Voltage

Voltage defines the power to surpass a resistance. The higher the voltage, the more resistance can be overcome.

That’s why voltages above 30V are considered dangerous: higher voltages can surpass the resistance of the human skin and drive a harmful current through your body. Voltages below 30V cannot do this and are considered safe. That’s why DIY projects typically use voltages well below 30V.

30V is a conservative threshold. Healthy and dry skin will not conduct electricity up until 50V. However, when skin gets wet or has lacerations, even voltage below 40V can be lethal. By keeping voltages below 30V, you make sure electricity cannot penetrate your skin even under the most adverse conditions.

Voltage and Current

The total electric power (in Watts) is defined as voltage x current. This total power is the energy that can do work for you.

Voltage influences the total power in two ways:

• The higher the voltage, the more power can be driven through a resistance
• If the voltage is too low to penetrate a given resistance, no current will flow at all, and the total power is null.

To clarify: if you touch wires connected to a 5V voltage, no current at all will flow through your body because the voltage is too low to drive electrons through the resistance of your skin.
To operate devices, you also need a certain minimum voltage. For example, LED have an internal resistance, similar to your skin. LED define a forward voltage (minimum voltage) that must be present for current to flow. Voltages below their forward voltage will keep LED dark.

AC and DC

In AC, the polarity changes continuously many times per second, and current flows back and forth. The voltage is not constant but rather an average voltage, and the electrons in a wire rather vibrate than flow. Typically, electrical household power is AC (due to the way how it is generated and transported to you).

In DC, the polarity does not change, and current always flows in the same direction. Typically, battery power and electrical power provided by power adapters is DC.

If you rub something, it gets hot: you applied a force. It does not really matter though whether you rub back and forth (AC), or drag something behind you in one direcion (DC).
In either way, friction (resistance) was overcome, and this applied force produces heat.
In the same way, both AC and DC can perform work. It does not matter whether electrons travel from A to B, or whether they just vibrate, as long as they move.

In most hobbyist projects, you use DC and therefore need a battery, or a power adapter to convert AC to DC.

The reason AC exists at all is that it can be easily produced using magnetic fields in generators and turbines, and that AC can be efficiently transmitted over long distances because AC voltage can be easily raised or lowered using transformers.

To transport energy, the higher the voltage, the lower the current can be (because the total energy is voltage x current). This reduces losses through heat and is the reason why electrical transmission lines use super high voltages like 1.200kV and more.

Voltage Sources

Voltage exists whenever there is a difference in electric charge between two points: this creates an electric field between these two points that can in turn move (apply force or induce energy into) electrically charged particles (electrons).

This potential difference can exist in a number of ways:

• Battery: a battery uses chemistry to store more electrons on one side than on the other. It is a DC source. Since a battery stores electrons to produce the voltage, its supply is limited and deplets over time, and while you use it, the voltage slowly decreases because the difference in electrons on both poles of the battery decreases.
• Power Supply: electrical energy is produced elsewhere (AC) and delivered to your home. A power supply converts the energy from AC to DC and lowers the voltage to safe levels that you then can use. Power supplies deliver endless energy with a constant voltage (as long as you pay your bills).
• Generator: similar to a power plant, you, too, can generate electrical energy by converting physical movement. A dynamo or generator moves magnets embedded in a coil to produce alternating magnetic fields that induce electrical current. Like a power plant, this is AC.
• Photovoltaic: Solar cells use substrates that convert the photon energy of natural light into electrical power and produce DC. The voltage depends largely on the amount of light which is why you cannot use solar cells directly. You need a voltage regulator to convert the wide range of incoming voltage to a fixed output voltage that you can use.
• Static electricity and Lightning: when air is dry (and less conductive), huge amounts of electrons can gather on surfaces. When enough electrons cumulate, the resulting voltage can be so incredibly high that even very high resistances such as air start to conduct at this voltage. The results are little sparks (with electrostatic discharges) and huge thunderbolts (with lightning). Even though electrostatic discharge is not dangerous to you due to the low current, it may well be dangerous enough to sensitive semiconductor components that you play with, and can destroy them. That’s why you shouldn’t touch their pins or use ground straps to prevent electrons from building up.

Hydraulic Analogy

A good way of picturing voltage is a hydraulic analogy. When a pump (power source) is driving water through a closed circuit of pipework, then voltage is the speed of the water (depending on the speed of the pump), and current is the amount of water (depending on the pipe diameter and the volume of the pump).

Both the speed (voltage) and the amount (current) of water together determine the total amount of water (energy) delivered.

This also explains the minimum voltage required to pass a resistance: if there is a bottleneck (resistor, load, paddle wheel) in the pipework, more pressure (voltage) is needed to drive water through it or overcome the initial resistance of the load/paddle wheel. If the bottleneck is severe (heavy paddle wheel, high resistance) and the pressure is low (low voltage), no water flows at all (the paddle wheel won’t move even a bit). A minimum pressure (minimum voltage) is required for any given resistance.

