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Reading Schematic Diagrams.
By Brian, MW0GKX.

What is a Schematic Diagram? Well if you don't know then should you be reading this? The answer is yes, otherwise how will you learn? This article will give you the basics of reading Schematic Diagrams.

What is a Schematic Diagram?

A Schematic Diagram is the technical drawing that shows people how to make up an electronic circuit, rather like a recipe tells you how to make a cake.

Unlike a block diagram, which shows how the various circuit blocks interact with each other, or layout diagram, which shows the physical placement of the components, a schematic diagram shows the actual electronic connections being used to connect the components up to make the circuit.

How do they "work"?

Schematic diagrams use symbols to show what type of component is at that point in the circuit and they can be very confusing when you first see a schematic with it's various little symbols and other information (numbers, letters etc.).

The symbols have differed from country to country and have changed over time, but are now to a large extent internationally standardised (although there tend to be US and European standards that do differ slightly I try to cover both here).

It is a usual, although not universal, convention that schematic drawings are organized on the page from left to right and top to bottom in the same sequence as the flow of the main signal or power path. For example, a schematic for a radio receiver might start with the antenna input at the left of the page and end with the loudspeaker at the right. Positive power supply connections for each stage would be shown towards the top of the page, with grounds, negative supplies, or other return paths towards the bottom. If the schematic has Positive voltages 0 volts and Negetive voltages then the 0 volts line is usually thicker and toward the centre of the diagram (0 volts may be thicker wherever it is, but this, sadly, does not seem to be standard).

Schematic diagrams intended for maintenance may have the principle signal paths highlighted and various voltages marked to assist in trouble-shooting the circuit. More complex devices have multi-page schematics and must rely on cross-reference symbols to show the flow of signals between the different sheets of the drawing.

Lets look at a simple schematic diagram:

Sample schem

I have labeled the components 1 - 7 with the same type of component using the same number. This diagram is actually for a small, unregulated, DC power supply.

The input is on the left (~, the symbol for Alternating Current) with the DC output on the right.

What symbols are used?

The two tables below represent the most popular component symbols used in electronics today. I have not shown any IC's, just passive and active discrete components. The difference between active and passive is that active components require a power source to work, whereas passive components do not.

This first table shows passive components, that is resistors, capacitors and inductors, and also electrical components such as switches and relays (although technically, as a relay requires a power source to operate, it should be an active device). Also shown are the symbols for wires that are not joined (no electrical connection) and wires that are joined (an electrical connection) speaker etc. In Europe the symbol for the resistor is a rectangle, in the US symbol for the resistor is a zigzag, therefore you may come across either in schematics depending whether they were drawn with European or US symbols, but it is unusal to find both on one schematic.

Passive and electrical components

This next table shows some active components. At one time vacuum tubes were being replaced by the smaller transistor and integrated circuit, but they are finding their way back into electronics for use in professional audio equipment and some radio transceivers. I would like to point out that the demise of these thermionic devices was in no way due to poor performance, it was because of the physical size, weight and power consumption when compared to transistorised equipment. The US symbol set has a circle around most active components, whereas the European symbol set does not.

Active components

To see more component symbols see: Component Recognition page.

Simple test:

Sample schem

See if, using the tables above, you can name the components numbered 1 - 7.









Simple test Answers:

  1. Switch.
  2. Fuse.
  3. Transformer.
  4. Diode, four of them in a bridge rectifier configuration.
  5. Electrolytic capacitor.
  6. Resistor.
  7. Light Emitting Diode (LED).

Well, how did you do?
Here is the same diagram with component labels and values, more like you would see on a schematic for the equipment:

Sample schem2

Often, but by no means always, there is a seperate component list with the values for the components shown on the schematic, other times you will find the values marked on the diagram.

Standard Component name Abbreviations:

There are a set of widely used abbreviations that are used on schematics but, unfortunately, there is no World Standard for the abbreviations. I will list the commonest ones here to give an idea of what you will come across.

Well that's the basics. Using the information on this page and the Component Recognition page should be enough to get you going with understanding "Circuit Recipes".

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