Physics 4 Winter 1998
Lab 1 - The R-C Neon Oscillator
Theory
The circuit for the neon oscillator is shown in Figure (1) below. The operation of this circuit
depends on the unusual switching properties of the neon lamp which alternately allows the capacitor
to charge through the resistor and then to discharge through the lamp. The waveform, its
amplitude, and the frequency generated can all be observed using an oscilloscope.
Figure 1
A neon lamp requires that a certain minimum potential, Vf , be placed across its terminals before
it will change from a non- conducting to a conducting state and allow the flow of electricity. Once
the lamp is conducting however, the potential may be reduced considerably below Vf before the
lamp is extinguished and returns to its non-conducting state. The voltage at which the lamp ceases
to conduct is called the quenching voltage, Vq . In order to refire the lamp, the voltage across its
terminals must again be raised to Vf .
This operation may be partly understood in the following manner. Initially, the gas in the neon
lamp is un-ionized; that is, almost all of the electrons and ions in the gas are bound together into
stable neon atoms and very few charged particles capable of being affected by an electric field are
present. When a small potential is first applied, the only thing which happens is that the minute
quantities of ions and electrons present are attracted to the electrodes (the electrons toward the
positive electrode and the ions toward the negative electrode) and a very minute current flows. This
current is much too small to measure by any ordinary techniques and has no effect on the operatioin
of the circuit. One can say that the resistance of the neon lamp in this condition is essentially
infinite.
If the potential is raised to a high enough value, Vf , some of the charge carriers in the gas
actually gain enough energy in being pulled toward the electrodes to ionize a significant number of
otherwise neutral atoms; that is, they again enough energy to actually knock off some of the
electrons from the ions to which they are bound. In neon, this takes some 20 eV per atom. Once
this condition is reached the number of charge carriers multiples very rapidly and the neon gas
becomes a very good conductor of electricity.
Once the conduction condition has been achieved, the voltage across th electrodes may be
lowered below Vf and the gas will still continue to conduct. This is simply because there are many
ions and electrons present, a sufficient number of them having energy enough to continue the
ionization and to make up for the losses. Eventually, however, as the voltage is lowered, the
number of new ions and electrons being created is not large enough to make up for those which are
being removed from the gas and the neon lamp again becomes non-conducting.
Consider the circuit shown and assume at t = 0 the switch is closed. Before t = 0, Vc = 0, but
immediately after the switch is closed current begins to flow through R, and C begins to charge.
The neon lamp is non-conducting and it may be considered an open circuit (i.e. no current flows
through it). The potential across the capacitor as a function of time is given by the relation
Vc = Vb 1 - e - t/ RC
(1)
When Vc reaches Vf , the neon lamp fires and the capacitor rapidly discharges through the lamp until
Vc falls to Vq and the lamp ceases to conduct. The capacitor then charges again through R until Vc =
Vf and the cycle repeats itself. The waveform of the voltage Vc is shown in Figure 2 below.
Figure 2
[Note: The interval (t3 - t2 ) is usually much less than (t2 - t1 ), and is shown exaggerated here.] The
circuit is seen to generate a non-sinusoidal periodic waveform with a period T.
The time it takes for C to discharge through the neon lamp is small compared with the time it
takes C to charge through R if the resistance of the neon lamp when conducting is << R. It can be
shown from Eq. (1) that the difference t2 - t1 (see Figure 2) is
t 2 - t 1 = RC loge
Vb - Vq
Vb - Vf
.
(2)
The time difference t3 - t2 is the time for the capacitor C to discharge from Vf to Vq through the
neon bulb. If the bulb has resistance Rb (or if the bulb is in series with a resistance Rb which is
much greater than the actual bulb resistance) then
t 3 - t 2 = RbC log Vf
Vq
(3)
and the total period is t3 - t1 , or
T = RC log
Vb - Vq
+ RbC log Vf
Vb - Vf
Vq
(4)
References
Experimental Purpuse
The purpose of this laboratory is to use the oscilloscope to observe the operation of a simple
oscillator circuit in which the voltage varies as a function of time. The time variation and the
amplitude of voltage is determined by various circuit elements and the applied d-c voltage. The
operation of this circuit can be described in terms of the basic physics of resistive and capacitive
circuits, plus the properties of electrical breakdown in gases (the neon tube).
Procedure
A word of caution: The voltage on many of the contacts in this experiment will be 200 volts
or more, which can give a nasty shock. Do not touch the exposed wires unless the voltage control
on the power supply has been turned full counterclockwise to "zero", and (as a extra precaution)
the standby switch is on "off".
1. On the power supply: (1)
(2)
(3)
(4)
Set the voltage knob full CCW.
Set the VOLTS-MA switch to VOLTS
Set the standby switch to OFF
Set the power switch to ON
2. Construct the circuit as shown below.
Set the resistance to 1M (106 ohms) and the capacitance to .01 µf. When the multi meter is
connected across the power supply (it's needed because the meter on the supply is not very
accurate) it should be set on the 250 or 1000 volt scale, Volts d-c. The larger part of the 10 meg
probe should be hooked onto the positive side of the neon lamp (the purpose of the probe is to
isolate the scope from the circuit; it reduces the measured voltage by a factor of 10:1).
3. Turn the standby switch to ON, increase the power supply voltage until the neon tube fires and
a sawtooth waveform similar to Figure 2 is observed. (Does it look exactly like Figure 2 ?)
4. Measure the period of one full cycle, and observe what fraction of this period is devoted to
charging the capacitor.
5. Measure Vf by increasing and decreasing the supply voltage, and measure Vf - Vq off the scope.
6. Repeat step 4 for different values of C, R, and V b . If C and R are changed so as to keep the
product RC constant, does the period change? Measure T for at least three different
combinations.
7. Calculate the theoretical value of the period T using Eq. (4).
Lab Report
As in previous labs, you should use the labbook format given to you by your TA at the
beginning of the term. Your lab report should include the answers to all of the questions asked in
the introduction or procedure, all raw and derived data, and an estimate of the magnitude and
sources of error in any data recorded. When answering any question or when giving any
comparison or explanation, always refer to specific data to support your statements. For this lab,
also include the following:
1. Derive Eq. (2) from Eq. (1).
2. Tabulate your results giving Vf , Vq (both of these will presumably be constant throughout),
Vb , R, C, Ttheo , and Texp .
3. Comment on the accuracy of the measurement of Vf , based on the reproducibility of your
results.
4. Given the tolerance on R and C is 10%, are your results in the expected range?