Tutorial: How to design a transistor circuit that controls low-power devices--- make money online

hey guys been here I thought I'd make a

quick video that shows how I normally

design circuits with transistors so

today we'll focus on just switching

things on and off like actuators most of

the time you have a microcontroller and

want to control something like a motor

or solenoid or something so this video

will show you how to size the transistor

and set it up so let's zoom in and I'll

show you what I've got okay so one of

the most common tasks that I need to do

with the transistor is just to turn

something on or off so not really

amplifying a signal it's it's just

acting as a switch so I drew a couple

diagrams here today we're going to be

controlling this which is an air valve a

solenoid air valve and it's a 12-volt

device I just put power on it and it

opens pretty simple so I just drew it in

this diagram as a coil which is a good

way to represent it it's like an iron

core coil so to turn the thing on and

off we could either put a switch between

the coil and ground or between the coil

and our voltage source it doesn't really

make any difference to the coil but it

will make a difference to us later if we

choose a switching component so I listed

off a few of them and I'll describe why

I didn't pick why I did or didn't pick

them for this project we could have used

a relay it's a bit overkill in fact this

one is quite overkill it's big and

expensive it's mechanical it doesn't

switch very fast the best things about

relays are that they can handle very

high currents and they isolate the

circuit they isolate what is being

switched from the control circuit so

we'll save that for another day

similarly a solid-state relay also a

very cool component mostly used for

isolation and high voltages or high

currents so save that won a MOSFET or an

IGBT also for much much higher currents

they don't isolate I guess they sort of

could but we're not going to use these

today either a Darlington transistor

this is actually not a bad choice today

we don't need to use a Darlington but I

think I'll probably do a video on these

later because they

actually are quite useful in some

circuits generally for higher current

stuff today that that solenoid is

doesn't really use that much current so

we're going to use this little guy a

standard bipolar transistor bipolar is

just a plain transistor that's that's

what bipolar means and this one happens

to be a PN twenty to twenty two a and

this little tiny case here is called a

teo ninety two case so the next thing

the first thing to do when picking a

bipolar transistor is to decide whether

we're going to use NPN or PNP you may

have heard these terms floating around

so I made a little diagram here to help

us decide which one we're going to use

first off the schematic symbols the

arrow in the in the symbol always is in

the direction of the conventional

current flow and the arrow always points

to the N material so NPN or PNP so

that's one way to remember it

also watch out I switched the emitter

and collector on this between these two

in the in the diagram here so it's

collector base emitter emitter base

collector so in a PNP the conventional

current flow goes from the emitter to

collector and in an NPN it goes from

collector to emitter so you may be

wondering well you know what what can we

do with all this basically what we're

going to do is put one of these in place

of the switch here and the two terminals

of the switch are going to be the

collector and emitter or emitter and

collector depending where we put it in

the circuit here and the way that we get

these switches to turn on or off is by

either pouring current into the base in

the case of an NPN or drawing current

out of the base in the case of a PNP so

it's it's a switch that is controlled by

a current flow okay so a minute ago I

said that

a transistor is a switch that is

controlled by a current flow and that we

were trying to decide between PNP and

NPN for our circuit so how do we

actually decide in this case the thing

that's going to make the decision for us

is our control signal and in this case

it's going to be our favorite 0 to 5

volt logic signal that this could come

from a microcontroller or a PC parallel

port or a number of different things so

let's look at each case if we went for a

PNP transistor and we have this either a

0 volt or 5 volt control signal we don't

have an easy way of controlling how much

current is going to flow out of the base

of the transistor to turn it on and off

and the reason for that is if the base

is at 5 volts or we have a 5 volt signal

and then there's some sort of circuitry

between it and the transistor there's

going to be a current flowing from our

supply voltage in this case it's like 12

but it could be a number of different

things and and in in the two cases of

the base being 0 or 5 there isn't really

an easy piece of circuitry we can put

here that will allow us to control the

base current flowing from the emitter to

the base in the case of the PNP however

in an NPN transistor we have a really

great option it's just a resistor so

when this is at 5 volts we have a

current flowing from the base to the

emitter and that's going to turn the

transistor on and when this is zero no

current is going to flow and the

transistor will be off so this is

perfect so in this case the NPN is an

obvious choice in fact in all of my

projects I probably use NPN transistors

95% of the time in switching

applications like this I think you would

almost always want to use an NPN so just

keep that in mind it's I think in

general NPN s are quite a bit more

common so let's talk about actually

choosing a transistor and setting it up

with this solenoid valve so we've

decided to go the NPN route because it's

easier for us to control the base

currents with an NPN in the type of

circuit that we're using here

and I said that a transistor is a

current amplifier or a current

controlled switch so if we pour some

