A step down transformer is a device which reduces a higher AC potential to a lower AC potential as per its winding ratio and specifications.
In this article we are going to discuss how to design and construct a basic step down transformer which are typically applied in mains-operated power supplies.
Introduction
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
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
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
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.
Hello Swag,
Sincerely appreciate gentlemen like you who help tinkerers like us! I just bought a kiln requiring 208volts and my service is 242volts. The kiln manufacturer tells me I can run the kiln, but the elements will fail prematurely, replacement cost for 240 elements is $700+. Would you be so kind as to help me create a drop down transformer either for each leg of 17 or so volts or in combo of about 32 volts, please? I can make the steel core parts and wind the whole thing by hand – I am very meticulous. Have more time than money! Hahahaha!!!! I am not the best at math so doing the calcs myself without guidance from a seasoned vet like you, would not be smart on my part. I don’t want to blow up the kiln I just invested $500 in. Thank you very much – in advance!
Thank you Ralph, A transformer based design may not be so appropriate since it will be bulky, difficult to calculate and prone to mistakes. The easiest and the ideal way to provide a controlled power to a kiln element or any resistive load is through a triac based phase chopper circuit as explained in the following article:
https://www.homemade-circuits.com/how-to-make-simplest-triac-flasher/
You can use any of the the last two designs for controlling your kiln element heat very efficiently and provide a long life to the element.
Hey Swag,
Greatly appreciate the speedy reply. I reviewed your recommendation and think the second design would be best with just a couple increments (resistors) rather than 4, but I have some concerns of which may or may not be warranted based on my limited knowledge on the subject… namely – “minimum transients and spikes” & “generation of lots of RF.” The kiln draws 53 amps and has a digital controller that ramps up temp, holding times, etc. Will there be varied voltages, RF emissions that interfere with the controller and are the components capable of handling 53 amps? Thank you!
Thanks Ralph, there will be RF emission when the load is operated with reduced power, and the Rf will decrease as the power on the load is increased using the potentiometer. However the L1, C1 is specifically to cancel these RF, so if these are correctly optimized, the RF emission can be expected to be minimum. The L1, C1 will also work to absorb spikes and transients.
53 amps looks very high, so the triac will need to be rated accordingly. Finding a 60 to 100 amp triacs is not easy, so this may be an issue.
To solve this a simpler design could be as shown below:
This design might also work as efficiently as one can expect. The bridge rectifier will also need to be rated at 60 to 100 amps
Thank you Swagatam
Hi Swagatam,
I intend to build my own transformer winding Machine. I find it difficult to find plans to do this even on youtube. Could you perhaps suggest where I can get the necessary info.
Your assistance will be much appreciated.
Regards
Jan
Sorry Jan, presently I do not have any information regarding transformer winding machine. If I happen to find it I’ll surely update in my blog.
What type of a transformer is used in a mosquito bat ? its a small black with yellow jacket around it.
It is a calculated ferrite core transformer. The calculation formulas are explained in this article:
https://www.homemade-circuits.com/how-to-design-and-calculate-ferrite-core-transformers-for-inverters/
Good day sir
What is the acceptable gap between the lamination of a transformer unit of a 630kva oil cooled minisub and the bottom of the tank, the voltage is 6600/550v
Good day Ceaser, sorry I have no idea about it…
Thanks, and also swagtam i have an e core measuring 1.6cm tongue width and 3.3 tongue depth, i use the turn per volt formula and i get 8.53turns per volt but i think its wrong. Please help me calculate the turn per volt using my details, the frequency is 50hz, i just want to know if my answer is right. Please help me calculate it.
Hi Sam, This article is about iron-core step down transformer, it’s not for ecore.
Please swagtam what unit is there after the 1.152 in the core area formula, help me write it out in english, my phone displayed the unit as a tiny box (my phone cannot read the unit). Thanks for the previous reply.
Sam, That’s a square root symbol.
Hi swagtam. I used the turn per voltage formula and fixed these values: 1/(4.44*10^-4*50hz*528mm^2*1.1) giving me a total of 1/12.89376, equals 0.077556895turns per voltage, please help me calculate it using my values. The 528mm^2 is the core area, and i got it from tongue width: 1.6cm * tongue depth: 3.3cm. Please help me out
Hi Sam, we have solve it from top towards bottom, meaning start with current and the voltage of the primary and secondary and so on, it can be very difficult to do the opposite way.
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.
I made a 78va capacity trafo inverter, i load it With 110w appliances, what may bẹ the effect technically.
Output voltage will drop proportionately
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.
Thanks Christo,
I am glad you found it useful…please keep up the good work!
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.
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
Thank you Dev, you can read the following post:
https://www.homemade-circuits.com/how-to-design-and-calculate-ferrite-core-transformers-for-inverters/
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
Hello Osei, The formulas are applicable for 5 kW but the tables are not, they are limited to 10 amps max.
Thanks teacher Swagatam
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?
regards.
Hi Paaker, probably the only way to know is by connecting a correctly rated ammeter directly across the output wires of the transformer and check the measurement. Do it only for a couple of seconds, otherwise the transformer or the meter or both could start fuming.
fine.
its possible to make a Battery charger with this transformer( 8v-0v-8v) to charge 12V 150A battery? I thing my transformer is around 40-45A. coz the wire I use to wind it was SWG 10.
yes its possible. use the outer wire to get 16V, and then add a voltage regulator to stabilize at 14.2V
Fine,
can you please suggest me a perfect circuit for this purpose.
Regards.
Here’s a simplest design:
https://www.homemade-circuits.com/wp-content/uploads/2018/08/DC-motor-mosfet-speed-manage.jpg
Replace the motor with a 1K resistor, adjust the pot to get exactly 14.2V across the 1K
That’s all…now you can connect your battery and let it charge until its terminals reach 14.2V.
For better protection connect a 0.3 ohm 25 watt resistor in series with the positive line, put this resistor over a large heatsink, and do the same to the mosfet also
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.
That’s great Sherwin, I am glad you could do it so easily…keep up the good work
This was useful.Hope you can make a similar post on ferrite core transformers and its switching supply circuits.
I am glad you liked it, if I happen to find the ferrite core formulas, will surely update it here…
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 ?
hello cristian, you can use any set-reset circuit using IC or transistor, and use the “set” points at the bottom of the ground tank and use the “reset” points at the brim of the overhead tank, and wire the relay accordingly.
yes 220V/30 amp relay will work
i show this but not look ic model
https://http2.mlstatic.com/tablero-con-automatico-para-tanque-cisterna-listo-para-usar-D_NQ_NP_675905-MLA25094466995_102016-F.jpg
the design is correct, may be the ICs are IC 555
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