Articles, Blog

Electronic Circuits | Transformers

December 29, 2019


Peter Wheeler: Okay, hi. Hello again. Today I’d like to talk with you about transformers. These are not those toy robots that turn into
monster machines, these are devices used in electronics and electrical circuits in order
to transform, change, voltages, currents, and we’ll discuss it today, impedances. There’s an interesting physical situation
that occurs when you take a conductor. Let’s take a conductor and let’s put a plus
and a minus here. When you move charge, so we talk about current,
it moves for example in this direction, from positive to negative. Some people talk about electrical movements,
and that’s fine, we can even do that as well. Let me do this for a second, let’s clean this
up a little. Okay. Electrical, electrons move from minus to plus,
charge, current moves from plus to minus. Now if you’ve got current moving, for example
let’s look at the end of this conductor, if we look at that and here it’s coming at you. We use our right hand and we put the right
hand, so it’s coming at me, we put the right hand thumb in the direction of the current. What you’ll notice is your fingers will curl
in the direction of a magnetic field. You’re saying, “Okay, but is that the same
for electrical, electron flow?” Electron flow does the same things except
what you do is you use your left hand. If we look at the flow, because it’s leaving
us, I’m going to show it like this. I use my left hand, my thumb pointing to you,
guess what? My fingers also curl in the same direction
of the magnetic field. The magnetic field whether it’s formed in
your mind by current flow or electron flow, the magnetic field will still be in the same
direction. Now the interesting thing is we’re just moving
charge here. The other aspect of magnetic fields and current
or electron flow is that if a magnetic field cuts this conductor, if we have a magnetic
field cut this conductor, we will actually cause charge to flow in that conductor, or
electrons to flow in the other direction, what have you. There’s an interesting relationship between
charge moving and a magnetic field being formed, and then vice verse, a magnetic field cutting
a conductor and the charge being required to move as a result of that magnetic field. It’s a very tight interaction between these
two physical phenomena. The property that we describe in being able
to create a magnetic field by moving charge is, the property of the conductor is called
inductance. Inductance is signified by the letter, the
symbol, L. The units for inductance are Henrys. We measure inductance in Henrys, and the symbol
for Henrys is H. We notice that there’s a situation that arises
here, okay, what can we do with this? What can we do with this? We’ve got a long length of wire, we push a
charge through it. We can create a magnetic field. We know that if we have a magnetic field moving
and we have say a long length of wire, we should be able to move a lot of charge as
well. Why don’t we create a device? Why don’t we build a device here? What I’m going to do is I’m going to take
a lot of wire and I’m going to coil it up. Why? Because I want to stay tight with my device,
so if I coil it up I can make it occupy a very small space. I’m going to push, in a second I’m going to
push a changing current through this coil of wire. Here, this is a direct current, it’s just
going in one direction. What I want to do is put a changing current
through this device. As well I don’t want the magnetic field just
to be out where it wants to be, I’d like to concentrate it. What I’m going to do is I’m going to wrap
these wires around an iron core, a type of [toroid 00:04:41], bobbin let’s say. On the other side then I’ll wrap a smaller
set of wires, in this case here fewer turns, on this device. I’ve got more turns, fewer turns. Now, what I need to do is, it’s one thing
to setup the field and it stays there, but I need to cut the other side of these turns
here with my magnetic field. Which means what I’m required to put in here
is an alternating signal, AC signal. It’s like we’re going to take our charge and
we’re going to move it, and then we’re going to slow down and are going to reverse direction
and then we’re going to slow down, and I’m going to move it again, and one more time
I’m going to do this. I’m going to put a lot more charge in, there
it is, it’s bubbling out, and then I slow down and then here I’m going to put more charge
in, let’s bubble out and then slow down, and back and forth. I represented this as a voltage changing in
time, in amplitude and a time scale here. What’s going to happen? We’re going to, right hand rule if you want
to do plus and minus, or left hand rule if you want to do minus to plus, you’re going
to create a magnetic field that is going to cut the secondary turn. This side of this device, which is a transformer,
we call these primary. This is our primary side, PRI-mary, primary,
and this, secondary. We have our primary turns and we have our
secondary turns. There is a relationship between this signal
that I’m putting in and this signal that I’m going to put out, take out. In fact if I look with the oscilloscope and
