Brown University
Physics Department
PHYS 0060
LAB B - 160
•
A glass tube containing neon gas will give off its characteristic light when the voltage across the tube exceeds a certain value. The value corresponds to the geometry of the tube (and of course the energy levels of neon) and physically signals the “breakdown” of the gas - an electron free in the gas acquires enough energy between collisions that it liberates several more electrons when it does collide. As a result of this “avalanche” effect, a large number of atoms are excited to higher energy levels, from which they return to the ground state with the emission of visible, characteristic light.
This study uses a small neon tube, together with an RC circuit to build a “relaxation oscillator.”
The R and C combine to build the voltage across the tube with their characteristic time, but when the tube begins conducting it discharges the capacitor, and so shuts itself off. The cycle then repeats itself, producing a free running oscillator.
•
A neon tube consists of two electrodes separated by a gap of a few millimeters in an atmosphere of neon. As you might guess, a neon tube is not a simple circuit element obeying
Ohm’s law. In fact, it is a nonlinear device. On application of a small voltage to the tube, no current passes through the tube (it is like an open circuit with infinite resistance). As the voltage exceeds a certain threshold voltage, V
T1
, the gas “breaks down” and a large current starts to pass through the tube (now it is like a wire with very small resistance). Once the current is started it may be maintained by a lower voltage than V
T1 and we will denote this minimum voltage necessary to maintain conduction by V
T2
. The voltage—current characteristics of the tube can therefore be represented as shown in figure 1.
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Brown University
Physics Department
I I
PHYS 0060
LAB B - 160
V V
V
T1
V
T2
Ohmic Device
Figure 1
Neon Tube
Because of its peculiar voltage—current characteristics, the neon tube may be used to make an oscillator. Consider the circuit shown in Figure 2. (V
0 is the voltage across the supply and V is the voltage across the neon tube.)
R
𝑉 𝑜
C
Figure 2
V
Neon  Tube
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Brown University
Physics Department
PHYS 0060
LAB B - 160
Suppose we have just switched on the power supply and the capacitor, C, is uncharged.
The voltage V will increase as the capacitor charges through the resistor R. When V exceeds V
T 1 the neon tube will suddenly start to conduct and will discharge the capacitor until V
T 2
. This will happen in a very short time. The capacitor will then start charging again through R and the whole cycle will be repeated.
V
0
V
T1
V
T2
Figure 3
When a capacitor is being charged, the charge on it is given by:
Q = CV
0
(1-e -t/RC )
Since Q = CV for a capacitor, we have
V = V
0
(1-e -t/RC )
Using this we can easily obtain an expression for the period of the oscillator, τ . If we assume that the discharge is instantaneous, then
τ
will be equal to the time it takes to charge the capacitor from V
T2 to V
T1
. Letting
V
T2
= V
0
(1 — e -t
2
/ RC )
V
T1
= V
0
(1 — e -t
1
/ RC ) and solving for t
1 and t
2
,
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Brown University
Physics Department
PHYS 0060
LAB B - 160 t t
2
= -RC ln (1-V
T2
/V
0
)
1
= -RC ln (1-V
T1
/V
0
) we have for:
τ
= t
1
-t
2
= RC ln((V
0
-V
T2
)/(V
0
-V
T2
))
•
volts.
Set up the circuit shown in figure 2 with R = 150K, C = 0.1
µ F and V
0
= 100 to 110
We will monitor the voltage across the neon tube with the oscilloscope. We want to use the scope to measure the period, VT
1 and VT
2
. To measure voltages with respect to ground, use a digital scope such as the Tektronix 2012. The outer rings of its input terminals are internally grounded. It is recommended you review Introduction to the Oscilloscope in the Brown Physics
60 Lab Manual.
Using the horizontal sensitivity, vertical sensitivity and the trigger level controls, obtain a stable wave form as in Figure 3. Measure the period of the oscillation and V
T1 and V
T2 with respect to ground, i.e., V = 0. The expression for τ is rather sensitive to the values of R and V
0
.
Using a digital multimeter, measure these.
Substituting the measured quantities into the expression for τ , compare the theoretical and experimental values for τ . Note that you can save your waveform data by using the Tektronix Open Choice Desktop Software available on the lab computers (extremely useful for this experiment).
•
V
0
___________ V
T1
____________ V
T2
____________ R___________
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Brown University
Physics Department
Periods of Oscillation:
PHYS 0060
LAB B - 160
τ
(measured)__________
τ
(theoretical)__________
If your construction checks out, you can proceed to variations in R and C and observe how oscillators of different, or variable frequency could be made.
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