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Make Step Down Transformers with Calculations

Make Step Down Transformers with Calculations

Nearly every single electronic circuit requires a separate power supply, which is often in the form of a battery or a rectified power supply. In this article we are going to discuss how to make small basic transformers which are typically applied in mains-operated power supplies, such as step down transformers


This will likely help electronic hobbyists to develop and build their very own transformers based on their particular demands. Within the next pages, a simplified layout method is presented in order to achieve satisfactorily developed transformers. On the other hand, the design process may be a subject of some experimentation.

The tables presented in this article trim computations short which help the designer to find the appropriate size of wire or even core lamination. Exclusively pertinent data and calculations are supplied here to ensure that the designer is absolutely not baffled by unwanted details.

Here we will specifically discuss about transformers which possesses 2 or more winding of insulated copper wire around an iron core. These are: one primary winding and one or maybe more secondary winding.

Each winding is electrically isolated from the other however are magnetically connected by using a laminated iron core. Small transformers possess a shell style structure, i.e. the winding are encircled by the core as demonstrated in Fig. 1. The power supplied by the secondary is in fact transmitted from the primary, although at a voltage level dependent on the winding ratio of the a pair of winding.

Video Interpretation

Basic Transformer Design

As the initial phase towards the design of a transformer, the primary and secondary voltage evaluations and the secondary ampere rating has to be distinctly expressed.

After that determine the core content to be employed: ordinary steel stamping or cold rolled grain oriented (CRGO) stamping. CRGO features a greater allowable flux density and reduced losses.

The best possible cross-sectional part of the core is roughly assigned by:

Core Area: 1.152 x √(output voltage x output current) sq cm.

With regard to transformers having several secondaries, the sum of the the output volt-amp product of each winding needs to be considered.

The quantity of turns on the primary and secondary winding is determined using the formula for turns per volt ratio as:

Turns per volt = 1/ (4.44  x 10-4 frequency x core area x flux density)

Here, the frequency is usually 50Hz for Indian household mains source. The flux density could be considered as approximately 1.0 Weber/ sq. m. intended for ordinary steel stamping and approximately 1.3 Weber/ sq. m. for CRGO stamping.

Calculating Primary Winding

The current in the primary'winding is presented by the formula:

Primary Current = Sum of o/p Volt and o/p Amp divided by Primary Volts x efficiency

The efficiency of small transformers can deviate between 0.8 to 0.§6. A value of 0.87 works extremely well for regular transformers.

The appropriate wire size needs to be determined for the winding. The wire diameter is dependent upon the current rated for the winding and also the permitted current density of the wire.

The current density could be as tall as 233 amps/ sq. cm. in small transformers and as minimal as 155 amps/ sq. cm. in big ones.

Winding Data

enameled data on copper winding

Typically, a value of 200 amps/ sq. cm. may be considered, according to which Table#1 is created. The amount of turns in the primary winding is presented by the formula:

Primary Turns = Turns per Volt x Primary Volts

The room consumed by the winding is determined by the insulation density, technique of winding and the wire diameter.

Table#1 provides the estimated values of the turns per square cm. through which we are able to calculate the window area consumed by the primary winding.

Primary winding Area = Primary turns / Turns per sq. cm from Table#1

Calculating Secondary Winding

Considering that we have the assumed secondary current rating, we are able to determine the wire size for the secondary winding simply by going through Table#1 directly.

The quantity of turns on the secondary is calculated in the identical method when it comes to primary, but around 3% excess turns should be included to reimburse for the internal drop of secondary winding voltage of the transformer, upon loading. Hence,

Secondary turns = 1.03 (turns per volt x secondary volts)

The window area necessary for secondary winding is identified from Table#2 as

Secondary window area = Secondary turns / Turns per sq. cm. (from Table#2 below)

Calculating Core size

The principal qualifying measure in picking the core could be the total window area of winding space accessible.

Total window area = Primary window area + sum of secondary window areas + space for former & insulation.

A little extra space is necessary to support the former and insulation in between winding. The specific quantity of extra area may differ, even though 30% could be considered to begin with although this may need to be customized later on.

Table Dimension of Transformer Stamping

transformer stamping dimension

The perfect core sizes possessing a more substantial window space are generally determined from Table#2 taking into consideration the gap between lamination while stacking them (the core stacking element may be taken as 0.9), we now have

Gross core area = Core Area / 0.9 sq cm. In general, a square central limb is preferred.

For this, the width of the tongue of lamination is

Tongue width =  √Gross core area (sq.cm)

Now refer to Table#2 once again and as a final point find the appropriate core size, having adequate window area and a nearby value of the tongue width as calculated. Modify thel stack height as needed to acquire the intended core section.

