An 800-Cycle Transformer Design

Primary 120 volts 800 cycles

Rectifier to deliver 0.2 amp at +450 volts using 5U4G in single-phase full-wave circuit with 0.5-μfd capacitor input filter.

Figures 51 and 52 tell whether the product ωCR_L will produce the necessary d-c output without exceeding the rectifier tube peak inverse voltage rating and peak current rating.

ωCR_L = 6.28 ∙ 800 ∙ 0.5 ∙ 10^-6 ∙ (450/0.2) = 5.65

For R_S assume a peak current of 0.5 amp. Average anode characteristics show 97 volts tube drop, or 97 / 0.5 = 194 ohms at peak current. R_s/R_L = 194/2,250 = 0.086. Add 5 per cent for transformer windings; estimated R_s/R_L = 13.6 per cent.

Check on Peak Current from Fig. 52.

the peak value assumed. Rms current in tube plates and secondary windings is 2 ∙ 0.1 = 0.2 amp. Output voltage, from Fig. 51, is 0.69 peak a-c voltage per side. Hence secondary rms voltage per side is 450 ∙ 0.707 / 0.69 = 460 volts, and secondary volt-amperes = 2 ∙ 460 ∙ 0.2 = 184. The anode transformer must deliver 2 ∙ 460 = 920 volts at 0.2 amp rms. Primary volt-amperes = 0.707 ∙ 184 = 130.

Inverse peak voltage is the peak value of this voltage plus the d-c output, because the tube filament is at d-c value, plus a small amount of ripple, while one anode has a maximum of peak negative voltage, during the non-conducting interval. Thus peak inverse voltage is 460 ∙ 1.41 + 450 = 1,100 volts, which is within the tube rating.

Choice of core for this transformer is governed by size and cost considerations. Assume that the core works at 8,500 gauss. The loss per pound for 0.005-in. silicon steel and grain-oriented steel is 12.2 and 6.6, respectively. (See Fig. 62.) But punchings have 80 per cent stacking factor, whereas the type C core has 90 per cent. In this thickness 0.005-in. grain-oriented steel compares still better with ordinary silicon steel than Fig. 62 would indicate and so will be used for the core.

Let two type C cores be used with the following dimensions:

Strip width	3/4 in.	Window height	5/8 in.
Build	3/8 in.	Window width	1 1/2 in.
Total net core area	0.506 sq in.	Core weight	0.75 lb

Turns could be figured from equation 32, except that the induction is in gauss. Since many core data are given in gauss, equation 32 is changed for convenience to

[34]

where dimensions are in inches and B is in gauss. Primary turns are then

Final design figures are:

Primary 122 turns	No. 26 glass-covered wire	d-c resistance 1.8 ohms
Secondary 900 turns	No. 29 glass-covered wire	d-c resistance 38 ohms

Primary copper loss at 100°C	=	3.35 watts
Secondary copper loss at 100°C	=	2.04 watts
Core loss 6.6 · 0.75	=	4.95 watts
		_________
Total losses		10.34 watts

With an open-type mounting and mica insulation this transformer has a temperature rise of 75 centigrade degrees.