current into the base and it flows from

the base to the emitter that will turn

the transistor on and allow a large

current to flow from the collector to

the emitter and the amount of

amplification is known as the

transistors gain so a transistor with a

high amount of gain will allow a large

collector current to flow with just a

small base current and a transistor with

not much gained will not have as much

amplification and that amplification

factor is known as hf e or it's also

sometimes called beta

that's the gain of the transistor at DC

i should also point out that we're not

talking about AC circuitry here at all

this is all just steady-state DC

analysis to make our lives easy so what

we're going to do here is pick a

transistor and this is a little bit

where you know experience comes into

play I mean there's thousands of

transistors available how do you pick

one well you know for Hobby projects in

sort of small-scale design like this

there's really only a handful that come

up so I'm just going to say that we're

going to pick the PN 2222 a which is a

really great low medium power very

inexpensive easy to use switching

transistor and it has a beta of about a

hundred we could be more conserved in

that but we'll just say beta is a

hundred for this particular transistor

it varies quite a lot and I mentioned

Darlington transistors earlier

Darlington transistors are basically two

two transistors ganged together to

increase the gain so those can have

gains of like you know ten thousand or

something and that could be quite useful

for switching higher current loads while

keeping your in your base current low so

anyway so let's let's get back to the

design that we're actually doing today

this solenoid valve this is a Clippard

solenoid valve and if we get the spec

sheet in here

there's not very much for electrical

specifications

it just says 0.67 watts at 12 volts they

make this in 12 or 24 volts this is a 12

volt variant so we've got our our

schematic representation here 0.67 watts

at 12 volts well we want to know the

current so we use Ohm's law which is

current times voltage equals power or in

this case we want to go in reverse point

six seven Watts over 12 volts put some

units on there for you I just type this

one up is 0.05 five eight amps or about

fifty-six milliamps okay so if we didn't

know which transistor we were going to

use you could just start looking down

the list of transistors in a catalog and

think to yourself well the transistor is

at least going to have to handle 56

milliamps although you'll find it pretty

much every single transistor can handle

more than this this is not a very big

requirement here so what we know at this

point is that we're going to use an NPN

transistor it has to handle 56 milliamps

from the collector to the emitter and we

need now we need some way of turning the

thing on and off and if we did decide to

use the PN 2222 a we know that its beta

is 100 so what we have here is our

transistor and IB the base current is

flowing like this this is grounded this

is the coil with our voltage source and

this is the collector current flowing

through the coil to ground and I said

that beta is 100 meaning that this is a

hundred times less than the collector

current so IB is equal to 56 milliamps

over a hundred or 0.56 milliamps so now

the question is how do we how do we get

point five six milliamps to flow into

the base luckily there's a really easy

way we can just add a resistor so if we

put a resistor here and then send this

over to +5 volts if we sized the

resistor correctly we'll get exactly 0.5

6 milliamps to flow through here the

transistor will multiply it by a hundred

and we'll get the current to flow

through the coil so let me draw this a

little bit more clearly and then we can

choose our resistor value so our circuit

is almost done this is a pretty good

almost finished version of it here we've

got a resistor on the base where we're

going to put our 0 to 5 volt signal and

the coil is connected here the solenoid

is connected here so that when the input

is 5 volts we get a current flowing

through this resistor which turns the

transistor on and allows a large current

to flow through the coil to ground so

you're thinking yeah but I live in a

voltage world how do we actually get

this thing to turn on and off so we put

this resistor here to convert our input

voltage to a current and you might be

thinking well that's easy I'll just use

Ohm's law five volts over the resistor

value gives me the current almost

there's another little twist here the

base current is actually five minus 0.7

divided by the base resistor and this

point seven is a an intrinsic value two

silicon transistors this is an NPN so

there's a PN Junction here and silicon

PN junctions will drop a voltage and for

pretty much all transistors I think it's

about 0.7 volts I think it's point six

volts for diodes I'm not exactly sure

why that is but and you can look that up

and so we know that to get our coil

working we needed 56 milliamps to flow

so the IC has to be let's say equal to

56 milliamps

and I said that the transistor

amplifiers current by a hundred so we

need at least point five six milliamps

to flow through the base but it's

actually a better design practice to

shoot a little bit higher because we

want to make sure that the transistor is

fully on what would happen if we gave it

less base current let's say we only gave

it point one milli amps in the base so

you'd multiply that by a hundred and we

get ten milliamps flowing through here

and that would not be enough to activate

our solenoid so the way we have it

designed now if we chose a base resistor

that only allowed 56 milliamps to flow

through the collector the circuit would

just be on the cusp of working so we

want to design it to be a little bit

safer so let's just say we want to shoot

for a base current of 1 milliamp so we

use the multiplication factor of a

hundred and we get a hundred milliamps

through here so you might be thinking

we'll wait a minute how do we get a

