This document provides a guide for designing a simple transformer with step-by-step calculations. It outlines determining the load power and primary/secondary currents based on the voltage. Wire gauges are selected based on current capacities. The core size is calculated based on the power. Finally, the number of turns for the primary and secondary windings are calculated based on the core size and voltages. Key materials include copper wire, silicon-iron sheets, and insulation to prevent short circuits between windings.

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DESIGN A TRANSFORMER
This paper guides a simple and practical calculation in designing a transformer. The hands-on work can
be understood from this video http://www.youtube.com/watch?v=s6NyTprQCBI
MATERIALS
Masking tape, wax paper, copper cable, magnet copper wire, sandpaper, silicon-iron sheet, formwork,
soldering wire
CALCULATION
1. Determine the load power, P.
Let say,
P=300W
2. Find the primary winding current. Malaysia using 230 VAC, +10%,-6%, thus the current would be;
Ip = P/V
Ip = 200W/230V
= 0.87 A, thus the wire that we will using for primary winding is 0.26 mm2
Let say the secondary winding voltage, Vs = 40 VAC
Is = 200W/40V
= 5 A, thus the wire that we will using for secondary windings is 1.31 mm2
3. Find the size of the cable for primary windings and secondary windings.
By referring to the table below we can determine the size of primary and secondary coil (magnet wire).
Table: cable size for safe current carrying capacity
American Wire Gauge (AWG) Area size (mm2) Current (max)
30 0.05 0.22
29 0.06 0.29
28 0.08 0.3
27 0.1 0.4

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26 0.14 0.5
25 0.16 0.6
24 0.25 0.8
23 0.26 1.0
22 0.34 1.2
21 0.38 1.6
20 0.5 2
19 0.65 2.6
18 0.75 3.2
17 1.04 4.1
16 1.31 5.2
15 1.65 6.6
14 2.5 8.3
13 2.6 5.1
12 4 5.13
11 4.15 6.16
10 6 2.21
9 6.62 5.26
8 8.36 33.3
7 10.6 44.2
4. Find the section kernel
Section kernel = 𝑃
= 200
= 14.4 cm2
Thus, the size of the centre core is 14.4 cm2. We can use any combination of length and width to get the
area. For example, 3.8 cm x 3.8 cm = 14.4 cm2
L= 3.8 cm
W= 3.8 cm

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Width,W
Length,L
5. Calculate the number of turn for primary windings.
Turn per volt = 42/section kernel
*42 is a constant
= 3 turns/volt
No of turn (primary) , Np = 3 x 230
= 690 turn
6. Calculate the no. of turn for secondary windings (refer table above)
No of turn (primary) , Ns = 3 x 40
= 120 turn
SELECTED Q & A
1. Why primary windings and secondary windings of the transformer didn’t short circuit?
The windings will act as an inductor when AC current flow in it, blocking the high current. Though
voltage is applied, current flowing, impedance also exists. That's why the transformer doesn't get short
circuited. Insufficient number of coil due to miscalculation will result in short circuit.

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2. Is a transformer is obeying ohm's law or not?
Ohms law is for determining the behavior of electricity through a load (a resistor). Transformer is
nothing more than a pair of inductor. A transformer by itself is not a load. You apply Ohms law
separately to the primary circuit and to the secondary circuit of a transformer. You do not use Ohms law
to learn how the primary induces current in the secondary. The current that we calculate is the
maximum (short circuit) or ideal transformer current. If we put resistor at any side it still obeys ohm’s
law.
3. Why we usually use magnetic copper wire for the windings?
Magnetic copper have insulation. To build a transformer, solenoid or inductor each turning wire cannot
short with the other from the side.
REFERENCE
1. http://www.construyasuvideorockola.com/construc01.php
2. http://answers.yahoo.com/question/index?qid=20090705145110AAjSE33
3. Suruhanjaya Tenaga Malaysia (ST).