The reference point in diode and other tube circuits is usually a common point between the
individual circuits within the tube. The reference point (common) in figure 1-5 is the conductor between
the bottom of the transformer secondary and the negative side of the filament battery. Note that one side
of each voltmeter is connected to this point.
Name the two series circuits that exist in a diode circuit.
Before a diode will conduct, the cathode must be what polarity relative to the plate?
Diode Operation with an Alternating Voltage on the Plate
After experimenting with a positive plate and a negative plate, Fleming replaced the direct voltage of
the battery with an alternating voltage. In our explanation, well use a transformer as the source of
alternating voltage. The circuit is shown in figure 1-5.
Note that the only real difference in this circuit from the previous ones is the transformer. The
transformer secondary is connected in series with the plate circuitwhere the plate battery was
Remember from your study of transformers that the secondary (output) of a transformer always
produces an alternating voltage. The secondary voltage is a sine wave as shown in the figure.
You'll remember that the sine wave is a visual picture, a graph of the change in alternating voltage as
it builds from zero to a maximum value (positive) and then drops to zero again as it decreases to its
minimum value (negative) in the cycle.
Assume that the polarity across the secondary during the first half-cycle of the input ac voltage is as
shown in the figure. During this entire first half-cycle period, the plate's polarity will be POSITIVE.
Under this condition, plate current flows, as shown by the ammeter.
The plate current will rise and fall because the voltage on the plate is rising and falling. Remember
that current in a given circuit is directly proportional to voltage.
During the second half-cycle period, plate's polarity will be NEGATIVE. Under this condition, for
this entire period, the diode will not conduct. If our ammeter could respond rapidly, it would drop to zero.
The plate-current waveform (Ip) in figure 1-5 shows zero current during this period.
Here is a summary of effects of applying alternating voltage to the plate of the diode:
1. Diode plate current flows during the positive half-cycle. It changes value as the plate voltage rises
2. The diode cuts off plate current during the entire period of the negative half-cycle.
3. Diode plate current flows in PULSES because the diode cuts off half the time.
4. Diode plate current can flow in only one direction. It is always a direct current. (In this case
PULSATING DCone that flows in pulses.)
5. In effect, the diode has caused an alternating voltage to produce a direct current.
The ability to obtain direct current from an ac source is very important and one function of a diode
that you will see again and again wherever you work in electronics.