Transformers
What is a transformer?
A transformer is a device that transfers electrical energy from one circuit to another through inductively
coupled conductors—the transformer's coils. A physical and schematic diagram of a transformer is
shown. The primary winding is a coil of wire as is the
secondary winding.
Transformer Action
An AC voltage connected to the first or primary winding creates a varying current in the primary winding
and thus creates a varying (expanding and contracting) magnetic field in the transformer's core and this
varying magnetic field induces a voltage in the secondary winding. If the secondary winding has a load
connected to it then a secondary current will flow. It is important to note that this process occurs in
spite of the fact that there is no direct electrical connection between the primary and secondary. The
electrical energy is inductively coupled from the primary to the secondary – transformers do not work
with DC voltages.
Transformer Construction
The number of turns of wire on the primary winding is designated NP and for the secondary NS. The
number of turns of wire for each winding determines whether the output voltage is larger or smaller
than the input voltage. Clearly if the secondary has more turns than the primary, the secondary voltage
will be larger than the primary.
Question: What happens if the primary has more turns than the secondary?
Turns Ratio
The Turns Ratio describes the ratio of the number of turns of wire on the secondary to the number of
turns of wire on the primary and is represented by the Greek letter η (eta)
η = NS/NP
The turns ratio η determines the relationship between the primary and secondary voltages.
η = NS/NP = VS/VP
Step Down Transformer
A Step Down transformer is one where NP > NS
so that VP > VS
Step Up Transformer
A Step Down transformer is one where NP < NS
so that VP < VS
Isolation Transformer
This is a special kind of transformer where NP = NS
electric isolation between two electrical circuits.
so that VP = VS and its purpose is to provide
The 3 sample diagrams illustrate these cases
Examples
1.
A transformer has 200 turns of wire on the primary and 20 turns of wire on the secondary and
the primary is connected to 120 VAC. What is the turns ratio? What is the secondary voltage?
Turns Ratio
η = 20/200 = 1/10 = 1:10
Secondary Voltage VS = turns ratio x VP = (1/10) x 120 = 12 VAC
This is a Step Down Transformer
2. A transformer has 100 turns of wire on the primary and 250 turns of wire on the secondary and
the primary is connected to 28 VAC. What is the turns ratio? What is the secondary voltage?
Turns Ratio
η = 250/100 = 5/2 = 2.5:1
Secondary Voltage VS = turns ratio x VP = 2.5 x 28 = 70 VAC
This is a Step Up Transformer
Conservation of Power
In an ideal situation the Power at the Input of the transformer matches the power at the output of the
device.
PIN = POUT
so VIN x IIN = VOUT x IOUT = VP x IP = VS x IS
So we can deduce from this that
η = NS/NP = VS/VP = IP/IS
Examples
3. A transformer has 200 turns of wire on the primary and 20 turns of wire on the secondary and
the primary is connected to 120 VAC. A load resistor of 100 Ω is connected to the secondary.
What is the secondary current? What is the primary current?
From Example 1 above
Turns Ratio
η = 20/200 = 1/10 = 1:10
Secondary Voltage VS = turns ratio x VP = (1/10) x 120 = 12 VAC
Secondary Current IS = 12 VAC/100 Ω = 120 mA
Primary Current IP = 120 mA x (1/10) = 12 mA
4. A transformer has 100 turns of wire on the primary and 250 turns of wire on the secondary and
the primary is connected to 28 VAC. A load of 1000 Ω is connected to the secondary. What are
the secondary and primary currents?
From Example 2 above
Turns Ratio
η = 250/100 = 5/2 = 2.5:1
Secondary Voltage VS = turns ratio x VP = 2.5 x 28 = 70 VAC
Secondary Current IS = 70 VAC/1000 Ω = 70 mA
Primary Current IP = 70 mA x (2.5) = 175 mA
Transformers Cores
Like inductors, a transformer can have an air core or cores made of various other materials such as iron
or ferrites. The core of the transformer enhances the magnetic properties of the device. The vertical
lines between the winding indicate a non-air or iron core.
Transformer Voltage Phase Relationships
Phasing Dots are used to indicate how the windings of the transformer are configured to each other.
The black phasing dot at the top of the primary and secondary windings tell the user that if you connect
the red lead of an oscilloscope to the top of the winding and the black lead to the bottom then the
primary and secondary voltages will be in phase. If the scope leads are reversed on the secondary, the
voltages will now be 180° out of phase.
Transformers with Multiple Secondaries
Manufacturers often make transformers with several secondary winding to accommodate the need for
different voltages in a piece of electronic equipment. Each secondary will have a separate turns ratio.
Question
Determine the 3 turns ratios for the transformer shown?
Secondary A
η = VS/VP = 50 V/120 V = 1/2.4 = 1:2.4
Secondary B
η = VS/VP = 30 V/120 V = 1/4 = 1:4
Secondary C
η = VS/VP = 10 V/120 V = 1/12 = 1:12
Center Tapped Transformers
A common transformer configuration occurs when the secondary winding is divided in half as shown
below. The center tap of the transformer gives a little more flexibility with respect to rectifier circuits.
Testing a Transformer
Basic testing of a transformer can be done with an Ohmmeter. Each of the primary and secondary
windings are constructed from wire so their resistance should be a low (but non-zero) value of
resistance. Exactly how low the resistance will be depends on the length and cross-sectional area of the
wire used. The resistance R of a length of wire is determined by:
R = ρ L/A
where R is the wire resistance in Ohms
ρ is the resistivity of the wire (Cu, Ag or Fe etc)
L is the wire length
A is the cross-sectional area (diameter)of the wire*
If the Ohmmeter reads exactly 0 ohms then we would conclude that the winding is shorted. If the
Ohmmeter reads ∞ ohms then we would conclude that the winding is open-circuited (wire is broken).
If a measurement was done between the primary and secondary winding the Ohmmeter should read ∞
ohms. The primary and secondary are not electrically connected or touching.
Questions
1.
One transformer winding is constructed from #16 wire and another winding is made from # 22
wire. Each winding has the same length and is made of the same material. Which winding has
the highest resistance?
2. If a measurement was done between the primary and secondary winding and the Ohmmeter
reads 0 ohms, what is the fault?
*American Wire Gauge - AWG – is a standard for specifying wire size (diameter or area and
resistance/length). The larger the AWG number the smaller the wire diameter and the larger the
resistance; the smaller the AWG number the larger the wire diameter and the smaller the resistance.
This diagram shows a tool for measuring the gauge of a wire.
Click out this web page for an AWG table
http://en.wikipedia.org/wiki/American_wire_gauge