I am trying to make a transformer design but don't have much experience. I am using a lamination type core. My input voltage is 120 Vac at 400 Hz, and my desired output voltage is 26 volts with a load of 85 ohms, and a power rating of 8 VA. Using a Magnetic Metals 25EI core with a 0.5" stack height, I calculate the primary turns should be about 322 with B = 13 k Gauss. I plan to use 30 AWG magnet wire for both primary and secondary. Does anyone know how I can calculate the secondary turns to meet the 26 volt full load spec? (No spec for No load output voltage is given - I know it would be 70 turns if it was 26 Volts out at no load) If so, could you show me how, including the formulas? Also, I would like to be able to calculate the temperature rise for this design. If someone knows how, please help. Thanks

hi dwprep, i should be posting this in the "transformer topic" anyway here it is. DESIGN FORMULAS FOR FULL WAVE EI CORE TRANSFORMER I. computation of DC output voltage across secondary Es = 2.35 * Edc 2.35 = twice the ratio of the RMS to the average value + 5% regulation II. DC current (Idc) computation Is = K * Idc = Is = 0.707 * Idc non-bridge bridge full wave 0.707 1.06 half wave 1.40 2.20 III. Output power computation: (Pout) Pout = Es * Is Pout = Es1 * Is1 + Es2 * Is2 (this is used in multi-tap) IV. Power input in Volt-ampere (VA) comp. VA = Pout/ efficiency % V. Tongue Area (A) comp. in sq. inch A = sq. root of VA / 5.58 (5.58 is a given factor) VI. Tongue Area (A), tongue width (tw), stacking height (g) computation: g = tw = sq. root of A Note: get the next highest value if sq. root of A is higher than standard VII. Window width (W), tongue lenght (tl), in relation to tongue width (tw) W = 0.5 * tw tl = (3/2) * tw VIII. Primary (Np) & Secondary (Ns) turns comp. Np = 3.49 * Ep * l^6 / f * B * A Ns = 1.05 * Np * Es / Ep Note: 3.49 & 1.05 are given factors, f = frequency A = standard tw B = flux density in gauss IX. Magnetic wire size comp & Ip comp; Ip = VA / source line (Eac) dp = 1.13 * [(sq root of Ip / 2470 - 585) * (log VA)] ds = 1.13 * [(sq. root of Is / 2470 - 585) * log VA)] Note: diameter result refer to commercial size & use the next highest size if not of same value X. Winding lenght (wl) comp; wl = tl - (2 * 0.125) - (2 * 0.032) XI. turn / layer (T/L) & # of layers (L#) comp. T/L = wl / dp (w/insulation) T/L = wl / ds (w/insulation) L# p = Np / (T/L) L#s = Ns / (T/L) XII. Winding build - up (WB) comp. WBp = L#p * (dp + 0.002) WBs = L#s * (ds + 0.002) WBtotal = 1.1 [(0.095 + WBp + WBs + (2 * 0.002)] Note: compare WBtotal against Window width (W). it should not exceed 90% of W XIII. Lenght (L) of magnet wire to be used for computation thru mean lenght of turn (MLT) of each winding MLTp = 2 (tw + g + 4b) + (pi * WBp) MLTs = 2 (tw + g + 4b) + [pi * (2WBp + WBs)] Lp = MLTp * Np Ls = MLTs * Ns XIV. Wire weight (Ww) comp. Wwp = (Lp / 12) * (f/1000) Wws = (Ls / 12) * (f / 1000) Note: f = weight in pounds / 1000 ft. all weight results should be rounded off to the next highest weight in terms of 1/8, 1/4, 1/2, 3/4, 1, 1 1/8 etc. XV. computation of coild resistance (Rl), copper losses (Lcu), Core loss (Lc), voltage drop (Vdp) Rlp = (Lp/12) * (R / 1000) Rls = (Ls / 12) * (R / 1000) Note: R is resistance / 1000 ft. Lcup = Ip squared * Rclp Lcus = Is squared * Rcls Core loss (Lc) = (Approximate core loss) * (Core weight) / lb. Lc = (w/lb) * (lb.) / lb. Voltage drop prim. (Vdp) = Ip * Rclp Vds = Is * Rcls XVI. Efficiency (n) & voltage regulation (Vr) comp. n = Pout / Pout + Lc + Lcu Vr = (Is * Rls) + [(Ns + Np)squared] * Rlp / Es hope this can help you. heat rise will entirely depend on current drawn.

Whilst I have no problem with two threads existing on this topic, I thought I would provide a link to the existing Transformer Project thread in the Projects Forum.