What is Voltage

Voltage is the key to understanding electronics.   Without voltage, nothing happens in electronics.   What is it?   Nobody knows.   We know how to generate it.   We know what it does.   We know how to measure it, but nobody knows what it actually is.

It is also called “Electro Motive Force” or “EMF” which is no help whatsoever in knowing what it is.   That, is roughly equivalent to saying “the thing that pushes is the thing that pushes” – very true but absolutely no help whatsoever.   OK, having admitted that we really don’t know what it is, we can start to say the things we do know about it:

A new battery has a voltage between its terminals.   This voltage is said to cause a current to flow through any complete electrical circuit placed across it.   The current flowing through the circuit can cause various things to happen such as creating light, creating sound, creating heat, creating magnetism, creating movement, creating sparks, etc., etc.

By using the current caused by a voltage, a device called a ‘Voltmeter’ can indicate how big the voltage is.   The bigger the voltage, the bigger the current and the bigger the display on the voltmeter.   The voltmeter can have a numerical display where you read the voltage directly from the display, or it can be an ‘analogue’ voltmeter where the voltage is shown by the position of a needle on a scale.   The size of the voltage is stated in ‘Volts’ which is a unit of measurement named after the man Volta who introduced voltage to the world (it was always there, we just did not know about it).

Voltages add up if they are connected the same way round, i.e. with the + terminals all facing the same way:

The physical size of the battery usually determines the length of time it can supply any given current – the bigger the battery, the longer it can provide any given current.   A battery is constructed from a number of ‘cells’.   The number of cells in the battery controls the voltage of the battery.   For example, an ‘AA’ size battery (what used to be called a ‘penlight’ battery) has a single ‘cell’ and so produces 1.5 Volts when new.   The very much larger and heavier ‘D’ battery also has just one cell and so it also produces 1.5 Volts when new.   The difference (apart from the higher cost of the ‘D’ cell) is that the larger cell can provide a much higher current if both batteries are discharged over the same period of time.

There are several different types of battery construction.   A rechargeable NiCad battery has a single cell but its construction method means that it produces about 1.35 Volts when fully charged.   In passing, NiCad batteries have a ‘memory’ characteristic which means that if they are recharged before they are fully discharged, then the next time they are discharged they run out of power at the voltage level it had when the last charging was started.   Consequently, it is a good idea to fully discharge a NiCad battery before charging it again.

Car and motorcycle batteries are described as Lead/Acid batteries.   This type of construction is not very convenient being large, heavy and potentially corrosive.   The big advantages are the ability to provide very high currents and giving 2.0 Volts per cell.   These batteries are normally produced as 6 Volt or 12 Volt units.   The Amp-Hours for lead/acid car batteries is usually quoted for a 20 hour discharge period, so a fully charged, new, 20 AHr battery can provide 1 Amp for 20 hours of continuous use.   That battery loaded to give 5 Amps, will not provide that current for 4 hours but might only last 2 hours, or perhaps a little better.   The manufacturers literature should give an indication of the performance, but if it is important, run your own test to see how the battery actually works in practice.

“Mains units” are known in the electronics world as “Power Supply Units” or “PSUs” for short.   These convert the mains voltage (220 Volts in UK, 110 Volts in USA) to some convenient low voltage; 12 Volts, 9 Volts, 6 Volts, or whatever is needed.   A mains unit can provide several different voltages simultaneously.

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