How to Read Schematics

hi guys welcome to an electronics repair I'm gonna tackle a big topic now this is not going to be one video it's going to be two or three but I'm going to try and publish these over the next few days

let's get this one through let's do the whole thing and a few chunks and hopefully at the end of this video you will be able to read and understand schematics

I'm not saying you'll be an expert at this there will be times when you are still puzzled by the operation of a circuit but this is a good place to start

so first off the obvious question what is a schematic well a schematic is a diagram a representation of a circuit

it's not the same thing as a physical way out of a circuit on a circuit board it's more a diagram that shows how components are interconnected rather

than how they are placed emulations to each other on the screen behind me you can see a schematic this is in fact from an audio

mixer and if we have a look we will see our schematic is made up of symbols and the lines

the symbols represent components so each of these symbols is a component the different symbols are different

types of components and also marked on and around them you'll see some numbers so for example we see here or Ted a

I'll show you what all these symbols mean at the moment but in fact this one is a resistor or and the 10A refers to the position of that component on the

circuit board so if you look on the circuit board you'll see often in the silk screen printed on there the locations of the components with the numbers so these numbers reference the

location on the circuit board let me show you it makes it much easier here we have a typical circuit board and you will see on the silk screen that's

the white writing on here we have numbers and letters printed so for example here we have r108 through to r111 pointing to these components here

we have some more down the side or 160 and not in order particularly 164 three two one one five eight and they refer to the components nearby you can usually

work out by the fact there are seven numbers there and there are seven components there so in order are one six one one six two one six four one five

eight and so on we will find that some components have different letters so we see this that and the other all this that and the other

these ones are marked I see so I see 401 I see 400 okay these letters and numbers are not the

value of the components these are the reference this is the number we see on the schematic against that component and this is to help you identify the

components on the circuit board during this video we will learn to understand the symbols what symbols represent what components and the common

lettering systems that are used as well it's not always possible to identify a component by the lettering system but it often is here's another circuit board this is

being salvaged for some parts but it's another good example so again we have a similar lettering a numbering system or

63 c83 d23 d22 these Q Q8 there's another q something there D21 and so on some things like

this wire and marked as PG there's a reason for that also during this video you will learn how to identify components by the lettering system you often can so certain letters

are used for certain types of components very often we will learn that and how to recognize the symbols on the schematic this is our first step in understanding

schematics the symbols are like the letters of the alphabet you have to understand the alphabet before you can understand the words okay so in this video part one

we're going to look at symbols in addition to symbols on our schematic you see we have lines these are connections these are wires or tracks on

the printed circuit board these are connections between the components you will also notice that some of these lines have names prefab one disk one

disk two and so on you can see there are names on some of these lines and if we look at some other schematics you will see that sort of quite different from this one

so for example this contains digital and analog circuitry it works different but this is a schematic again this schematic contains some other

features that we're learning about for example some of these names have lines over the top of them okay well by the side in that case what the writing is

sideways so we have these things on our schematic we have more symbols we can learn and this is another schematic the last one I will show you is an example

this represents part of the circuitry on the motherboard and again you can see we have a similar thing but now we have arrows on the lines

we have more labels on the symbols and this is a different type of layout which you can see this type of schematic does not consist of a lot of lines

interconnecting all the components it's laid out in a different way so all of these things we're going to learn about in part one this part

two we will look at the way components are connected and start to understand the building blocks of the circuit if you like if this part is the alphabet of

schematics the second part will be wording the vocabulary of schematics in this video I'm going to use a method I normally use which is pen and paper so

no fancy animated or static graphics on this just pen and paper the old-fashioned way okay so first of all we need to learn the alphabet we need to learn the symbols

there are probably hundreds if not thousands of symbols using schematics and you will not win all of them I don't know all of them but there are certain

ones that you come across much more often than others so some of the most common symbols we need to know one symbol represents what electronic components

and the electronic components broadly fall into two classes passive components and Active Components the passive components consist of

resistors passages and inductors post a type of an inductor called Transformers and then there are a number of

components that are not exactly these classic passive components but they kind of fall in the same group so we have for example fuses

switches relays batteries power sources and nodes we'll put them on here power sources and nodes okay

