Voltage
Overview
The term "Voltage" is used to describe the potential of an electric current or field.
This subject is covered extensively by other sources. The following discussion is an overview in layman’s terms which may contain technical inaccuracies. The goal is to provide a basic understanding for audio enthusiasts and Lavry product owners.
*The following discussion applies to AC power typical in the USA. Other countries with single-phase 230VAC are similar to 115VAC in the USA.
Basics
The potential of an electric current or field is similar to the potential energy stored in water under pressure. One easy way to picture this is to take a known amount of water and raise it above the ground, like in town’s water tower. It takes energy to counteract the effects of gravity and get the water up to the storage tank at the top of the water tower.
This energy exists in the water as “potential energy” because it only manifests itself when the water moves to a lower height above the ground through a vertical column of water. In a vertical column of water the pressure increases with the depth of the water. In the following discussion we are ignoring any effect the depth of the water stored in the top of the tower has on the water pressure within the tank.
The water staying in the top of the water tower is somewhat analogous to a “field” as the potential energy exists in the water as a function of the distance separating it from the ground, but it does no work.
Release of the potential energy can produce power to do work. In electrical applications, the amount of work also depends on the electrical current; which is similar to the volume of water (the quantity in gallons or liters). The combination of significant pressure and volume can be used to generate electric power by turning large water turbines connected to electric generators in hydroelectric facilities.
- The electrical potential is measured in Volts, and the current is measured in Amperes or “Amps.”
- The amount of power is equal to the voltage multiplied by the current or V times A (VA).
The power is used in the “load,” which can be a device like a light bulb, an electric motor, or a heating coil. These devices transform the power supplied by the electric current into light, mechanical motion, or heat.
Loads like an incandescent light bulb or heating element are “purely” resistive, so the voltage and current change in-phase. In this type of load the VA equals Watts.
In inductive loads like electric motors and switching power supplies, the complex impedance results in the current changing out-of-phase with the voltage, so the power is measured in VA.
Power
- Because power is a function of VA, a large current with a low voltage can produce the same power as a small current with a high voltage.
The application can influence decisions on the right combination of voltage and current. For example, to transmit power from where it is generated to the end-user, tall very high voltage transmission towers are used to suspend relatively thin cables high above the ground (for safety reasons). With a very high voltage the current required to transmit the power is relatively small, and losses due to resistance of the cable are reduced. When the current reaches the end user, a power transformer is used to reduce the voltage while supplying a much larger current. This process actually happens in stages, with power system “sub-stations” reducing the very high voltage to a high voltage for local distribution on smaller power poles or, in some cases, via underground cables.
An example of the other end of the spectrum is the wiring in an automobile. The thickness of the cable from the 12 volt battery to the starter motor is almost as large as the cable on the high voltage transmission tower! It must be to carry the 100 amps needed to generate enough power to start the car in cold weather.
So what is the difference? The size of the cable required is determined by the current it must carry, and to some extent, the distance. But the car’s 12V times 100 amps is 1,200 VA and the transmission line’s 120,000V times 100A is 12 million VA. The power of 1,200 VA can start a car, but it can’t power 1,000 homes!
Power Dissipation
One of the laws of physics deals with the conservation of energy. Energy can be transformed in various manners and even stored in atomic bonds, but it must end up somewhere. For example, to slow down a car or truck, the driver applies the brakes. The brakes transform the energy stored in the momentum of the vehicle into heat, slowing the vehicle in the process. Where did the energy come from? Burning gasoline or diesel fuel breaks atomic bonds releasing heat that generates high pressure, and the engine transforms the pressure on the pistons into mechanical energy used to put the vehicle in motion.
Where did the energy in the atomic bonds come from? The sun, which provided the energy to create the atomic bonds in growing plants millions of years ago. The plant matter was transformed over time into petroleum (bio-fuel excepted!)
Power Dissipation in Electronics
Power dissipation is one of the most important limiting factors in the miniaturization of digital electronics. The power dissipated in the electronic circuitry must go somewhere; it is released as heat.
Early digital “logic” circuits operated on 5VDC, and required substantial power to operate. Due to reasons beyond the scope of this discussion, the power required is relative to the speed of the switching inside the logic circuits. As the speed of operation increases and the size decreases, there is a limit imposed by the power that must be dissipated as heat.
Stray capacitance Effects
A different consideration is that the maximum speed a specific device can switch states is also limited by the operating voltage; but this is not directly related to the dissipation of power. It is related to existence of “stray capacitance” in semiconductor circuitry; a voltage “field effect.” Although small in value, it takes some energy to charge this capacitance in the process of the logic circuit changing states. Lowering the voltage reduces the power and time it takes to charge the “stray capacitor,” allowing the same circuit to switch faster while using less power. These factors, in addition to the demand for portable electronics, have resulted in newer circuit designs that operate on lower and lower voltages. The first big change was to 3.3 VDC, but newer logic IC designs can operate, at least internally, on voltages as low as 2.2, 1.8, and 1.2 VDC.
