voltage terminal becomes a point with an even greater potential difference relatively.
Suppose you have a battery rated as 3 V. The potential difference between the positive (+) and negative terminals (-) is 3 volts. But if these same terminals are rated as + 3 V and – 3 V, then the potential difference in voltage is 6 V. The true ground is 0 V, of course.
Therefore, current or electricity flows from a terminal with a sufficiently positive voltage to the ground or a negative terminal, and further if there is a terminal with an even greater negative voltage.
Power supplies with negative voltage can facilitate the flow of electricity from a positive terminal to the ground and then to the negative terminal. If the ground or 0 V is the positive terminal for negative voltage, current will flow through power supplies from 0 V to the negative terminal.
5 Reasons Why PSUs Have Negative Voltage
Let me use the example of ATX 2.2 power supply units (PSUs) that have the following negative voltages:
- -5 VDC
- -12 VDC
-5 VDC isn’t necessarily used by most motherboards nowadays, but the power supply unit still supports it for configurations that require such compatibility. The -5 and -12 negative voltages are for low-power hardware components you may use with a selected motherboard.
Just as the positive voltages of power supplies cater to the amperage requirements of different hardware, a negative voltage like -12 DC regulates a low current flow to integrated and other components that demand it. Apart from that, negative voltage has a few other utilities.
1. ATX Standard
Power supply units that adhere to older ATX standards have negative voltage. Otherwise, any PSU won’t be considered to be compliant with a particular or quoted ATX standard. This is why most power supplies have negative voltage, regardless of whether or not such outputs are used.
Let me use the example of the latest ATX standard, i.e., the Version 3.0 specs released in 2022.
According to Intel, a power supply unit can have a -12 VDC-regulated output. This regulated DC output with negative voltage is intended to power hardware components that require as little as 0.1 amps (A) of electric current. However, the -12 VDC output is optional for the latest standard, and ATX 3.0 doesn’t have any criteria or provisions for the erstwhile -5 VDC output regulation.
That said, you will still find many power supplies featuring both -5 V and -12 V outputs to comply with older ATX versions so that the specs are per the evolving standards’ criteria. -5 V has been a redundant DC output criterion for a while now. But backward compatibility is one of the reasons for power supply unit manufacturers retaining the -5 VDC regulated output.
Previous ATX standards required negative voltage output regulation to power a few hardware components, many of which are no longer common, and some aren’t used.
2. ISA Expansion Slot
ISA expansion slots in desktop or personal computers use positive and negative voltages as regulated DC outputs based on hardware requirements, such as the following:
- Extra serial port
- Modems
- Network cards
- Sound cards
- Video cards
While ISA expansion slots were invaluable for PCs during the 1980s and 1990s and still are for many industries, they became obsolete in personal computers since PCI became the norm. No peripheral card for desktop PCs uses an ISA expansion slot nowadays.
However, the first two major ATX versions featured specs to support ISA expansion slots, mainly for backward compatibility and PC build possibilities.
3. RS-232 and RS-485
Like ISA expansion slots, the RS-232 port was the standard for connecting various peripherals. The RS-232 ports required the following negative voltages for different hardware:
- -3 DC
- -5 DC
- -12 DC
Unlike ISA expansion slots, the RS-232 port is not exactly obsolete or redundant, but the current versions don’t use negative voltage. Similarly, the RS-485 port continues to be a common serial interface. But the dependence of these ports on negative voltage is almost nil in modern PCs.
The legacy or older standards for RS-232 and RS-485 made negative voltage necessary, so all power supplies had to feature not only -5 VDC and -12 VDC but also -3 VDC in many cases. -3 DC has been archaic for a long time, -5 DC is rarely used, and -12 DC is losing significance.
4. Sound Cards
A negative voltage is often necessary for operational amplifiers and to offset voltage to stabilize amplifier gain. Thus, power supplies have negative voltage to be more compatible with various amplifiers, sound cards, etc. Here are some of the benefits of using negative voltage:
- Improved frequency response
- Increased circuit stability
- Increased input impedance
- Reduced non-linear distortion
- Reduced output impedance
- Reduction in noise
Negative voltage offers these benefits with feedback and how a circuit regulates its inputs and outputs. The negative voltage reference is also critical to ensuring the tolerance range for each regulated input and output isn’t breached.
5. Voltage Reference
Like a power supply fan always runs to proactively keep the PSU cool or at least prevent it from overheating, motherboards and many hardware components should be protected from temperature spikes due to high amp flows or voltage fluctuations beyond the tolerance range.
The ATX standard for positive voltage regulation accounts for a +/- 5% tolerance. Negative voltage can fluctuate by +/- 10%. Regardless of the difference, power supplies must be able to figure out the exact voltage fluctuation to ensure that the system is functional and safe.
Earth is true ground, but the commons in power supplies may not be 0 V in real-time. Voltage reference using positive and negative potential differences, not polarity, enables power supplies to use two known potential differences to regulate current flow within tolerance levels.
Therefore, power supplies may cut off the amps to a component or system by leveraging the negative voltage terminal, which can prevent overheating in the event of a major spike. Power supplies can also establish and sustain true ground or 0 V with positive and negative voltage.
Conclusion
Power supplies have negative voltage for some low-power hardware, operational amplifiers, and reference; however, -3 V and -5 V are no longer essential to contemporary motherboards and PCs. The -12 V output may also become redundant in the foreseeable future.