so let's start with resistors these are a very common type of component we find them on basically every schematic and the symbol for a resistor actually

has two different variations we get this variation how do we get this variation they're interchangeable you can use either of them as you wish

and the symbol doesn't have to be horizontally in the schematic I guess it could be this way around and the same for this type so the orientation of the symbol doesn't

matter these are all resistors still we get a whole variety of specialized ones if you like and all of the specialized ones are based on either these symbols and the symbol is

interchangeable so I'll show you with the next type but after that we'll just stick with one of these but bear in mind we can use either bases for all of these

symbols so the next one we'll use is variable resistor I won't write the word resistor out again otherwise I'm writing a lot of

times here but a variable resistor symbol can be like this or it can be like this and to show its variable we have the slider or the track if you

like okay this can be a rotary variable resistor it can be a slider variable resistor it can be either of them the variable resistor is also there's a

potentiometer that's another term for this there is actually a difference a potentiometer is wired in such a way

that it uses all three pins effectively a voltage on one pen or a signal not on the other ground on the other and the slider will effectively move from one to

the other and we can have a variable resistor it's the same components why the different way and the variable resistor we just use two pins one end of the track and the side so the variable

resistor can also be drawn like this foreign end of the carbon track that forms available resistor under the slider okay likewise it could be drawn like that and

also likewise it could be drawn vertically okay so those are resistors and variable resistors now as I mentioned we have a number of specialized resistors if you

like and I will just use the zigzag symbol but it could always be the other one okay another type of resistor which you don't find very much you'll find this if it's

all in vintage equipment is a tapped resistor like so very effectively it's like a variable is just about fixed in a certain position and you can have more than one tap

like so for example so this is a tapped resistor this is one you very unlikely to come across these days unless you're working on very old equipment

we also have some specialized resistors if you like so we have the very step sometimes also called a voltage dependent resistor or

mov which stands for metal oxide varista this is a type of resistor that changes its value its resistance with voltage

okay and there's a similar one if you like which is the thermistor and this one is a resistor that changes its value with temperature

the varmester has a symbol that looks like this and again we can use a zigzag all the box with the same lines going through it

so these terms are all synonymous with each other and this is the symbol the thermistor is similar so don't mistake them the thermistor symbol

is this okay now with the thermostat there are two main varieties we have thermistors that increase in resistance

with temperature and the ones that decrease resistance with temperature and they both have different uses and circuits now the symbol for both types is the same

but often in the schematic you can't tell which was which because buy them you will either see NTC or PTC written

on the schematic NTC is a negative temperature coefficient which means that as the temperature increases the value decreases becomes more negative with

temperature if you are here and the PTC becomes higher resistance more positive resistance with temperature however you like to think about it these decrease in

resistance as they get hotter and these increase in resistance as they get hotter there's also a variation on the variable resistor called a preset or trim

potentiometrical trim a pot these are the little ones you find on circuit boards you adjust with a screwdriver they're not meant to be continually adjusted by the operator they are meant

for setting the circuit up and then you weave them in the position they set they have a very similar symbol to this one actually and of course with the two

variants so we have this type with looks like a long letter T through it and of course we can have that with the square box as well

oh and we have and all the variants again with both the square box or the zigzag and this is like so

all for that matter and I'll draw this one square like so I like to think of this as looking like a letter T and this component is usually

called a trimmer or a trim a pot there's one other type actually which comes to mind and that is the ldr so

this is a resistor that changes its value with light you shine a light on and it changes resistance normally they become lower in resistance so these are

not common components they really obsolete now we use these a lot of the 1970s and little projects we could make where the effect of what you shot a light on and it sounded a buzzer or something

similar all things are a lot of fun to school kids basically the ldr has this symbol

and yes that can be the other way and an interesting thing about this symbol is the use of these arrows pointing inwards

this is a common thing in schematics and it's used in various components so any symbol you see which has arrows pointing

in towards it like this is wide dependent or white sensitive okay so you can get photo diodes photo

transistors and they will all use Photo Triax as another one photo thyristors all these photosensitive components symbol will be different but in common

with each other they will have these arrows pointing inwards the direction of the arrows is not important this symbol could just as easily be

like so four from this side whichever okay and there are variations on all of these

