Physical Principles of Electron Microscopy: An Introduction to TEM ...

The Transmission Electron Microscope 69
To provide direct (rather than alternating) high voltage, the current from
the transformer secondary is rectified by means of a series of solid-state
diodes, which allow electrical current to pass only in one direction, and
smoothed to remove its alternating component (ripple). Smoothing is
achieved largely by the HV cable, which has enough capacitance between its
inner conductor and the grounded outer sheath to short-circuit the ac-ripple
component
to ground at the operating frequency of the transformer output.
Because the output of the oscillator circuit is linearly related to its input
and rectification is also a linear process, the magnitude of the resulting
accelerating voltage is given by:
V0 = GVi (3.8)
V+
Vi +
-
voltagecontrolled
oscillator
I f
voltagecontrolled
oscillator
I f +I e
R f
diodes
filament
R b
Wehnelt
-V0 HV
tank
Figure 3-6. Schematic diagram of a TEM high-voltage generator. The meter connected to the
bottom end of the secondary of the high-voltage transformer reads the sum of the emission
and feedback currents. The arrow indicates the direction of electron flow (opposite to that of
the conventional current). Components within the dashed rectangle operate at high voltage.