What are voltage and power rails?

Voltage is the push that moves electricity; rails are the supply lines that carry it. More push allows more speed but costs much more power.

1. Voltage is a push
Voltage is the push that moves electricity, like water pressure in a pipe. More push lets circuits switch faster.
2. Making the exact voltage
A regulator on the main circuit board lowers the supply voltage to what the chip needs, often about 1 volt (1 V).
3. Separate rails for the main parts
Each supply line is called a rail. The CPU, graphics and memory often get separate rails at different voltages.
4. Asking for a new voltage
The chip sends the regulator a number, a voltage ID (VID), asking for more voltage for heavy work, less when idle. The change takes microseconds.
5. A sudden dip
When work starts suddenly, current jumps and the voltage dips, like water pressure when many taps open. This dip is called droop; if it goes too low, the chip makes mistakes.
6. More push costs much more power
Switching power, used each time circuits switch, grows with voltage times voltage. So 10% less voltage saves about 19% of it at the same clock speed.

Words to know

Voltage: The push that moves electricity, measured in volts (V).
Current: How much electricity flows each second, like how much water flows through a pipe each second. Measured in amps (A).
Regulator: A part on the board that makes an exact voltage.
Rail: One supply line carrying one voltage.
VID (voltage ID): The number the chip sends to ask for a voltage.
Microsecond: One millionth of a second.
Droop: A short voltage dip when the current suddenly jumps.
Switching power: The power used each time circuits switch. Also called dynamic power.

When a chip uses more or less power than expected, testers check each rail's voltage and current first.

Why does a small voltage cut save so much power?Think first, then tap to see the answer.

Switching power grows with voltage times voltage: 10% less saves about 19% at the same clock speed.