symbols so for example you'll sometimes see the light dependent resistor like so without the circle around it you do fine variations these components have

been around for decades and things symbols if you like have come in and out of fashion basically during that time so yes you will find variations but really once you know the symbol it

shouldn't be too difficult to work out what that one is okay or even this one yeah so that's all of the resistor types I

can think of maybe there are more but I'm not certain I know of them okay so let's have a look now at the next common passive component which is capacitors

with capacitors there are probably less symbols and variants that you are likely to see but there are some pastors generally come into three types

so you have fixed non-powerized capacitors we have polarized these are your electrolytic capacitors

and we have variable these are the main ones variable capacitors you only find these are radio frequency circuits really

fixed and polarized capacitors are fixed capacitors they have a fixed value the main difference is that these have a positive and negative term no one must be connected the correct way around and

these don'ts so your basic symbol for a capacitor this okay again it could be horizontal what's this

and this is your fixed non-polarized capacitor we then have the polarized ones so these are your electrolytic and however it's

semi-capacitors now these ones they do have variations this is the symbol I normally use and this end is the positive on the schematics sometimes you will see the

plus sign and sometimes you will not but there are a number of variants on this in particular the United States variants the very common one is this

okay and again this end is a positive and likewise you may or may not see the plus side on the schematic on the symbol there are a few older versions of this

in particular this one you may well come across on Old schematics okay again this is the positive and the similar in between if you like

is the non-polarized electrolytic capacitors these have a symbol like this so it's similar to this one this has the

thicker eye compared to that one and by the way I will just mention these can be also draw with the bar filled in okay

hope so again this is the power rise these two are the non-power guys these are the electrolytic capacitors but as I say they don't have a poster minus you find

these particularly in audio crossover circuits and that sort of thing so those are the symbols you are most likely to come across

if you're working on radio frequency circuits particularly vintage radio you will find variable capacitors there are some variants again I won't show all of

them but this is a variable capacitor that looks similar to the variable resistor this arrow through the diagonal Arrow suggests something is variable okay whether it's a resistor or other

capacitor it uses the same methods if you start to realize these things it becomes very easy to analyze these symbols to what they are

it'll be no surprise then that the trim a capacitor this is a preset variable one will have the T like so okay there is an unusual variant of this by

the way I'll just show it to you in case you ever come across this one and that's that symbol variable capacitor

this is obsolete I would say but you may come across it so now you've seen it if you remember you'll know what it is and we can get what I'll call gangs or

dual capacitors or even triple variable capacitors so you get symbols like this where we have two capacitors together variable

variable and then we have a dotted line so between the two wipers you have a dotted wire wipes so that tells you that these are ganned so they basically both adjust

together okay they don't adjust separately the angels together that's what's called ganged okay and it will come as no surprise now

we were talking about resistors that you can get guns resistors these for example like your stereo controls on an amplifier so you have two variable

resistors on the same shaft they vary at the same time and assemble for that one is the same as this really so you have your two variables

and the dotted line shows a guns the other day of mechanically interconnected all for that matter okay

but this is a less common way of doing it another way of doing this and you may see this on capacitors resistors and doctors for that matter is they will use

the markings on the schematic the component reference so you'll see something like we'll use a resistor because this as well be a capacitor

VR 2 a for example variable resistor 2 a v r 2 b and this AV business tells you this is

both parts of VR2 A and B and they don't need to be next to each other on the schematic they could be completely different positions on the schematic but from this reference component reference

number and you can imagine here we could have like a VC one a this VC one b rather than showing this gang thing okay

so that's another way of doing that but you'll be happy to know that that's pretty much all we need to do with capacitor symbols and you can see now I hope the correlation between these types

of symbols between resistance capacitors and the next one we're going to do is induct us and you will see this again and while we're doing this we will look at the close related Transformers

there are many many variations on these symbols again but the first thing I will mention is the basis of the symbol just like the resistor we had

the zigzag well we had the boxes are basically the symbol with inductors we have four variants

would you believe it okay so we have this one which is an inductor we also have this one was the one I tend to use

an inductor we also have a box like the resistor but filled in

I get a filled box that's an inductor and we have I've never particularly seen this one but it exists it's out there we have a box with a letter L in L being

