The core material selected dictates parameters such as core loss and magnetic saturation. For this project, we will select microsil material (aka 33XX, M6, and grain oriented silicon steel). This is a commonly selected material for power transformers since it provides relatively low losses at an affordable price. The lamination size must be determined based on the desired VA (aka Volt-Amp) capacity of the design. The number of primary turns must be calculated such that the core can support the desired voltage without saturating. The secondary turns will be calculated such that the proper voltage will appear on the secondary winding of the transformer.

• SPECIFY DESIGN REQUIREMENTS

  Ep=120VAC

  Es=12VAC

  Is=1A

  f=60Hz

  S=1700 (current density)

  Bmax=17000 Gauss, max flux density for microsil

• PERFORM CALCULATIONS

  Calculate VA requirement

  VA=Ep*Ip=Es*Is=(12VAC)*(1A)=12VA

  Calculate min WindowArea*CoreArea product

  min WAxCA=(VA*10^8)/(4.55*S*Bmax*freq)=(12*10^8)/(4.55*1700*17000*60)=0.152 in^4

  Select lamination

  The author happens to have 68EI3314 laminations available from previous projects and therefore will use those for this project. The manufacturer page for this lamination is here. Note that the WAxCA product of 0.167in^4 for a square stack of this lamination exceeds our minimum requirement.

  Calculate Primary Turns

  Npri=(Ep*10^8)/(4.44*freq*Bmax*Asqcm)=(120*10^8)/(4.44*60*17000*3.05*0.9)=965 turns

  Calculate Primary Wire Size (using industry accepted value of 500 cir mils/amp

  cir mils= I[Amp]*(cir mils/amp)=(VA/Ep)*(cir mils/amp)=(12/120)*500=50 cir mils

  From a wire table, like this one, one can see that 33AWG is the proper size for our primary winding.

  Calculate Secondary Turns - ignoring core loss

  Npri=(Np*Es)/Ep=(965*12)/120)=97 turns

  Calculate Secondary Wire Size (using industry accepted value of 500 cir mils/amp

  cir mils= I[Amp]*(cir mils/amp)=(Is)*(cir mils/amp)=(1)*500=500 cir mils

  From a wire table, like this one, one can see that 23AWG is the proper size for our secondary winding.

  Summary

  We will build a transformer using a square stack (0.687" stack height) of 68EI3314 with Np=965 turns 33AWG and Ns=97 turns of 23AWG.

• 68EI3314 transformer laminations
• 33 AWG magnet wire
• 23 AWG magnet wire
• transformer tape. Thin electrical tape is a good substitute.
• bobbin on which to wind the wire. A bobbin can be made from cardboard if not available. (see sketch below)

1. Acquire plastic bobbin or make a cardboard one from attached sketch.

   To create cardboard bobbin, follow these directions. Draw sketch to appropriate size. Cut along solid lines. Fold along dotted lines.

2. Wind the primary winding on the bobbin. This winding can be performed manually. However, to place a large number of turns on a bobbin, it is better to automate this. You can do this by fixing the bobbin to the shaft of a motor (such as a grinding wheel) and run the motor at a very slow controlled speed such that you can control the feeding of the copper wire as it is wound onto the bobbin.
3. Place a single layer of transformer tape over the primary.
4. Wind the secondary winding on the bobbin on top of the primary using the same technique as used with the primary.
5. Place a single layer of transformer tape over the secondary.
6. Assemble the laminations into the bobbin using the interleave technique. In other words place an "E" lamination from one side followed by an "E" lamination from the other side with an "I" lamination to complete that layer. Repeat this process until you can no longer fit laminations in the bobbin.

• http://www.howstuffworks.com/inside-transformer.htm
• Practical Transformer Design Handbook - by Eric Lowdon

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