(source: Electronics World, Dec. 1971)
By RICHARD M CARTOSCELLI
The use of a unijunction transistor with a v.t.v.m. or scope
permits accurate visualization of the circuit time constant.
OFTEN it is very difficult to learn the correlation between RC time constants
and sawtooth waveforms. The advent of the unijunction transistor (UJT)
has changed this entirely. The UJT will permit visualization of an extremely
slow time constant with a v.t.v.m. (or solidstate v.o.m.), or a very
rapid one with the oscilloscope.
A conventional circuit which has been used as a sawtooth wave generator
is shown in Fig. 1A. by making either R or C larger or smaller, the charge
time of the capacitor can be increased or decreased, respectively. What
results is a rapid or slow blinking interval of the NE2 neon lamp. The
operation of the circuit is quite simple. The capacitor charges exponentionally
until the firing voltage of the NE2 is reached. Once the NE2 has come
on, it appears as a short circuit across the capacitor, thus discharging
it instantaneously. The charging again repeats itself. Either a scope
or v.t.v.m. can be placed across the capacitor to monitor the exponential
voltage rise or sawtooth waveform.
The main drawback to the circuit of Fig. 1A is the high d.c. voltage
required. The NE2 has a firing potential of nearly 90 volts d.c. Additionally,
the circuit does not lend itself to many experimental uses. On the other
hand, the UJT circuit to be described and depicted in Fig. 1B permits
several sophisticated techniques which can be integrated as a part of
mathematical timeconstant computations.
The operation of the circuit in Fig. 1B is as follows: Capacitor C charges
exponentially through resistor R until the peak firing potential of the
UJT is reached. At this point the base1 resistance of the UJT approaches
zero and C discharges nearly instantaneously through the UJT and RBI.
Once C has discharged, the UJT's peak firing potential is no longer present
and the UJT's base1 resistance returns to its normal value. Thus what
exists across capacitor C is an exponential sawtooth waveform. The values
of C and R in Fig. 1B permit either a "slow sawtooth," capable
of being observed with a v.t.v.m., or a highfrequency sawtooth which
can be monitored with an oscilloscope.
To initially orient readers to the exponential properties of charging
time, a "slow sawtooth" is used. Values of R and C in the UJT
circuit are 270 k ohms and 20 F. Monitoring the voltagerise across C
with a v.t.v.m. reveals in initial fast movement of the meter's needle
followed by a constantly reducing movement speed. This continues until
the UJT's VP is reached, whereupon the meter's needle drops instantaneously
toward zero. However, the capacitor discharge time is so rapid that the
initial part of the next sawtooth appears again, and the mechanical travel
time of the meter's needle is not capable of keeping pace. What virtually
results is the meter's needle returning to approximately halfway between
zero and the V, that was reached.
For a "fast sawtooth," values of C and R in the UJT circuit
are reduced to 0.2 µF and 10k ohms, respectively. If a common 5 or 10
percent tolerance resistor is used, a careful measurement of its value
should be made and recorded.
The same is true for the value of capacitor C. Once the circuit is set
up into operation, a careful measurement of the time for one sawtooth
is taken with an oscilloscope and recorded. Next the instantaneous voltage
across C is calculated by the exponential formula: e= Emax(1E_t' ) where:
Emax= VBB, 10 volts; t= time of one sawtooth, seconds; R= actual value
of R, ohms; and C= actual value of C, farads. A careful check should be
made to determine if the VBB is set as closely as possible to 10.0 volts.
The actual intrinsic standoff ratio (eta) of the particular UJT being
used must be determined. This can easily be done with the experimental
circuit shown in Fig. 2 and the procedure that follows. The Cal button
is depressed and R3 is adjusted to make the meter read fullscale. The
Cal button is then released and the value of is read directly from the
meter (1.0 full scale). The eta for the recommended 2N4891 UJT will be
somewhere between 0.55 and 0.82.
The next calculation involves the peak firing potential of the UJT. The
formula used is:
VP= VBB_eta + V_ F
where: VBB_eta =10.0 volts; eta = intrinsic standoff ratio of particular
UJT being used; and VF= 0.56 volt at 25 °C. Following these calculations,
the peak potential of the sawtooth wave is measured. The measurement is
then compared with the two calculations. If the VBB has been accurately
set to 10.0 volts and the UJT's eta was precisely measured, the calculation
involved to find V, can stand as a point of reference. If both calculations
and the measured value of the sawtooth check within 0.2 volt, the oscilloscope
can be considered accurately calibrated on both horizontal and vertical
inputs. In the event the measured the calculated values do not check,
further calculations can be made to determine which scope axis is in need
of calibration.
The time of one sawtooth is calculated by the following formula where
t is solved for:
e = Emax(1e^t/RC)
where: e= VP, in volts (VP= VBB_eta + VF); Emax =10.0
volts; R= actual value of R, ohms; and C= actual value of C, farads.
The time derived should agree with the measured time of one sawtooth.
If the V, measured does not agree with the value calculated by Vp= VBB_eta
+ VF, the vertical calibration is not correct.
Fig. 1. (A) Neonlamp and (B) UJT sawtooth generators.
Fig. 2. Test circuit for UJT intrinsic standoff ratio.
