Almost every electronic circuit needs a separate power supply, which may be a battery or a rectified power supply. Here we will deal with the design and making of small transformers that are normally used in conjunction with mains-operated power supplies.
This will help electronic hobbyists to design and construct their own transformers according to their needs. In the following pages, a simplified design procedure is given so as to obtain satisfactorily designed transformers. However, the design procedure is often a matter of trial and error.
The tables provided here cut calculations short and help the designer to choose the proper size of wire or core lamination. Only relevant data and calculations are provided so that the designer is not confused by unnecessary details.
The transformer
A transformer has two or more windings of insulated copper wire over an iron core. They are: one primary winding and one or more secondary windings. Each winding is electrically isolated from the other, but they are magnetically coupled with the help of a laminated iron core. Small transforrners have a shell type construction, i.e. the windings are surrounded by the core as shown in Fig. 1. The power delivered by the secondary is actually transferred from the primary, but at a voltage level determined by the turns ratio of the two windings.
Making the Preliminary Transformer design
As the first step to the design of a transformer, the primary and secondary voltage ratings and the secondary current rating must be clearly stated. Then decide on the core material to be used: ordinary steel stampings or cold rolled grain oriented (CRGO) stampings. CRGO has a higher allowable flux density and lower losses.
Core area = 1.l52 x root of (Output voltage x output current) sq. cm.
For transformers with multiple secondaries, the sum of the output volt-amp. product of each winding is to be used. The number of tums on the primary and secondary windings is decided by the turns per volt ratio as:
Turns per volt = 1/ (4.44 X l0^-4 frequency x core area x flux density)
Here, the frequency is 50Hz for Indian domestic mains supply. The flux density can be taken as about 1.0 Weber/ sq. m. for ordinary steel stampings and about 1.3 Weber/ sq. m. for CRGO stampings.
Primary winding design
The current in the primary winding is given by:
Primary Current = Sum of (output volts x output amps)/(Primary volts x efficiency)
The efficiency of small transformers varies between 0.8 to 0.96. A value of 0.87 can be used for ordinary transformers. The proper wire size has to be selected for the winding. The wire diameter depends on the current to be supplied by the winding and the allowable current density of the wire. The current density may be as high as 233 amps/ sq. cm. in small transformers and as low as 155 amps/sq. cm. in large ones. Usually, a value of 200 amps/ sq. cm. can be taken, on whose basis Table 1 is given.
The number of tums in the primary winding is given by:
Primary turns = Turns per volt x primary volts.
The space taken up by the winding will depend on the insulation thickness, method of winding and the wire diameter. Table I gives the approximate values of the turns per square cm. from which we can estimate the window area occupied by the primary winding.
Primary Winding Area = Primary turns/Turns per sq. cm. from Table1
Secondary winding design
Since we have assumed that we know the secondary current rating, we can find out the wire size for the secondary winding by referring to Table 1 directly.
The number of turns on the secondary is calculated in the same way as for the primary, but about 3% extra turns are to be added to compensate for the internal drop of secondary voltage of the transformer, upon loading. Thus,
Secondary Turns = 1.03 (turns per volt x secondary volts)
The window area required for secondary winding is found from Table1 as
Secondary window area = Secondary Turns/Turns per sq. cm from Table1
Core size
The main criterion in selecting the core is the total window area of winding space available.
Total window area = Primary window area + sum of secondary window areas + space for former and insulation
Some extra area is required to accommodate the former and insulation between windings. The actual amount of extra area varies, although 30% may be taken to start with but may have to be modified later. The suitable core sizes having a larger window area are selected from Table ll. . Taking into account the gap between laminations while stacking them (the core stacking factor taken as 0.9), we have
Gross core area = Core Area/0.9 sq.cm.
ln general, a square central limb is preferred. For this, the width of the tongue of lamination is
Tongue width = root of Gross core area cm.
Now refer to Table ll again and finally select the proper i core size, with sufficient window area and a close value of the tongue width as calculated. Adjust the stack height as required to obtain the required core section.
Stack height = Gros core area/Actual tongue width cm.
The stack should not be much less than the tongue width but may be more. However, it should not be more than 1% times the tongue width.
How to Design and Make a Transformer finally
The windings are wound on an insulating former which .fits over the center limb of the core.The primary is usually wound first, then the secondary, with insulation between windings, A final insulating layer is provided over the windings to protect them from mechanical damage. When thin wires are used, their ends must be soldered to thicker wires for bringing the terminals outside the former. The laminations are assembled over the former with alternate laminations reversed in assembly. The laminations must be held together tightly by a suitable clamping frame or by screws (if holes are provided in the laminations).
Shield
lt is a good practice to use an electrostatic shield between the primary and secondary windings to prevent disturbances from passing through to the secondary from the primary. The shield is made out of a copper foil which is wound between the two windings for slightly over a tum. Insulation must be provided along the length of the foil and care taken so that the- two ends of the foil do not touch each other. A wire soldered to the foil is bought out and connected to the ground.
A practical design example can be witnessed below:
with packaging, and 8
