What is RMS Voltage?
RMS voltage is a comparison to what the DC Voltage of the same value could do.
RMS stands for Root-Mean-Square. It's a mathematical formula for the continually changing voltage (AC, or unsteady DC). It shows as an RMS Voltage, not just the instantaneous voltage, but over short sample time, it shows how much equivalent DC Voltage is required to transfer the same quantity of power.
To mathematically solve the numbers, the actual formula itself requires calculus. However, using some simple circuitry inside a hand-held voltmeter, the RMS voltage is obtainable.
But we're getting ahead of ourselves. Before understanding RMS Voltage, we have to realize that in a circuit, electricity isn't power; electricity in a circuit is just one method of transporting power from one place to another.
Electrical Current Flow versus Power Flow
In a circuit, current flows, and the current carries power. The amount of power being carried can be calculated as the voltage times the current. When calculating power, the actual formula for power is
W = E x I. Using words, it's
Wattage (in Watts) equals
Electromotive Force (in Volts) times the
Intensity of Current (in Amps).
When looking at a circuit, though, power flowing from the source to the load is a different concept from electrical current flowing around the circuit. Current flow, by itself, shows the direction of electrical current flowing in a circuit wire; power flow shows the direction that power flows in a circuit using the same two wires.
Electrical Current Flows in a Circle; A Circuit is a Circle
In a circuit, electrical current flows in a circle:
- Through the first wire
- Through the load
- Through the second wire
- Through the source of electricity, going back to the first wire
Power Travels from the Source to the Load; Power Only Flows One Way
Power always goes from the source of the power to the load.
The method carrying power could be:
- Electrons traveling in traditionally wired electrical circuit
- Electromagnetic signals in a wave guide
- Electromagnetic signals transmitted from a radio station antenna
- Light signals traveling in fiber optics
- Water under pressure spraying out of a broken pipe
The result is that power is transferred from one place to another.
DC versus AC
The word DC stands for Direct Current: the current always goes one direction. However, the term DC is also applied to voltage, so DC Voltage is always positive or always negative.
The word AC stands for Alternating Current: the current keeps reversing direction. The current goes one direction, then reverses and goes the other direction, then reverses and goes the first direction, then . . . You get the drift. However, the term AC is also applied to voltage, so AC Voltage also keeps reversing.
Steady DC Voltage
Steady DC voltage is the voltage of a battery. No matter when the DC voltage is measured, the DC voltage will always be the same. Whether the time taken to sample the voltage is instantaneous, or the time taken to sample the voltage is several minutes long, the voltage will always be the same.
It's easy to figure out how much power can be transferred because, when a battery is providing power to a resistor, the voltage never changes, and therefore the current never changes.
Watts Law,
W = E x I, is easy to figure out because the voltage, and therefore the current, is the always the same, no matter when the voltage and current measurements take place.
AC Voltage and Unsteady DC Voltage
When the voltage isn't steady, or worse, when the voltage keeps reversing (AC), the voltage is always varying. When making instantaneous measurements, the voltage is often not the same from one instant to the next instant. Power can't be figured out when the measurements keep changing.
Remember. It's the power being transferred that is the concern with RMS voltage readings, not the direction of current.
Unsteady DC Voltage: Power will always go from the source to the load.
If the real voltage is always positive or always negative, the resulting measurement is a DC RMS Voltage value that takes into account the amount of power that can be transferred. The power that DC RMS Voltage can transfer is equivalent to what a steady DC Voltage can transfer.
AC Voltage: Power will always go from the source to the load.
The amount of power that can be transferred during the time the voltage is positive is added the amount of power that can be transferred during the time the voltage is negative. This gives a total amount of power.
Even though the voltage is sometimes positive and sometimes negative, the resulting AC RMS Voltage value takes into account the total amount of power that can be transferred. The power that AC RMS Voltage can transfer is equivalent to what a steady DC Voltage can transfer.
RMS Voltage is Equivalent to Steady DC Voltage
Whether it's AC RMS Voltage, unsteady DC RMS Voltage, or steady DC Voltage, the concern is the amount of power that is transferred from the source of the power to the load.
Steady DC Voltage is the reference. The power that the AC RMS Voltage can transfer, and the power that the unsteady DC RMS Voltage can transfer, is the same amount power that a steady DC Voltage of the same value will transfer.
As a side note: AC Current, unsteady DC Current, or steady DC Current have the same relationship to each other as do the voltages. The RMS Current transfers the same power from the source of power to the load as DC Current of the same value.
Warning when Using RMS Voltage Readings
An RMS voltage measurement is showing the voltage of a varying voltage signal. In some ways, RMS is similar to average: sometimes the RMS voltage will be higher than the amount shown on the meter, sometimes the RMS voltage will be lower than the amount shown on the meter.
The concern is that an RMS reading isn't the highest, or peak, voltage. The real peak voltage will
ALWAYS be higher than the RMS voltage.
A 24-volt RMS volt measurement from a plug-in transformer, for instance, will have a peak voltage of 33.94 volts.
An example of the concern is with wire insulation. Yes, I've had someone ask if wire, rated to handle 30 volts, could be used with a 24-volt transformer. A 24-volt RMS transformer has a peak voltage of 33.94 volts. The 33.94 volts-peak exceeds 30 volts, so the 30-volt wire can't be used with a 24-volt transformer.
If the signal being measured isn't the pure sinewave from the utility's power source, the peak voltage may be lower than a pure sinewave, or much higher than a pure sinewave. The bottom line, though, is that the real peak voltage is always higher than the RMS voltage, never lower.
Douglas Krantz
