Harold Hall


Basic Electrical Theory DC     Harold Hall

A contributor to the Model Engineers’ Workshop magazine commented that he thought some readers do not fully follow articles on an electrical topic due to a lack of understanding of basic electrical theory. Because of this, I was asked to write a short series in an endeavour to improve the reader's knowledge. The following is an edited version.


It is not my expectation that these pages will enable viewers to design their own electrical circuits, though for a few it just may tip the balance in this direction. For most, hopefully, it will help them understand other people's designs and to fault find at least on simpler equipment.


It was however, a contributor to the ME forum asking how to calculate the result of placing capacitors in parallel and in series that prompted me to place these pages on my website.


The Supply

Electrical power is available in two forms, Direct Current(DC) and Alternating Current(AC). Most common will be the supply from a battery (DC) and the household supply (AC).


What then is the difference between AC and DC supplies? Direct current is where the supply voltage is both fixed in value and polarity, whilst alternating current rises rapidly from zero to a maximum then back down to zero, repeating this but with the polarity reversed, SK 1.

Workshop Data

In the UK, and many other countries, this cycle is repeated 50 times a second, known as 50 Hertz, 60 Hertz also being common.


In very basic terms, much theory is equally applicable to both AC and DC circuits, typically, as the voltage increases so does the current proportionally. Unfortunately, however, additional factors become important in AC circuits that complicate the issues considerably. Because of this I am dealing with AC circuits in a separate set of pages. I must though first clarify the difference between voltage and current for those who need this, doing this using the frequently adopted comparison with the flow of water.


Voltage / Current

Sketch 2 shows a tank of water feeding two outlets, each controlled by a tap. One tap is fully open, the other opened only a little, the flow from one is therefore greater than from the other even though the head of water and therefore the pressure is the same for both.


In electrical terms the water pressure equates to voltage and the flow to current. The control over the flow provided by the taps equate to the resistance (impedance in AC circuits) provided by two differing loads.

Another point worth noting is that with the tap fully closed the pressure is still there even though there is no flow. Similarly in an electrical circuit the voltage is still available even when no current is being demanded.



If you would like just the formula rather than all the theory then you may find the “Metalworkers Data Book” helpful.


As the title says the content of these pages only cover the subject at a basic level. They should though help with fault finding and designing simple control systems. Also, understanding other web sites and magazine articles having an electrical content. If you would like to go beyond that I list on the last page some web sites and books that may be of help.