hundred milliamps flowing through this

the answer is that we actually don't get

a full 100 milliamps because the current

is limited by the solenoid itself if we

were to short this or if we were to

connect the solenoid directly to 12

volts and ground it's only going to flow

as much current as the manufacturer is

rated 56 milliamps so a transistor can't

allow more current to flow through the

device than it would normally use on its

own but it can restrict the current okay

so let's let's solve this guy we've got

IB is equal to five minus 0.7 over RB

and I know the IB is one milliamp so

when doing the calculations always use

the standard units so enter this is

point zero zero one amps

4300 ohms

well they don't make 4300 ohm resistors

so we could either use a 4.7 K and since

we were pretty safe on our factor of

choosing one Billy amp for the bass this

4.7 K resistor will flow a little bit

less than one milliamp but we'd still be

pretty much okay so there's one other

thing I wanted to add here this for this

particular circuit since we're switching

a coil it's actually very important that

we add a clamp diode and that's going to

look like this so you might be thinking

that's stupid but having the diode is

pointing into our voltage source how

could this ever get higher than plus 12

and the answer is that when the

transistor turns off all of that current

that was flowing through this coil has

created a magnetic field in here and

when the transistor turns off that

magnetic field collapses and the voltage

is going to rise at this point it could

be quite significant if the coil has a

lot of inductance the voltage could get

to be you know hundreds of volts here

high enough where it would actually blow

out the transistor so it's extremely

good design practice to put a die a

clamp diode in there it could be a real

problem because you might design this

and switch it 20 times on the test bench

and it works fine and then you go out

and put the circuit into service and it

switches and it gets to be just high

enough and it blows your transistor out

so it definitely put a diode there and

you can also take it one step further

and add a capacitor in there as well

like this and this can be small like

point zero zero one micro farad's and

the capacitor will smooth out the pulse

here and give the diode time to turn on

and make sure that there's absolutely no

spiking or voltage rising going on in

here so let's build it and test it and

prove that the thing actually works

okay so here's the circuit yellow is

plus twelve volts black is a common

ground and red is plus five we've got

our transistor here the PN 2222 a 4.7 K

base resistor and our clamp diode so if

we put five volts into the resistor

control the valve quite nicely so what

we've done here is taken a very very

small signal five volts at one milliamp

or less and are able to control

admittedly a pretty small signal but

we've taken care of the inductive kick

from the solenoid valve so I thought of

just a couple other things you might be

wondering about throughout this I was

kind of trying to get the base resistor

value chosen and you might be thinking

well screw it we'll just put a zero ohm

resistor we'll just short this thing out

and make sure we have enough base

current but unfortunately you can't do

that because the transistor will draw so

much current through the base that will

destroy itself so so you have to have a

resistor here around the transistor will

self-destruct the very first time you

use it in fact when I was getting

started in electronics you know I

thought a transistor was basically just

a voltage controlled device and so yeah

I put five volts on that base and oddly

enough it didn't do anything it didn't

switch didn't allow current to flow so I

tried another one the same thing and I

thought man these transistors are just

totally defective but as it turns out

they were just being destroyed instantly

so yeah you have to have a base resistor

here also there's going to be a voltage

drop across here so so I said that

there's a point seven voltage drop here

so if we measure all these voltages

we've got five volts here we got zero

volts here we've got point seven volts

here that's you know five - point seven

over RB across this resistor gives the

current flow through here but what's the

voltage right here this is actually

pretty interesting this this voltage

could be

close to zero or it could be about point

one or point two volts depending on the

transistor and how much current is

flowing through it so I said that the

reason we have a point seven volt drop

from the base to the emitter is because

that's the way silicon semiconductors

are built we've got a PN Junction here

throughout the from from the collector

to emitter there's a PN and an NP

Junction so that point seven volts

cancels itself out so if the transistor

is perfectly built the voltage here will

be zero when the transistor is fully

conducting so but you know the problems

in manufacturing and I think they might

even build them asymmetrically on

purpose or something so actually let's

measure it and see what we've got

turn our circuit on okay sorry about

that guys I I had the wrong voltage set

on the supply here so we've actually got

0.1 1 7 volts alright see this thing

right about there we've got 0.1 1 7

volts which is what you would normally

expect for a circuit designed like this

so keep that in mind here if your device

requires exactly 12 volts across it

you're not going to get 4 full 12 volts

with a transistor like this you're going

to lose a tenth of a volt in the

transistor so I hope that about covers

it

obviously transistors get quite a bit

more complicated when talking about

switching things at AC when you start

talking about frequency gain and stuff

it adds a whole nother level of

complexity but I hope this gets you

started with DC design so let me know

what you think let me know if you have

any questions and I will see you next

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