this is my signal here, what I’m going to see on this side is something like this. Now you see this is smaller than this, from
a voltage perspective the amplitude here is smaller. That is because I have fewer turns in my secondary
to my primary. I could flip this around, I could make this
my primary, this my secondary, and then I’d have a small signal going in and a big signal
coming out. The thing about in and out on these devices
is that the energy in this world is finite, we cannot create anything or destroy anything,
we can only change its format, maybe light into heat and such. If we look on this side here, then our power
on our primary side is equal to our V-primary times the I-primary, power is V times I. Therefor,
this also has to equal power in the secondary, which is V-secondary, I-secondary. Now here’s an interesting, if I use this configuration
here, my voltage will go down. If my voltage goes down and this side power
has to equal this side power, primary equals secondary, what’s going to happen is my current
will go up to compensate. Power in the secondary, power in the primary
have to be the same, which is an interesting now, because if we look at the voltage in
the secondary and we divide that by the voltage in the primary, it’s the same as saying that
you had the secondary turns ratio divided by the primary. In this case here, this is a, let’s say a
two to one step down transformer, you can calculate how much you’ll step down by measuring
the voltage in the secondary, dividing by the voltage in the primary, and it’s the same
as the turns ratio. Not always present to you in a datasheet,
and we’ll show you a datasheet in a minute, is the actual turns ratio that way. There’s another way of determining turns ratio. If we go back to our inductance side here,
the fact that I’m trying to create a magnetic field, this inductance causes a small problem
in how fast the charge can move. I can put the voltage there but the charge
can’t move right away because it’s somewhat pushing to create this magnetic field. That concept is called reactance. Reactance is the pushback of the current with
respect to the voltage that’s applied. If we actually showed on a scope the different
between the two, or in a phasor diagram, we see that the voltage is leading the current. The current will catch up but the voltages
will be in front of it as we continue to change it back and forth, back and forth. Now that is reactance, but there’s another
concept because this winding, this winding too, as will this one, have a small, very
small, but will have a small resistance in it. By the way, just to clarify something, these
two are separate. There is no electrical connection between
the primary side and the secondary side, putting an Ohm meter across here will show you an
infinite or open circuit. We’re coupling our signal as a result of changing
an electrical field to a magnetic field, back to an electrical field. Let’s get back here, because like I said not
all datasheets will give you turns ratios. Reactance is the pushback to the current flow
when the voltage is applied. In this case here we have resistance, this
is pure play resistance, secondary resistance, primary resistance. The entire circuit pushing back on what we’re
trying to apply, that is called impedance. We denote that with the letter Z, impedance. If we actually then look at a datasheet, more
often than not they will give you the impedance at some fixed frequency of this primary circuit
and the impedance of the secondary circuit at that same fixed frequency. If we do this, if we take the impedance of
the primary and divide it by the impedance of the secondary, we will also get then our
turns ratio, our primary to secondary turns ratio. That’s another way of calculating. Notice this is slightly different, this is
secondary, secondary, primary, that’s this way. This is reversed as far as the impedance,
as far as what’s in the numerator and the denominator. That’s it. Reactance plus resistance is impedance, although
measured in Ohms in both cases here. Because the voltage is leading the current
in the inductor this is known as an inductor, but in the resistor the voltage and the current
are in the same phase for the resistance, then in order to calculate the two, and we
won’t spend much time on it here, what we do is we say the impedance is equal to the
square root of the sum of the squares of the resistance and the reactance. Again, I’m not really trying to cover that
today, what I’m trying to explain to is this connection from one side of a magnetic circuit,
electromagnetic circuit, to the other side. One last point before I leave this, normally
the manufacturer will put what is called a phasing dot on there, which means that if
it’s going up here, the signal on the primary, it will go up on the secondary. If you reverse your leads, what you’ll find
is the signal will be 180 degrees out of phase. This is showing in phase, this is showing
out of phase. I think we’ve got it.

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1 Comment

  • Reply Luqman Hakim September 22, 2018 at 8:42 am

    thank you.

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