Combined Anode and Filament Transformers

Anode and filament windings are combined into a single transformer mainly in low-power ratings such as those in receivers and grid bias power supplies.

One widely used combination includes the anode and filament windings for a rectifier and a filament winding for the amplifier tubes.

Fig. 60. Power supply transformer.

Fig. 61. Winding arrangement to save insulation.

Figures 60 and 61 show how winding insulation sometimes may be graded to require a minimum of insulation and space. The high-voltage filament winding Si is placed over the coil form to take advantage of its thick insulation. Layer insulation is sufficient between Si and S₂, and between S₂ and S₃. Over and under the primary winding is 115-volt insulation. Thus Fig. 61 is a high-voltage transformer with no high-voltage insulation in it except what is incidental to the coil form.

Combined anode and filament transformers are difficult to test for regulation or output voltage aside from operation in the rectifier circuit itself, because a-c loads do not duplicate rectifier action. Most transformers of this kind are used in rectifiers with capacitor-input filters or with fixed loads in which regulation is not important.

Ratings are easier to predict. Anode secondary v-a rating is the product of rms voltage and current, but the corresponding portion of primary v-a rating depends on the rectifier and is found as mentioned in Rectifiers with Reactor-Input Filters and Rectifiers with Capacitor-Input Filters. To this is added the sum of filament winding v-a ratings, and the primary current can then be calculated from the total volt-amperes.