Electrical Transformer Tutorial

Uploaded by Afrotechmods on 07.11.2010

In this video I am going to explain how transformers work and show you how to
wire one up to step mains voltages down to lower voltages
that your projects may need.
In my project we will step down 120V to 24V
but the same principles will apply to most transformers.
I'll also give you a quick peek at how you can go from mains electricity
to creating a dual rail DC power supply.
So what is an electrical transformer?
A transformer is a device that takes electricity,
turns into a magnetic field,
then turns it back into electricity.
Depending on how the transformer is designed,
you can use them to step up voltages
or step down voltages
or even keep the voltages the same if all you want is electrical isolation
from your wall outlet.
So basically transformers help you get from one voltage to another.
And when you remove the protective casing
all transformers end up looking like this...
with a bunch of wires wound around an iron or ferrite core.
Next I want to talk a little bit more about how transformers work.
Transformers only work with AC.
An AC voltage on the input creates an expanding and collapsing magnetic field
within the transformer's core.
Now when you put this alternating magnetic field in the presence of
another coil,
the moving magnetic flux will induce electrical current
into the secondary coil
so you will get another AC voltage on the output.
Here I have a transformer that takes 120V on the input
and gives you 12V on the output...
so we call it a step-down transformer.
Now just for fun I am going to use a crappy magnetic field probe,
and if I move it near a transformer that's in use
you can see the magnetic field expanding and collapsing at 60 Hz.
That's because stray magnetism is inducing current into my probe.
The same thing is happening on the secondary side of the transformer,
but obviously more efficiently, so I get the full 12 volts.
But with DC, the magnetic field would stay still, so no current would be induced in the secondary coil
and nothing would happen.
Now let's move on to more conventional circuit diagram.
The simple theoretical model of the transformer will usually show you the turns ratio.
The turns ratio is how you know if you are stepping voltage up or down
and by how much.
And when I say turns, I mean literally the number of times the wire was wound
around the transformer's core.
The left side is called the primary side and this is where you connect your input voltage.
The right side is called the secondary side and this is where you connect your load.
Now in school they will tell you that the ratio of Vin to Vout is the same as
the ratio of primary turns to secondary turns.
So if you have a transformer with a ten to one ratio it will divide the input
voltage by ten, and that's what you'll get on the output.
With the simple theoretical model of transformers, the turns ratio is all
that matters, so here's an easy formula for you to use in your homework.
Now the problem is, if you actually want to build something, that's all bullshit!
Because if you try to build a transformer solely based on those equations,
all you will get is this.
In the real world, transformers behave a lot more like this,
or this,
or sometimes this.
It gets complicated.
In other words, transformers have all kinds of inductances,
resistances, capacitances
and magnetic hysteretic effects
and if you're new to working with transformers you should be buying a brand
new transformer specifically designed for your purposes.
Don't try to salvage old transformers from PCBs and expect them to work with
mains voltages.
Transformers tend to work best only at certain frequencies.
So let's talk about how to buy a transformer for your first project.
For the rest of this tutorial I'm going to be working with a transformer that
I bought to convert 120V AC at 60Hz
down to 24V AC.
You could use a transformer like this as part of a homemade guitar amplifier
or possibly a bench power supply.
You can get transformers like this at Jameco.com
And you can also get them at Parts Express.
And in Canada I got mine from Lee's electronic.
Just search for transformer and you'll see various options with different output
voltages and current ratings.
If you see term "CT",
or numbers like 12-0-12,
that means it's a center tapped transformer,
which means you will have a choice of having the full 24V AC
or splitting it up into two 12V supplies.
More on that later.
Now if your transformer is rated in VA instead of Amps, you might be
wondering what that means.
Well basically you can think of VA as being equal to the power rating of
the transformer in watts.
Now that's technically incorrect,
but for anything a beginner is designing, they might as well be the same thing.
So if you think your project will draw 50 watts,
buy a 75 VA transformer so you have a bit of a safety margin.
If you want more information on what VA means,
Google "Volt-Ampere" and "power factor".
Working backwards, for the 45VA transformer I bought here,
the maximum load that I would try put on it is about 30 watts just for the sake of
having a nice safety margin.
and I should probably keep an eye on the temperature too.
So pick your voltages, figure out how much current your transformer needs to handle,
and buy your transformer so I can show you how to wire it up.
Okay... got your transformer? Let's do a quick review of mains electricity.
