4-49
Figure 4-43B.—Positive-diode counter and waveform.
The initial heavy flow of current produces a large voltage across R1 which tapers off exponentially
as C1 charges. The charge on C1 is determined by the time constant of R1 and the conducting resistance
of the diode times the capacitance of C1. For ease of explanation, assume that C1 is charged to the peak
value before T1.
At T1 the input signal reverses polarity and becomes negative-going. Although the charge on
capacitor C1 cannot change instantly, the applied negative voltage is equal to or greater than the charge
on C1. This causes the anode of D2 to become negative and conduction ceases. When D2 stops
conducting e
out
is at 0. C1 quickly discharges through D1 since its cathode is now negative with respect to
ground. Between T1 and T2 the input pulse is again at the 0-volt level and D2 remains in a nonconducting
state. Since the very short time constant provided by the conduction resistance of D1 and C1 is so much
less than the long time constant offered by D2 and R1 during the conduction period, C1 is always
completely discharged between pulses. Thus, for each input pulse, a precise level of charge is deposited
on C1. For each charge of C1 an identical output pulse is produced by the flow of ic through R1. Since
this current flow always occurs in the direction indicated by the solid arrow, the dc output voltage is
positive.
At T2 the input signal again becomes positive and the cycle repeats. The time duration between
pulses is the interval represented by the period between T1 and T2 or between T3 and T4. If the input-
pulse frequency is reduced, these time periods become longer. On the other hand, if the frequency is
increased, these time intervals become shorter. With shorter periods, more pulses occur in a given length
of time and a higher average dc output voltage is produced; with longer periods, fewer pulses occur and a
lower average dc output voltage is produced. Thus, the dc output is directly proportional to the repetition
frequency of the input pulses. If the current and voltage are sufficiently large, a direct-reading meter can
be used to indicate the count. If they are not large enough to actuate a meter directly, a dc amplifier may
be added. In the latter case, a pi-type filter network is inserted at the output of R1 to absorb the
instantaneous pulse variations and produce a smooth direct current for amplification.
From the preceding discussion, you should see that the voltage across the output varies in direct
proportion to the input pulse repetition rate. Hence, if the repetition rate of the incoming pulses increases,
the voltage across R1 also increases. For the circuit to function as a frequency counter, some method must
