How to read circuit diagrams (Part 2) - component specific aspects

Polarised components

Certain components are polarity-sensitive, so it matters a lot which way round they’re connected: some parts will be destroyed if improperly connected so the circuit diagram will always show the correct orientation.

Examples include:

Diode, triac, thyristor symbolsDiodes and rectifiers: these are the most common polarised components. Usually we mark the cathode with a ‘k’ and the anode with ‘a’. Previously we used a + sign (only) to mark the cathode. This applies to diodes, rectifiers, LEDs, Zener and Schotkky diodes, bridge rectifiers, etc. Thyristors have terminals of anode (a), cathode (k) and gate (g), and triacs have a main terminal 1 (MT1), main terminal 2 (MT2) and gate (g) which we show clearly in our own circuit diagrams. Generally you’ll need to read the circuit diagram carefully to identify the pins.

Electrolytic symbolsElectrolytic capacitors: it can be highly dangerous to reverse-connect them so the polarity is always clearly marked in circuit diagrams, with a + sign showing the most positive connection. Both variations of symbols are shown (EN/ DIN/ BS and ANSI).

MOSFET and bipolar transistorsTransistors: bipolar (NPN or PNP) transistors have an emitter (e), base (b) and collector (c): at EPE Magazine we show these letters on our circuit diagrams, but usually you won’t see them in manufacturer’s diagrams.

Something to look for is the arrowhead in the transistor symbol: it denotes the emitter but sometimes in data sheets etc it might not be drawn too clearly. Furthermore, we clearly show the transistor pinouts in all our drawings, removing any scope for connecting them wrongly!

MOSFET transistors (n-channel or p-channel) have three terminals – drain (d), gate (g) and source (s) which again we show clearly in our own circuit diagrams to help beginners. However you’ll generally need to identify them yourself in circuit schematics.


How i.c. pin numbers are depicted in circuit diagramsIntegrated circuit connections are only identified by their pin-numbering sequence, and to mis-interpret them could be disastrous, so check the circuit diagrams closely.

The circuit (left) shows a 555 i.c. in a demo circuit with its pins numbered as shown. The positive supply rail is +9V and the 0V rail is also shown. The device's data sheet would explain the functions of each pin.

Sometimes an i.c. terminal (pin 4) might be shown with the letters “nc” – it's to give you confidence that there’s no connection: it’s been left as a “floating” pin.

Regulators usually have three terminals, Input, Output and Ground (or adjust), which are clearly labelled on our circuit diagrams. Regulators are easily destroyed by wrongly connecting them so it’s vital that the circuit diagram is clear in this respect.

Special features

Some components have particular features that we can emphasise clearly in our diagrams by using graphics. The best way to explain them is with some typical examples.

Multi-pole connector symbolConnectors come in all shapes and sizes, from single-pole test sockets to 1,000 pin sockets that connect to a computer microprocessor. A pole is an individual element in a connector, so a three-pole plug and socket carries three such connections simultaneously. We can show this clearly in circuit diagrams by using a dotted line to group them together.

The symbol for the receptacle or ‘socket’ (female connection) varies. We use a 'cup' as shown. In EPE we may annotate the receptacles as SK1a, b and c  and the matching plug as PL1a, b and c.  Connection systems used to interconnect circuit boards ('box headers') are classed by the number of “ways” instead.

Transformers vary from tiny RF types used in radio circuits up to large types operating at mains voltages or higher HT (high tension) levels. Usually their pinouts are clearly identified in circuit diagrams so that there can be no mistaking which way they are connected. Sometimes, circuits use a specially-made inductor or transformer, possibly wound by hand with  enamelled copper wire (magnet wire).

phase 'blobs' on a small transformerSome coil schematics show a blob marking the start of a winding. This ensures that the phase relationships of waveforms are maintained, so it’s necessary to construct the circuit with this in mind.

There may be special provisions for “screening” or metal shielding to prevent interference, or grounding the steel cores of mains transformers to a “safety earth”. The circuit diagram should show this clearly.

DPDT (double pole changeover) switchSwitches are classified by the number of “poles” and “ways”.  Circuit symbols for switches are easy to understand. Simpler types may be described by the number of “throws”. The simplest is an on-off switch, or  single-pole single-throw (s.p.s.t.) push button or toggle switch. Then there are changeover switches (single-pole double-throw or s.p.d.t.) that switch from one circuit to another.

A double-pole double throw (d.p.d.t.) has six terminals (photo) and changes two inputs between two possible states.

3-pole 4-way (3p4w) switched - dotted line shows it's 'ganged' By coupling or “ganging” several switches together, you can control multiple circuits at the same time, so a circuit diagram typically shows a mechanical connection with a dotted line.

This is true of any type of switch, whether toggle, slide, pushbutton or rotary. A 3-pole 4-way rotary switch (shown) has three “inputs” (poles) and four possible outputs (ways), and a dotted line shows how they are mechanically connected. The moving part is sometimes called the wiper (w).