Stack Height = Gross Core Area / Actual Tongue Width

The stack must not be a lot under the tongue width rather should be more. However, it must not be greater than 11/2 times the tongue width.

Core Assembly Diagram

lamination core assembly
Core lamination details

How to Assemble the Transformer

The winding are done over an insulating former or bobbin that fits on the middle pillar of the core lamination. The primary is generally wound first, and next it is the secondary, keeping an insulation between the two layers of the winding.

One last insulating layer is applied on top of the winding to safeguard all of them from mechanical and vibration deterioration. Whenever thin wires are employed, their particular ends needs to be soldered to heavier wires in order to bring the terminals outside the former.

The lamination are usually put together on the former by alternate lamination reversed in set up. The lamination has to be tightly bound together through an appropriate clamping framework or by using nuts and bolts (in case through holes are supplied within the lamination assembly).

How to Apply Shielding

This can be a wise idea to utilize an electrostatic shielding between the primary and secondary winding to circumvent electrical interference from moving across to the secondary from the primary.

The shield for step down transformers can be constructed from a copper foil which can be wound between the two winding for somewhat more than a tum. Insulation has to be presented across the entire foil and proper care taken in order that the two ends of the foil never come in contact with each other. Additionally  a wire could be soldered with this shielding field and connected with the ground line of the circuit or with the lamination of the transformer which may be clamped with the ground line of the circuit.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

29 thoughts on “Make Step Down Transformers with Calculations”

  1. I saw a 1.5kva/24v inverter, the trafo guage amperage was 21.2amps for the battery side and the ac output sise was 2.64amps. please explain how it is so. For this inverter, it is suppose to be 55amps for the battery side which is not feasible

    • Pleas multiply the primary side voltage with its current, and the secondary side voltage with its current to get the wattages at the relevant sides of the trafo.

  2. Hi Swag,

    As they say the truth, in simple English, shall set you free. Thank you. This is the first clear discussion I see on trafo’s, especially the relationship between changed voltages and currents. I was particularly concerned about the effect of the changes in current in step-down trafo’s. One thing that became very clear is that I can imagine any situation and start from there. I can then practice the formula’s in every way possible to understand all the effects of all the different designs. This also gives me the ability to reverse engineer existing trafo’s and understand things even better.

    Thank you, I like the way you explain things that I can follow your thinking. That makes it every easy to turn my own thinking onto something real.

  3. Good day sir, is it possible to wind like 10pcs of wire at same time for primary coil to make it faster and fewer number of turns and join end to start

    • Victory, the number of turns should be as per the calculations, adding more number of wires will increase the current handling capacity of the winding.

  4. Sir thanks for such a good circuit diagrams & details the one thing which is very important designing the transformer for SMPS Power supplies so sir is it possible to have some help from your side related to this topic as this is a very critical.
    Hope to receive the reply.
    Thanks & Regards

  5. teacher Swagatam I was reading your article on transformer calculations
    now how do I know out put current when calculating core area per your calculations. 2 is this formula applicable to E and I transformers that handle 5Kw of power thanks teacher

  6. Hi Swag,

    I got some microwave oven transformers. to rewind for battery charging / inverter purpose. I try to rewind this transformers secondary line 12-0- 12 my coil gauge was SWG 10 and I have rewinding it bifilar type finally I got
    8v-08v coz no more space to wind.and its working perfect. on this way I have rewind several transformers with different gauge wires.

    now my question is I know how much the voltage output from this transformer. but how to check how much ampere its giving. it is possible to check a transformers output current without connecting a load?


  7. Dear,
    Now I’ve started to build my own transformers (i’m describing an IRON CORE trafo). First i considered this difficult and never knew how to do so. Previously i had to compensate someone in the field to build as per my specs.
    Now I’ve cracked the way how to build my own only after seeing this page in your blog.

    I’ve got the stamping and frame ordered from Lamington road, purchased varnish and other basic materials required to build the transformer.
    Till date I’ve made just one successfully.

    I calculated all the electrical parameters involved in its construction and now its been constructed and working properly.

  8. hello friend long time

    i looking circuit for water level for up tank and down tank
    down tank is whit water and up is low level turn pump on and so

    and what y think i use a automotive rele 30amp to turn a 3hp 220 water pump ?

  9. This is a very interesting topic especially for me thank you for that
    But what in a case i have a transformer salvage from ups and i want to redesign it how do i go about the calculationt
    Thanks and best regards

    And as for the sinewave inverter using spwm i’m still on it when I’m done i will share the good news
    thank you once more for help God bless you real good

    • Thank you Faith, for modifying a readymade transformer you may have to go in the reverse manner and first identify the core size and the other parameters then compare them with the values given in the respective tables and subsequently evaluate the wire sizes and the number of turns

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