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What is a Transformer? An Electrical Transformer Tutorial

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 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… Nothing! 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.

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