Transformers
An Electronic Device, and also a Action Hero…
Transformers
• Transfers electrical energy from one circuit to
another.
• No physically electrical connection between the
circuits.
• May involve changes in voltage and current.
• Frequency does not change.
Construction
• Primary coil or winding.
• Secondary coil or winding.
• Neither are connected to each other.
Construction
• Why doesn’t current jump
from one winding to the
next in a transformer or
inductor?
• The wires inside a
transformer and inductor
are covered in a varnish or
enamel that insulates one
winding from the next.
How Transformers Function
•• The
primary
winding
receives
theit’s
source
ACfrom
The
secondary
winding
receives
energy
• The core material provides a path for the
and
creates aflux
changing
magnetic
flux. winding
the
magnetic
created
by
the
primary
magnetic lines of flux.
and sends it to the load.
Explaining how the magnetic field
“cuts” the windings in a
transformer. (again)
Coupling Circuits (continued)
The DC component is not transferred.
3 Types of Transformers
• Step-Up Transformer
▫ It delivers higher voltage than the source, aka, it
steps the voltage up. The voltage on the secondary
side is higher than the primary side.
• Step-Down Transformer
▫ It delivers lower voltage than the source, aka, it
steps the voltage down. The voltage on the
secondary side is lower than the primary side.
• One-to-One Isolation Transformer
▫ The voltage on the primary is the same as the
secondary. There is no increase or decrease in
voltage.
What would the output of a oneone transformer be if the input is
a square wave?
?
Transformer Example
The Core of a Transformer
• The material of the core can be made from a
variety of materials. It can also be made out of
air.
• Typically an air core is useful for high frequency
applications while iron cores are used for low
frequency applications.
If the core is made out of iron, it is
usually in the form of a shell core.
• Laminations are insulated from each other.
• This makes shells electrically insulated from
each other (good for less eddy currents) but not
magnetically isolated from each other.
Similar to a row of staples
in a stapler.
• Take 5 minutes and read section 19.2.2 to
yourselves.
What happens to the fuse on the primary side of a
transformer if there is a short in the secondary?
The fuse blows even though they aren’t connected!!!!!!!!!
RVOTD: Danger, Will Robinson!
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High Voltage Lineman at work
• https://www.youtube.com/watch?v=SkFH8lLv
KZ0
• Not sure if already shown
Transformer Simulation
• http://vcephysics.com/content/2012/07/transfo
rmer-simulation/
Transformer Ratios
• You can calculate if a transformer is a step-up,
step-down, or one-to-one by simply looking at
how many turns there are of wire on the primary
and secondary sides.
• It can also be given by the equation below
Where:
Np = number of turns in the primary
Ns = number of turns in the secondary
Vp = voltage applied to the primary
Vs = voltage induced in the secondary
Useful Ratio Form
• Sometimes you may be given a “turns” ratio.
This may be one number that represents:
such as 10.0, or a ratio like 10:1
Example #1:
Vs?
120V
50
200
The Turns Ratio
• Common ways to express the ratio
▫ 10:1
▫ or 1:10
▫ Which of these steps-up voltage?
• Ratio is always quoted “Primary to Secondary”
• Ratio does not indicate the number of turns
Vp?
Example #2:
• Step-down Transformer
• Aka “Buck” Transformer
5V
7:1
• Find:
5V
35V
120V
Example #3:
• “Boost” Transformer
Transformer labeled:
1:5
Vs?
• Find:
600V
• Voltage does not just magically increase out of
thin air. Something must be lost in the process,
or else we would be making energy from
nothing.
• What do we lose in the process?
▫ Current!
RVOTD
• http://www.youtube.com/watch?v=uXEPy6Za6
cI
• High Current, low voltage (1V)
• 50kAmp powersupply, shorting crow bars
• If we have a perfectly efficient transformer, the
power transferred from one side to the other
must be kept the same.
• Since P = VI, the Voltage multiplied by the
current on the primary must be equal to the
voltage multiplied by the current on the
secondary.
Pp = VpIp = VSIS = PS
Pp = V p I p = V S I S = P S
• Therefore, if the voltage increases then the current
must decrease to keep the power the same.
• So current can also be calculated using the number
of turns on the primary and secondary side.
• IpNp = ISNS
Example
IpNp = ISNS
• A transformer has 100 turns in the primary and
600 in the secondary. There is 3A of current in
the primary. What is the value of current in the
secondary?
Example
IpNp = ISNS
• A transformer with a turns ratio of 8:1 has 2A of
current in the primary. What is the value of
secondary current?
• IS = 16A
Example – try these on your own
Video of
• https://www.youtube.com/watch?v=vh_aCAHT
hTQ
• Just watch first 3 minutes. The rest is for 3
phase
RVOTD
• http://www.bbc.co.uk/news/magazine15069858
Ignition Coils are Transformers
• From 6:17 minutes til end of ignition coil segment
• http://digital.films.com/PortalViewVideo.aspx?xtid=401
95&psid=0&sid=0&State=&title=Ignition%20System%2
0Operation&IsSearch=N&parentSeriesID=16198&tScrip
t=0#
Type 40195
into search
box in films
on demand
Transformer Schematic Symbol
Sometimes there are taps off the secondary.
Sometimes there are multiple secondary windings.
Multiple Winding Transformers
used in power supplies
Variac or Auto-Transformers
Phase Dots
• Phase Dots refer to whether the voltage on the
primary side is in phase with voltage on the
secondary side. There are either in phase or 180˚
out of phase.
In Phase
Out of phase
Phase Dots
In Phase
Out of phase
Maximum Power Rating for a
Transformer
• Transformers have a maximum power rating.
• This means the wires can only handle so much combined
voltage and current before they break down.
• Transformers usually have their power rating printed
right on them.
• A typical transformer could be labeled 24VAC, 3A, 60Hz.
This implies a 24Vrms secondary, a 3A maximum
secondary current and 60Hz frequency application. (It is
understood the primary voltage is 120V.)
Measuring for transformer type.
• You can measure the actual physical resistance
on the primary winding and the secondary
winding.
• This will tell you if you have a step-up, stepdown, or one to one transformer.
• Do you think the windings in the secondary side
of a transformer has more or less resistance than
the primary for a step down transformer?
• Why – because there are more windings for higher
voltage and those windings are usually thinner
because they don’t have to handle as much current,
this causes the resistance to go up even more.
Review –
• What type of transformer has a larger voltage on
the secondary than the primary?
▫ Step up transformer
• In a step down transformer, what has more
resistance, the primary or the secondary?
▫ Primary
• Suppose you overpowered your transformer, what
do you think will happen?
▫ Melt wires, cause short between primary and
secondary, maybe fire.
• If the voltage is increased in the secondary side,
what is decreased?
▫ Current
• What are the phase dots used for?
▫ To determine if primary and secondary voltages
are in phase or out of phase
• What causes an induced voltage on the
secondary?
▫ A changing magnetic field cutting across the turns
of the secondary windings which was caused by a
changing current in the primary windings.
Do impedance matching
transformers on white board.
Misc Oscope Questions
• http://www.allaboutcircuits.com/worksheets/sc
ope1.html