2 pole 6 way switchAn old-fashioned “wafer” or “Maka” switch consisted of enormous banks of ganged switches (10 or 20 poles or more) which were mechanically linked in elaborate circuit diagrams (all hand-drawn!). Again, dotted lines show how the “wipers” of switches are rigidly interlocked: changing one switch position changes them all.

Photo (left) shows a 2 pole 6-way switch, the two wipers being visible in the centre.

Relay symbolRelays use an electromagnetic coil to activate some switch contacts. The circuit schematic usually identifies the ‘internals’ of a relay, a coil with its resistance value shown optionally, and the separate contacts that the relay coil operates magnetically.

typical relayMore complex relays have pin numbers which match the markings on the relay base itself, and these may be shown on the circuit diagram.

Otherwise you’ll have to refer to the relay manufacturer’s datasheet and compare it with the circuit diagram to figure out which solder terminal does what, inside the relay.

Variable resistors and capacitors are straightforward enough to interpret in circuit diagrams. Sometimes the circuit diagram shows the CW (clockwise) and CCW (counter-clockwise) ends of the carbon tracks, so when you study the diagram you can figure out the effect on the circuit when you rotate the control one way or the other. The same is true for “multi-turn” preset resistors, say a 10-turn trimmer control found on a circuit board to adjust or “trim” a voltage.

dual-ganged potentiometerYou might sometimes see “ganged” devices, such as a stereo volume control (i.e. a dual-ganged potentiometer). Just like a multi-way switch shown earlier, the movable wipers are shown on diagrams joined together by a dotted line, so when you rotate the shaft of the control, both potentiometers move at the same time.

The photo (left) shows a typical dual-ganded potentiometer used in a stereo amplifier. It's two seperate potentiometers that rotate at the same time when the shaft is turned.

Screened cables: some electronic equipment, such as an audio, radio or video unit, is likely to be sensitive to interference from external sources and so screened cables are used to shield the central signal wire from noise. They have a braided copper 'jacket' to protect the innermost signal wires. The outer screen is often grounded at either or both ends, and again the circuit diagram will show us the intentions of the designer.

showing a screened cable outputThis simple block diagram of a radio module shows how its output is intended to be connected using coaxial (screened) cable. This prevents interference or degradation of the output signal.

Note also the ground symbol on the "input" side of the module, and the aerial (antenna) symbol above it.

Some radio circuits do indeed have a physical connection to the soil, which acts as a "mirror" of the aerial, making it twice as effective.

screened output In an actual circuit diagram the screen (the braiding of a screened cable) will clearly be shown connected to, say, the 0V rail.

In the example shown (left) the output from this audio amplifier goes to a connector (such as a phono socket) and the outermost braid of the screened cable (arrow) is connected to the 0V rail. It's a classic way of showing a screened or shielded signal connection.

Test Point for voltage measurementTerminals and Test Points:  it sometimes helps with the process of debugging, troubleshooting or repairs if some test and measurement results are shown in key locations in the circuit diagram. An obvious one would be on the power supply rail(s), to show the voltage. A test engineer can then compare multimeter readings against the circuit diagram.

There appears to be no standard way of depicting them in schematics, but one sensible way is to use designations like “TP1” or “TP2” on the diagram and ideally on the silk-screen print (if any) of the PCB.

The sample shows a test point which in a fault-free circuit should give a reading on between +13.5 to +13.9V on amultimeter. For voltage measurements, unless otherwise stated, it’s universally understood that they are given with respect to the 0V rail.

Test reading for currentCurrent flow (symbol I) may be shown on circuit diagrams with an arrowhead showing the direction of current flow. Assuming the tester can break into the circuit with an ammeter, a possible test reading might be given. (Quiescent current is 'idling' or tickover' current.)


Schematic Standards

I’ll close our discussion by mentioning some of the industrial standards and differences that appear in circuit diagrams generally. Component symbols do vary slightly, depending on what standards are used. Circuit diagrams that typically originate from the USA or Far East use ANSI (American National Standards Institute) symbols, but alternative European-style symbols will be centred on DIN (Deutsches Institut für Normung) or EN (EuroNorm), or the British Standard 3939.

Capacitors EN/BS -ANSIresistors EN/ BS -ANSI I usually consider a circuit as “American” if the capacitor symbol is a straight line and curved line, but resistors can be either zig-zag (USA or European style circuits) or rectangular (generally seen on European circuitry).

It’s a good idea to be aware of variations of symbols that are commonly used in books and data sheets.


This article introduced circuit diagrams and illustrated some of the finer aspects of interpreting them. There’s no substitute for picking up an electronics magazine, downloading a manufacturer's data sheet or electronics data book and actually studying some circuitry, and with practice you can soon read them exactly like you would a roadmap. Alan Winstanley.

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