the symbol for inductance okay so we have those four bases of our symbols and these are interchangeable okay so this basis can be used for many

of the other symbols they're all interchangeable I'll show you what I mean with an inductor usually the coil the wine is wrapped around some sort of form

those formers generally are either solid iron or they are ferrite which is like a metal or iron oxide mixed with a resin

or similar type solution it could be a ceramic type actually so often the symbol will also tell us the type of core that the coil is wrapped around

so you will see for example this one with two solid lines this is an iron core and you will see

the same with a dotted line this is a ferrite core okay this is the gray black substance you often see coils

wrapped around basically iron usually in Transformers you can recognize it it's sheets of iron okay as I mentioned all four of these can be like this so I'll just draw one in just

but I'm sure you get the gist of what I was saying so you could just as easily have this type of symbol with the iron or with the

ferrite okay what other inductors do we have well we have variable ones there's a few symbols for this but this is the same basis as

those variable capacitors and variable resistors so this should come there's no surprise in fact you could probably draw these now without me even shooting you

what they are so a variable inductor I'll draw this type okay the capacity right the resistant preset

or we'll trim it like the T theories the same as resist same as a capacitor and you can combine all this so you can have a

variable and doctor with a ferrite core and any other combination thereof there is another way to draw variable ones and

really one of them Works a bit like that strange variable capacity we had and one of those most Market variable resistant so we could have this I'll use this type of stone it doesn't matter

we could have this okay we can have this this is rather like that capacitor is well remember the wallet huh

this on common I can't save everything I've seen that one but if you ever see it that's what it actually is and we can have tapped inductors so like the touch resistors that we had

they fixed tap for example there so effectively the position the tap should give you the idea of the ratio of voltages towards this end so if that is 50 volts that might be a 10 volt tap okay

I'm not saying it always does represent where it is on the winding but it could do and those are your inductors really and I'm hopeful you're seeing a pattern with

all this now okay Transformers Transformers are just more than one winding on a former this is a Transformer okay

this is a transform of two secondaries okay and by the way notice I didn't put the solid lines between there it's kind of optional sometimes they're not there at all okay

this is a Transformer with a center Taps secondary windy huh and one other thing I mentioned about

Transformer symbols is this you'll see this you'll see this definitely so for example in this case two quills Drive in one coil no reason why we can't do that and

you will see this why not put it there these dots represent the phase so they're saying that when this end of the winding is in

the positive half cycle AC yeah that this induced voltage will also be in the positive R cycle at this end

but when this winding is inducing voltage into this one when this end is positive the positive is at that end

so effectively this one is passing the AC through just increasing the voltage or decreasing the voltage okay this one is passing the voltage

through with increasing or decreasing but it's also inverting the phase so that's what those are they represent the phase

there are loads of esoteric weird and wonderful doctor and Transformer symbols but I don't think you'll come across them if you do you'll certainly know their

inductors or they are Transformers and you can go and look up the symbols yeah but there's one more I will do because you may see this this is a

motorized in three phases okay with the Santa tap you may see that so I think we'll put that one in and we'll call that complete okay so that's inductors

now you may think that resistors capacitors inductors that's the three types of passive components and to some extent you're correct but there are other components you will find on

schematics with symbols that although not exactly passive components they are not Active Components either let's look

at these other passive components and so crystals and resonators symbol for a crystal is this one

yeah resonator is this one again that usually this actually drawn horizontally fatal is a bit bigger so it's more clear okay using this is connected to ground

by the way and you may say ground on it and then we have another one very similar oh oops almost missed that capacity there

actually okay which is that one so these two are resonators and this one there's a crystal those are the symbols of those ones

also in these group of passive components we've got fuses so fuse is either like that or it's like this

that's a fuse often marked on the schematic with the value of the fuse similar these will often be marked with the frequency of the crystal

one megahertz but they don't have to be yeah but usually they are marked at least with the value we have batteries as a single cell battery

probably better to call the cell because of battery by definition it's more than one cell okay we've also what you mention is that I already got that first

this is the positive end so these are battery and you'll often find on schematics voltage and current sources constant

common sources so a voltage source and these are normal schematics where they try to teach the principle of the circuit rather than actual schematic of

