With this information, you can now calculate how many turns of wire can fit in each layer of winding for each different winding. Start by determining the winding width (ww) of the bobbin, the winding height (wh) of the bobbin, and the mean length of turns (mlt) from the mechanical drawing (see the figure). Now you need to see if the windings fit into your winding area and determine the actual losses of the coils. This simple view of a winding bobbin shows the dimensions that will be needed for the winding area. Use the same method to determine the secondary wire gauge(s) that you used for the primary. This percentage will vary depending on the characteristics of your design. As a rule of thumb again, I start with a 10% increase in the number of turns, assuming a 90% efficient transformer: N(s) x 1.10 = N Turns. This number then needs to be increased to account for the losses in the coils. The first step is to use formula 3 (N(s) = V(s) / V(p) x N(p)) to determine the turns for a perfect transformer. You now need to determine the number of turns that will be required for each secondary winding. Using the example above, 0.22 A x 500cm/A = 110cm I would start with a 29 gauge wire (127.7cm) for the primary. This number may be smaller for small transformers, and larger for large power transformers that decision is again up to the designer. The next step will be a subject for debate and adjustment depending on the transformer characteristics: I generally start at approximately 500 circular mills (cm) per amp to choose the starting wire gauge. For example, a transformer with a 12-V, 2-A output at 120 V input would be:ġ2 V x 2 A = 24 VA 24 VA x 1.10 (110%) = 26.4 VA needed in the primary winding Ģ6.4 VA/120 V = 0.22 A in the primary winding For power losses, I start at a 10% increase in the input power, assuming a 90% efficient transformer. The primary current will be equal to the total output power plus transformer power losses, divided by the primary voltage. The primary winding current and wire size needs to be determined. There are several variables to consider when using these formulas you will need to consult the core manufacturer’s data for answers to specifications such as flux density and stacking factor.ġ) N(p) = (V x 10 8) / (4.44 B A f K) sine waveĢ) N(p) = (V x 10 8) / (4 B A f K) square waveĦ) Open circuit voltage (Voc) = N(s)/N(p) x V(p)ħ) Loaded voltage (Vld) = Voc-Ĩ) Temp rise (T(C)) = (losses/(0.008 x surface area))Ī(l) = Inductance per turn squared for a given core Certain cores don’t require a bobbin, but we’ll save those for another discussion.Īfter selecting a core and bobbin, you need to calculate the correct number of primary turns needed using formula 1 or 2 (see “Basic Design Formulas,” below). Make sure a sufficient bobbin style and material is available, and that you have all of the mechanical measurements to determine winding details later on in the design. Most core types will also need a winding bobbin to fit the core that you choose, and possibly assist in the mounting of the finished product. The exact core chosen may depend on board spacing, location, mounting style, or any of a number of physical and electrical parameters that only you can decide. There are many other core types, and many sizes, shapes, and material grades within the cores listed above. Remember this is only a guideline it’s not uncommon to go outside of these ranges (e.g., audio transformers can use silicon steel laminations and operate from 20 to 20,000 Hz).
0 Comments
Leave a Reply. |