# Transformer design

#### dwprep

Joined Apr 2, 2004
1
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

#### mozikluv

Joined Jan 22, 2004
1,435
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

heat rise will entirely depend on current drawn.

#### Battousai

Joined Nov 14, 2003
141