Breakdown and Forward/Reverse Voltage

Both breakdown voltage and the terms forward voltage and reverse voltage describe the same phenomenon: it is the minimum voltage that causes a material to experience electrical breakdown and become electrically conductive. Put simple: it is the voltage at which a material changes from being an insulator to becoming a conductor.

For non-semiconducting materials like metal or plastics, breakdown voltage is all that matters, and there is no separate forward or reverse voltage because the direction of current makes no difference to them.

Semiconductors (like diodes or transistors) are different: as their name implies, they act differently depending on the direction of electric field (polarity).

That’s why semiconductors need two breakdown voltages, one for each direction of electric current. To better differentiate, their breakdown voltages are called forward voltage (for current flowing from + to -) and reverse voltage (for currents flowing the opposite direction). Occasionally, reverse voltage is called breakdown voltage.

• Forward Voltage: this is the voltage at which electric breakdown occurs for current in the forward direction. A diode for example is designed to conduct current only in one direction, however it does so only when the voltage exceeds its forward voltage. For silicon diodes, this is roughly 0.6-1.0V, and below this voltage, a diode would not conduct current in either direction.
• Reverse Voltage or Breakdown Voltage: this is the same, just for the other direction: a diode for example blocks current in reverse direction, yet when voltage exceeds its breakdown voltage, it becomes conductive nonetheless.

Materials are classified as insulators and conductors but this differentiation depends really only on the voltage applied (their breakdown voltage): an insulator (like plastic) is a material that conducts at a high voltage only (because it has a high breakdown voltage) whereas a conductor (like metal) has a much lower resistance (aka breakdown voltage) and conducts always, even at very low voltages.

This electrical phenomenon has practical importance in many cases, including these:

• Safe Voltages: The human skin has a breakdown voltage of 40-50V so playing with voltages below this threshold is considered safe: there is no risk of electrocution because at this low voltage, the human skin is an insulator.
• Insulator: An insulator insulates only up to a certain voltage. Once a voltage exceeds the breakdown voltage of the insulator, it is no longer an insulator and becomes a conductor instead. You need to carefully select the insulation material for higher voltages to make sure it really insulates. This also explains why switches are rated for certain maximum voltages: once voltages exceed the limit, the air insulating the small distance between the two contacts inside the switch becomes conductive, and sparks can form. And it explains why you can get electrocuted when playing directly with 110/220V household voltage while at the same time you can safely play with even the hugest batteries, generators or power supplies as long as they are below 30V.
• Diodes: Diodes are components that conduct only in one direction. If a large enough negative voltage is applied to the diode, though, a diode will give in and allow current to flow in the reverse direction. This large negative voltage is again the reverse voltage. Some diodes are actually designed to operate in the reverse breakdown region, but for most normal diodes it’s not good for them to be subjected to large negative voltages. For normal diodes this reverse voltage or breakdown voltage is around -50V to -100V, or even more negative.
• LED: A LED is an insulator at voltages below its forward voltage. Once voltage reaches or exceeds the forward voltage, the LED becomes a conductor and starts to emit light. At the same time, LED have a very low reverse voltage of just 5V: applying power in wrong polarity can destroy them.

How does electrical breakdown actually work?

Metals are considered “good” conductors (they conduct even at very low voltages), whereas plastics insulate. Why do these materials behave so differently?

In metals one or more of the negatively charged electrons in each atom (conduction electrons) are free to move about the crystal lattice. In materials like plastics and ceramics (“bad” conductors), all electrons are tightly bound to atoms.

When voltage is applied, it creates an electric field that applies force to electrically charged particles such as electrons.

• Metal: At a low voltage, the electric field is weak. Its force, albeit weak, is still strong enough to move the freely movable electrons in metals: a current flows, the metal conducts despite the low voltage.
• Plastics: The electrons in plastics or ceramics are tightly bound to their atoms. This force is much stronger than the weak force of the electric field at low voltage.

When the voltage exceeds a certain threshold (the breakdown voltage), the electric field becomes so strong that it exceeds the force of the atoms, and their electrons suddenly start to move. This moment is called electrical breakdown.

In essence, electrical breakdown in a material occurs when the electric field becomes strong enough to pull electrons from the molecules of the material, ionizing them.

Once electric breakdown occurs, the released electrons are suddenly accelerated by the field and strike other atoms, creating more free electrons and ions in a chain reaction, flooding the material with charged particles. This is why (almost) no current flows below a certain voltage, and why any material suddenly becomes conductive once voltage exceeds a certain threshold.

At which voltage this occurs is just a matter of the given material, its inner structure and how strongly its atoms bind the electrons. This is measured in volts per centimeter: it characterizes the dielectric strength (or conductivity) of a material.

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