Inverters

Author: J.B. Hoag

A rectifier circuit is used to convert alternating into direct current. An inverter circuit does just the inverse; it turns a direct current into an alternating current.

The principle of an inverter using gas-filled triodes is shown in Fig. 19 M. Please examine this figure for a moment.

Fig. 19 M. The principle of operation of an inverter

You will notice that the plates and grids of both the gas-filled triodes have the same d.c. voltages due to the common plate and grid batteries. The plate battery is the source of d.c. which is to be converted into an alternating current. Now, when a small a.c. voltage is applied to the transformer at the left of the circuit, the grid of tube 1 becomes less negative, sufficiently so as to start a current in its plate circuit and through the resistance R₁. This flow of current makes the top of R₁ negative and the bottom positive, which charges condenser C, with negative on the top. The current in R₁ continues to flow until the second half-cycle of the a.c. voltage makes the grid of tube 2 less negative. When the striking potential of tube 2 is reached, its plate current is turned on and continues to flow during the remainder of the half-cycle. The voltage drop in R₂ charges condenser C in the reverse direction, that is, with

negative on its bottom plate. This charging current has to pass through R₁. In so doing, it sets up a potential drop across R₁ of sufficient magnitude and of such polarity as to make the voltage on the plate of the upper tube zero, with the result that this tube shuts off. Thus the upper tube conducts during the positive half-cycles and the lower tube conducts during the negative half-cycles, each reversal of the a.c. serving to switch the current from tube 1 to tube 2 and back again.

We thus see that the current surges in and out of condenser C of Fig. 19 M. A more practical form of the inverter is shown in Fig. 19 N.

Fig. 19 N. The inverter changes d.c. to a.c.

In this circuit, the resistors R₁ and R₂ of Fig. 19 M have been replaced by the two legs of the primary of the transformer T₂. When the upper and lower tubes are alternately turned on and off by the grid control, the rise and fall of the magnetic fields in the primary of the output transformer induce alternating voltages in the secondary. By resonating the transformer to the grid control frequency (by means of condenser C), a comparatively pure sinusoidal output voltage is obtained.

Fig. 19 0. A " d.c. transformer " circuit

In Fig. 19 O, the fluctuating voltages on the grids of the tubes are obtained by feedback through the condensers C₁ from the plates of these tubes. In addition, the a.c. output from the inverter is passed

through a full-wave rectifier tube and then smoothed out with a filter circuit (not shown). Thus d.c. is converted into a.c, stepped up or down in voltage by a transformer and, by means of a rectifier, reconverted into a direct current. The circuit might be called a " d.c. transformer."