1-18
Peak Current Rating
The maximum instantaneous current that a tube can pass in the normal direction (cathode to plate)
without damage is called the PEAK CURRENT RATING. Peak current rating is determined by the
amount of electrons available from the cathode and the length of time plate current flows.
Peak Voltage Rating
This is the maximum instantaneous voltage that can be applied to a tube in the normal direction
without a breakdown.
Peak Inverse Voltage Rating
This is the maximum voltage that can be applied to a tube in the reverse direction (plate negative
with respect to the cathode)-exceeding this will cause arc-over from the plate to the cathode and will
damage the tube. PIV, as this is sometimes abbreviated, becomes very important in the rectifier circuit to
be discussed as a later major subject.
Transit Time
Things that happen in electricity and electronics are often explained as if they happen
instantaneously. As fast as electricity acts, however, the truth is that cause and effect are separated by a
certain amount of time.
Each tube has a factor called TRANSIT TIME, which is the time required for an individual electron
to move from the cathode to the plate. In certain applications involving high-frequency voltages, transit
time places a limitation on tubes. We will explain this limitation when we discuss the circuits it affects.
Summary of Diode Parameters and Limitations
You should now have a basic understanding of diodes, many of their characteristics, and some of
their limitations. One of the more important concepts that you should now understand is that most of
these characteristics influence each other. For example, practically all plate characteristics are
interrelated. Change one and the others change. Another example is heater voltage. Every tube parameter
affected by the cathode depends on proper heater voltage. Interrelationships such as these make
electronics both fascinating and, at times, frustrating.
Many of the limiting factors that we have discussed are the same ones found in other electrical
devices such as motors, stoves, toasters, and so on. Heating and overheating, insulation breakdown, and
excessive voltage and current are all limitations that you have noted before.
The point is that you can and should apply just about everything you have learned about electricity to
electron tubes. Little is new except the environment.
Q12.
A large negative voltage is applied to the plate of a diode, and a large positive voltage is
applied to the cathode. If the tube conducts, what tube parameter has been exceeded?
THE TRIODE
Diode electron tubes can be used as rectifiers, switches, and in many other useful applications. They
are still used in Fleming’s original application in some radio circuits. You will learn more of these