a device okay but the voltage source like so plus minus and of course AC

voltages and the constant covers similar the arrow donates the direction of the Covenant flow so it could be either way okay but it's donating the direction of

current flow and you will sometimes also see this one and then I go by the side of it quite often with a current Source by the symbol it would tell you what the

coverage is 50 milliamps for example okay and the same with the voltage source it may tell you the voltage so those are the other passive component

symbols that we're likely to come across let's talk now about switches and relays so with switches the symbol is very

simple under a number of variations and these variations will depend on how many switch positions there are one position two three six and how many

poles in the switch there are so a simple basic switch is this symbol but if it's a push button switch in particular

it could be this symbol okay so those are switches and we find the various types of contacts if you like so

this one is a single pole double throw it has two positions okay and this is abbreviated to SP single

pole DT double throw this is a single pole single through which has one active position the other one's just open circuit single pole single throw

you will find uh double pole double throw so this is this type of switch okay and the two are gone together so often you'll find the dashed line like

we were looking at before with ganged variables just as a gang the variable capacitors so that is in fact a double pole and we can get all sorts of different

ones you know we can have a single pole three-way sp3 through usually okay so those are your switch symbols there's many varieties of symbols as there are

switches but these are all switches so it's easy to recognize them and the relay is very similar to this so with the relay we have the coil

usually representing the core of the relay and then we have the contact so for example we could have like this

this would represent a single pole double throw so it's SP double throw and this represents the normally closed

contacts so you'll often see on the schematic and see with the relay or n o this doesn't mean no contact it means

normally closed when referring to a relay and normally open so you can gather from this that when the relay is not energized that contact is closed this one is open that's what it means by

normal it means when the relay is not powered and save me drawing the whole thing again we can just put some more onto this one so we now have a double pole double

really and again you can see the normally closed position let's see here normally open there are of course lots of variations on these symbols for switches and relays

but given the information you have here you should be able to work out what is happening on any schematic you find that has these components the final topic for this part of the

video is nodes nodes are not electronic components they are symbols on a schematic that tell you where certain components and the lines

which are tracks on the schematic are connected to so I give you a good example we have for example on a schematic a resistor

a capacitor and these are connected to a line on the schematic that tells us this end of each component is connected together now this

may be connected to a node typical would be ground so if we see this symbol it tells us this track this end of each of these components is connected to grounds or

Earth if you like there are other symbols symbols of this so you find variations for example you will find this one used as ground

you will find this one used as ground they're all synonymous with each other you'll also find another slight variation on this you may find this quite often

which is this one this is normally what is called hot ground so this is a ground a zero volts reference for your circuit but it's not

a safety ground this is typical in power supplies this would be connected to the negative end of the large smoothing capacitor and hence via the bridge rectifier to the mains

so those are all ground symbols these are safety ground symbols in some circuits you may find more than one ground so the power supply may have a

ground and for example the audio circuits an amplifier ever separate grounds so you will sometimes find symbols like this one [Music]

hey and that's an analog or audio ground and you will find in the same circuit probably a normal ground as well and normally the two are connected together

in some way viral or value resistor or capacitor even but you will find that so when you see this it tells you that every time you see the symbol on the schematic somewhere else

those points are connected to each other and in other parts of the circuit you may find for example the hot ground symbol then you may have

this here obviously not connected to that I will say and you know those points are connected together so this is what nodes are it's just a way to show things that are connected to each other

without drawing tracks all over the schematic Like A Rat's Nest all the common nodes you will find are going to positive supplies so

this for instance VCC or five volts is telling you at that point is connected to 5 volts a very common

where you will see this use I'll just give you an example we may have an integrated circuit that does something and

out wall house two and from the bottom you will see something like that which tells you this is ground for example paying for

this might be pin one two three and something like this VCC and this may be for example pin eight so you'll see those on schematics and

that's what they are they're nodes of course they may be drawn differently okay so now we have all of the passive components and pseudo passive components

symbols part two we'll look at all the Active Components that we're likely to come across and then we will have really the alphabet of schematics we will know all

of the symbols that we're likely to find part three then is as I said the vocabulary of schematics this is how these things join together how to

understand the circuits how to read them okay hope you enjoyed that part one part two I think tomorrow and I'll see you all then ciao for now guys