If you live in north America you will have power cables that look like this on
the inside.
The black wire is the live wire that carries electricity into your home.
The white wire is the neutral wire for the return path of the current.
And the green wire connects to the earth, or literally the ground,
and is used as a safety feature in some appliances.
For what we are doing today, you don't need to worry about the earth wire.
In Europe, the wires do the same thing, but the color convention is different.
The live wire is usually brown,
the neutral wire is usually blue,
and the earth wire is green and yellow.
Okay so let's do a quick test of the wiring.
I have my multimeter set to measure AC volts,
and I've double checked that the cable is in the volts jack, not the amps jack.
I'm making sure that there's no way that the wires could possibly touch
and short out while I'm measuring,
and I have clipped and taped off the earth wire because I'm just not using it for this project.
Finally remove any wire clippers from the area.
If I plug it in, I should see roughly 120 or 240 volts.
Good! It worked.
Once I'm done with that, I unplug things because it's not a good idea to leave
exposed power wires lying around more than necessary.
Now let's talk about how to connect those wires to the primary windings of
a step-down transformer.
Here's a diagram of my transformer and a picture of the real thing.
If you have 120V electricity and wire the transformer
up like this, it will work just fine.
You'll get your 24 volts out and it will be usable.
However, you can only use the transformer at half the power rating.
If you want to use your transformer at its full capacity
you'll have to wire two pairs of live and neutral wires to the primary.
Now why did they do that? Why did they make life difficult with those extra windings?
Well, if you have 240V electricity like in Europe and Asia,
you will have to wire things differently.
Connecting the wires like this will change the ratio of the transformer turns
so you'll still get 24 volts out instead of 48 volts out.
So basically this is a transformer that can be used at two different input voltages
by making connections to the primary coils differently.
But, not every transformer will be like this.
Some transformers will not give you the option of having multiple configurations
on the primary side or the secondary side.
I already showed you an example of a transformer I ripped out of an old boombox.
All it does is take 120 volts and spit out 12 volts.
This is a nice and easy solution but at least if you know how windings work
you will be able to use all kinds of transformers in future,
not just the easy stuff.
So let's solder things up.
Here's my soldering job,
and if you want you could also put a fuse in line with the live wire.
Okay so now the primary is wired up... let's put the safety goggles on,
stand back, plug it in and see what happens...
Excellent. Zero fatalities - a new record.
Now let's see what's happening on the secondary side.
On the outer terminals we're getting about 27 volts AC
and that should drop down to 24V once you put a load on it.
So if that's all you want you can just solder some wires to these terminals,
ignore the center terminal,
and you'll have 24 volts RMS AC for your project.
Also, since the secondary side is isolated from the mains, it's now safe for me to
clip on my oscilloscope probes to take a look at the actual waveform.
And for safety's sake do not connect your oscilloscope's ground clip
to the primary side of the transformer while you are doing this.
You will create a small explosion if you do this.
On the secondary side of things,
let's connect the bottom terminal to ground, and measure the voltage of the
top terminal.
We get a nice sinusoid as expected
and this is a good reminder that RMS voltage is not the same thing as
peak to peak voltage.
The 27V RMS figure is helpful for knowing how much power
you'll be able to deliver to resistive load
and the peak to peak voltage of 78 volts
tells me that the rest my circuit will need components they can handle 78 volts
later on.
For example, I will need 100 volt capacitors as an absolute minimum to handle those 78 volt peaks.
Now I promised to talk more about center tapped transformers so here we go.
Do you see how the label says 12 0 12 volts?
Well instead of calling the outer terminal ground
let's call the center terminal ground and see what's happening on the oscilloscope.
The two waveforms are in opposite polarity.
They are also about 39 volts peak to peak which is half of the 78 volts
that we were getting earlier.
This makes sense because we're effectively splitting the secondary coil
into two pieces
and measuring voltages with respect to the center tap.
So having a center tapped transformer
gives you the option to have a negative voltage at the exact same time that you
have a positive voltage. And this is very useful when designing dual rail
power supplies.
Now check this out.
If I add a couple of diodes and capacitors,
and measure the voltage coming off the capacitors,
now I have a dual rail +18V and -18V DC power supply.
It's a crude one but for low currents it will work.
So I'm going to talk more about diodes and power supplies in future videos
so stay subscribed and thank you for watching!
If you follow my advice, your experience with transformers should be better
than Bennet's was.