Чтение общенаучной литературы. Кытманова О.А. - 7 стр.

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8
In fact, no electrons should be able to reach the anode when the grid is at a
reasonably. high negative voltage. The grid cylinder, G, therefore almost
completely surrounds the cathode, leaving only a narrow aperture near the anode.
The critical grid voltage , Vgo, at which the valve begins to conduct (or" strikes" as
it is called) varies with the anode voltage Va.
However, with the valve containing mercury vapour, the critical grid voltage also
varies with the temperature, for the mercury-vapour pressure determines the onset
of ionization to a certain extent. For example, if Vec = —6.5 at Va= 1,000 and at
an effective temperature of 40° C, Vgc might be — 10 at a temperature of 70° С
and —3 at 35° C. This variation of a striking voltage with temperature can be a
grave disadvantage where large changes in temperature are experienced.
Rectifiers
Rectifiers are devices designed to convert alternating current into direct
current. For this purpose devices with asymmetrical conductance such as vacuum
and semiconductor diodes are used. Until the end of the twenties of this century
vacuum diodes (kenotrones) were the main rectifying devices. In the thirties high-
power kenotrones were
replaced by more efficient
mercury vapour rectifiers.
At present high-power
gas-filled and gas-control
tubes are used. Low-power
rectifiers mostly used in low
and medium power
rectifying systems are
replaced by highly efficient
and reliable semiconductor
rectifiers. Let us examine the
simplest types of rectifiers.
a) Half wave rectifiers (Fig. 12).
The load resistor R is connected in series with the transformer secondary and the
semiconductor rectifying diode D. The wave-form in graph 1 shows the change in
voltage across the transformer secondary. Due to inherent rectifying properties the
diode conducts current only during the positive half-waves of the voltage cycles.
The current I and the output voltage VR are of a pulsating pattern (graph 2). To
smooth these pulsations a capacitor С is connected across the load. When the
transformer output voltage goes negative or drops lower than the voltage applied to
the capacitor, the ion diode does not conduct current and the capacitor is
discharged through load resistor R (sections ВС, DE, FG).
b) Full-wave rectifiers can be considered as composed of two half-wave rectifiers
connected to one common load. Shown in the figure 12 is a full-wave rectifier with
two rectifying elements and a centre-tapped secondary winding on the transformer.
When the voltage on the upper terminal of the secondary is positive with respect to

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