Peak to Peak (p-p) versus RMS
When discussing AC voltage, one must consider the fact that the voltage is not constant; as it is with DC voltage. At any instant in time the voltage of a 120VAC waveform could be anything between plus 170 volts and minus 170 volts, including zero volts! So how do we arrive at 120VAC?
In order to determine the amount of power available from an AC power waveform, we have to look at the area “under the curve.” The mathematical process is similar in some ways to the process of rectifying AC to DC power. This is useful in determining the power available to the load.
A simple mathematical average (of the absolute value) is not accurate, so the “Root Mean Square” (R.M.S.) method is used. With few exceptions, the voltage of an AC power source is the RMS voltage as versus the Peak-to-Peak (p-p) voltage. For example, 120VAC rms = 340V p-p (not 2 times 120V or 240 V p-p!)
In audio level measurements, the same can be true. Audio measurements are typically made with analog audio sine waves, similar to the waveform of AC power. This is why it is important to know whether an audio specification is the RMS value or a p-p value. Most Professional audio equipment is specified in dBu, which is an RMS value. In contrast, Consumer audio equipment is often specified in V p-p.
The Consumer equivalent of dBu is dBV, which is referenced to a “zero” of 1Vrms. To add to the confusion, the zero reference for dBu is 0.775Vrms. Please see our WIKI page dB for more details.
Hot, Cold, and Neutral
AC Power Circuits
We return to AC power to look at one other important issue; the terms “hot” and “neutral.” In the USA, the vast majority of power is supplied to homes and smaller commercial buildings in the form of "two phase” 230VAC 60 Hz. The local power transformer has an output coil (or “secondary”) with a center-tap that is connected to a cable running down the power pole to a rod driven into the ground. The output of the transformer is referenced to electrical ground in this manner.
The voltage on each of the two outputs from the secondary carry 115VAC referenced to ground, but the waveform of the AC voltage on each output is of opposite polarity. Because the AC waveforms are of opposite polarity, load devices connected across the two outputs “see” 230VAC because the voltage of the two waveforms add in the load. This is similar to the way a “bridging” audio amplifier operates into a speaker load. Headphone amplifiers referred to as “balanced” are a form of bridging amplifier.
Each one of the two cables connected to the power transformer secondary is “hot” because they both have voltage on them when reference to ground. In the circuit breaker panel, each hot cable connects to a bar shaped conductor that feeds a row of circuit breakers. Load devices that operate on 230VAC are connected to two circuit breakers, one connected to each of two hot conductors.
Load devices that operate on 115VAC are connected to one circuit breaker and the Neutral bar so they “see” the voltage between one hot conductor and the Neutral. The Neutral is called this because it has no voltage on it due to its connection to the ground point in the building circuit breaker panel. There is always a cable connecting the electrical ground point in the breaker panel to a rod driven into the ground outside the building near the panel. The ground of the earth connects the rod at the pole to the rod at the building electrically, so both are referenced to the same ground (voltage).
- Why is there a separate Neutral and Ground? Although the neutral is held at ground (zero volts), it carries the entire current that flows through the 115VAC load.
For safety reasons, unless there is a fault condition there should be very little or no current on the ground conductor. This is the basis for the safety device known as the Ground Fault Interrupter (GFI). By measuring the current in the hot and neutral conductors, the GFI can determine if the current in the Neutral equals the current in the Hot conductor (normal operation). If there is a miss-match, there is a good chance some of the current in the Hot is finding a path to ground other than the Neutral (fault condition).
Audio Circuits
There is a parallel to the AC power scheme in audio; unbalanced and balanced circuitry.
- In unbalanced audio circuitry the signal is a voltage waveform carried by a single active conductor which is referenced to ground. The signal voltage varies above and below ground (positive and negative voltage) and a second conductor held at ground potential serves as the signal return. This conductor is analogous to the 115VAC power system’s Neutral, and is sometimes referred to as “cold” as versus the “hot” signal conductor.
- In balanced audio, there are two signal conductors that both carry the AC audio signal as a voltage, and these signals are of equal amplitude and opposite polarity. This is analogous to the 230VAC power system’s two “hot” conductors, where neither of the conductors is connected to ground. Because all of the current flows through the active conductors, no audio current flows in any ground conductors connecting the two pieces of audio gear. There can be other sources of ground currents such as leakage currents in the power supply, and these non-ideal currents can result in hum in the audio caused by ground loops.
This is the primary reason Lavry Engineering does not use the terms “hot” and “cold” in discussions of audio signal polarity or wiring. While the analogy may hold for unbalanced audio connections where one signal conductor is ground (the signal return), the analogy does not hold for balanced connections where both signal conductors are effectively “hot,” similar to 230VAC circuits.
For these reasons, in discussions of balanced connections the terms “plus” and “minus” are used to represent the polarity of the signal carried by the non-inverted and inverted signal conductors, respectively.
In unbalanced connections, the single conductor carrying the signal is the plus conductor and the signal return conductor is the minus conductor. Despite the fact that the signal voltage in the return conductor is held at ground potential, the signal current in the return flows with the opposite polarity of the current in the active (plus) conductor.