DIY 250VA Inverter Transformer

Introduction:

In this instructables, I am designing a 250VA inverter transformer for my future DIY Inverter project and other applications. This inverter transformer is very useful for making an inverter or a battery charger. It transforms 12V AC voltage into 220V mains AC voltage and vice-versa.

My transformer(that I am going to design here) has a no load output voltage of 243V. This is because when I connect a load to the output of a transformer its output voltage drops so when I connect a load to our transformer its output voltage become 220+/-5V.

Tools:

1. Screw driver
2. Pliers
3. Soldering iron
4. Hammer
5. Digital Vernier Caliper(we can also use a ruler here but I will recommend for Vernier Caliper).

Electrical Hardware:

1. 33 No. Transformer Core(with E and I laminations having width of 0.5mm)
2. Bobbin(1''x1.5'')
3. Copper wire(16 S.W.G. for Secondary winding and 27 S.W.G. for Primary winding)
4. Insulating sheet(both plastic and paper)
5. Tape
6. Sleeves
7. 2.5'' M4 nut and bolts with washers

After collecting all necessary parts we are ready to move towards the first step of design. In design I calculated the number of turns of the copper wire for primary and secondary windings. For this I shall use following formula.

[V2/V1 = N2/N1]

here,

V2 = Secondary voltage.

V1 = Primary voltage.

N2 = Number of turns of copper wire in secondary winding.

N1 = Number of turns of copper wire in primary winding.

In my DIY inverter transformer the primary winding will be the high voltage winding with no load output voltage of 243V. The secondary winding will be low voltage winding with input voltage of 12V. Since we want to transform the 12V AC voltage to 243V AC. Now we have to select proper wire gauge and number or turns for windings but I do not have a deeper experience in designing of transformers, so I collected two old computer ups because they have such a transformer that I want to design. Now I opened both ups and remove out their transformers then I separate their laminated core form bobbin and carefully unwind the primary and secondary windings and then remove the insulation of the copper wire by heating them in fire flame and then I measure the gauge of wires. I found the following gauge

Primary winding wire gauge = 27 S.W.G.

Secondary winding wire gauge = 16 S.W.G.

So I decided to use the copper wire of the same gauge as I found in old UPS transformers. I had also counted the number of turns of wire in secondary winding and I found that it consists of total 66 turns with a center tap after 33 turns. Now I can easily calculate the number of turns for primary winding using above formula.

For my transformer:

N1 = ?

N2 = 33

V1 = 240V

V2 = 12V

Now substituting the above values in the following formula:

N1 = [{V1/V2}*N2]

N1 = [{243/12}*33]

N1 = 668.25(very close to 668)

Now we have following data for designing our DIY inverter transformer

• V1 (Primary winding voltage) = 243V
• V2 (Secondary winding voltage) = 12V
• N1 (No. of turns in primary winding) = 668
• N2 (No. of turns in secondary winding) = 33 + 33
• Standard Core No. - 33
• Bobbin size = (1"X1.5")
• Primary winding wire gauge = 27 S.W.G.
• Secondary winding wire gauge = 16 S.W.G.

For primary winding I used 27 S.W.G. copper wire. Before starting first layer of the primary winding, I wrap an a layer of insulating paper on the bobbin and then start the winding. When the first layer of the primary winding is completed, I again wrap a layer of the insulating paper on the first layer of primary winding and then started the second layer. I continue the process until the number of turns of the primary winding become 668.

After completion of the primary winding, I covered it with insulating paper and tape. Now I am ready to start my secondary winding. first I cut out three 2" long pieces of 4mm sleeve and passed the starting end of the 16 S.W.G. wire inside the sleeve hole then I passed the same end of the wire through bobbin hole along with sleeve. Now I started wrapping the 16 S.W.G. wire and continued until the first layer of the secondary winding is completed. After completion of the first layer of the secondary winding I wrapped tape around it so that wire wound not leave the bobbin. I continued preparation of secondary winding and when 33 turns of the primary winding is completed, I cut the wire with a wire cutter or plier and passed the end of the wire through the second sleeve that I have cut in the past. I passed the end of the wire from the center hole of the front edge of the bobbin. Now I again wrap the remaining 33 turns of the secondary winding same as previous 33 turns. During preparing the secondary winding if our wire is coming out of the edges of the bobbin then I carefully pressed it by hammering softly otherwise my bobbin may get broke. After completion of both(primary and secondary) winding I wrapped a white sheet of plastic and wrapped transparent tape around it.

After preparation of my primary and secondary winding, I am ready to fill the core inside the bobbin area. To do this in a precise way I put the bobbin in the center and a half of E type Laminations to the right side of the bobbin and another half of E type laminations to the left side of the bobbin. Now I can easy insert the E type laminations. I insert the first E lamination from the right side and then second E lamination from left side and third E lamination from right side and I continue the process until all the area of the bobbin is not filled up. In the last I also faced some difficulties so I use a wooden hammer to fill the last three E laminations. After filling E type laminations, I filled the I type laminations and after completely filling all I laminations I tight the core with M4 nuts and bolts.

⚠ Warning:

The transformer that I am going to test here is powered by mains 220V mishandling of such a high voltage can lead to fatal injuries.

The Last step is testing of my DIY inverter transformer. So I solder the wire cables to the ends of the primary and secondary winding of the transformer. I can check my transformer by following three ways:

First way: In this way I connect the primary wires of my transformer to mains voltage(220V/50Hz) in series with a light bulb(220V/50Hz). When I short the wires of the secondary winding of my transformer the light bulb lights up. This indicates that my transformer is working.

Second way: in second way I measured the input voltage and output voltage of the transformer and I got input Voltage of 162V(which is less than 220V but I can test with it) and output voltage of 7.9V. I already know the number of turns in primary winding(N1 = 668) and secondary winding(N2 = 33 + 33). Now I calculated the theoretical value of the output voltage(secondary voltage) using the formula that I have used in step-1.

V2 = [(N2/N1)*V1]

I have,

N1 = 668

N2 = 33

V1 = 162

for these values, I got the theoretical value of 8V for secondary voltage which is very close to measured value(7.9V)

Third Way: In third way I connected a load such as a 12V/5W light bulb between center tap(Red) and one of the other two wires(Black) and I found nothing blows up. Now after performing all above testing ways my DIY inverter transformer has built up successfully. You can understand the testing more clearly after watching the video.