Peaking Transformers

It is stated in Electronic Control Circuits that a large angle of intersection between the grid firing voltage wave and critical grid voltage is desirable for accurate control. A grid wave form with vertical front edge would be ideal. To produce a steep peaked wave form for firing thyratron tubes, sometimes special transformers are used. Usually the design depends on the non-linearity of the magnetizing current.

Fig. 192. Peaking transformer.

Figure 192 shows a peaking transformer in which the magnetic core is made of special laminations. The primary is wound on the full-width left leg, and the secondary on the right leg which is made of a few laminations of small width. In the space between primary and secondary is a laminated shunt path with an air gap.

Fig. 193. Fluxes and secondary voltage in peaking transformer.

In Fig. 193 are shown the core fluxes Φ_m and Φ_s, linking the primary and secondary coils, respectively. At low inductions, the same flux links both coils. As the flux rises from zero in each cycle, at first all the flux links the secondary coil, but because of the smaller cross section of the right leg it saturates at the value Φ_s and the main flux Φ_m flows through the shunt path. Thus there is a long interval in each cycle during which the flux change is substantially zero, and no voltage is induced in the secondary coil. During the short period Θ_S, a voltage is induced in the secondary coil which has a very peaked wave form. This happens twice in each cycle.

Because of the shorter time for the change in Φ_s, dΦ/dt would remain nearly constant over the angle Φ_S if there were no leakage flux, and for 1:1 turns ratio there would be approximately equal volts in the primary and secondary coils. The secondary flux change takes place over a much shorter period of time, and the flux rises to only a fraction of its maximum value Φ_m. Therefore less core area is needed in the secondary leg to obtain the desired voltage e_s. This leads to the following approximate ratio.

[109]

where

A_S = core area in the secondary leg
A_p = core area in the primary leg.

This air gap should be no more than 5 to 10 per cent of the window height, to keep leakage flux from threading through the secondary coil and giving a less peaked wave form. With this length of air gap and a total window length of twice the window height, the secondary turns are, for a 1:1 voltage ratio,

[110]

where

N_P is the number of primary turns.