Greetings Douglas,

I'm confused. I know what DC voltage is, but what is RMS voltage. Is it AC voltage?

Thank You,**RG**

I'm confused. I know what DC voltage is, but what is RMS voltage. Is it AC voltage?

Thank You,

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.

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.

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 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.

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.

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,

Remember. It's the power being transferred that is the concern with RMS voltage readings, not the direction of current.

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.

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.

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.

The concern is that an RMS reading isn't the highest, or peak, voltage. The real peak voltage will

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.

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