3B Scientific Training Oscilloscope User Manual

3B SCIENTIFIC® PHYSICS

Demonstration oscilloscope U8481350

Instruction sheet

01/08 CW/ALF

Inputs:
1 Anode voltage
2 Cathode voltage
3 Wehnelt (focussing) voltage
4 Heater voltage (0)
5 Heater voltage (+)
6 Chassis ground
7 Deflecting plate (left)
8 Deflecting plate (right)

Outputs:
9 Saw-tooth generator (-)
10 Saw-tooth generator (+)

11 Deflecting coils
12 Circular magnet (obscured by

deflection coil)
13 Cathode ray tube
14 Metal ring
15 Coarse adjustment for saw-

1. Safety instructions

tooth frequency
16 Fine adjustment saw-tooth

frequency

2. Description

The demonstration oscilloscope is operated with volt­ages, some of which are above 60 V.

• Always turn off power supply before making con-

nections.

• Use safety leads.

Since the glass tube is evacuated, there is an implo­sion hazard.

• Do not subject the tube to sharp blows or me-

chanical stress.

In schools and training institutions, operation of the
device is to be responsibly supervised by trained per­sonnel.

The demonstration oscilloscope can be used to show
the deflection of an electron beam by electric and
magnetic fields, just as employed in TVs or conven­tional oscilloscopes. Essentially, it consists of a cath­ode ray tube that is supplied with voltage via 4-mm
plugs and is surrounded by a ring, to which deflecting
coils can be attached.

A cathode ray tube is an evacuated glass tube, the
neck of which contains a heated cathode and an an­ode in the shape of a disc with a hole in the middle,
separated by a distance of approximately half a cen­timetre. Electrons emitted from the heated cathode
are accelerated towards the anode. Some of them
pass through the hole to form a beam that strikes the
fluorescent screen (with zinc silicate coating) and thus
becomes visible as a green fluorescent dot. The beam
is focussed partly by a Wehnelt cylinder surrounding
the cathode, the potential of which is negative with

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respect to the cathode potential, and partly by gas
constriction as a result of the tube being filled with
neon at a pressure of 0.01 mm Hg that also renders
the beam visible inside the tube.

There are also two opposing deflection plates in the
tube, oriented parallel to the beam, which can be
connected to the integrated saw-tooth generator, or
to an external voltage supply. The generator supplies

saw-tooth voltage waveforms with a frequency range
of 3.5 to 650 Hz and an amplitude of 100 V relative to
the anode potential.

Tube:
17 Deflection plates
18 Anode
19 Wehnelt cylinder
20 Cathode
21 Heater
22 Fluorescent screen

Fig. 1: Cathode ray tube

3. Technical data

Anode voltage: 250 V DC
Anode current: 1 mA max.
Heater voltage: 6...8 V AC/DC
Wehnelt voltage: -50...0 V DC
Size of deflection plates: 12 x 20 mm²
Distance of deflection plates: 14 mm
Deflecting coils: 300 + 300 turns

R

= 4.2 Ω

i

L = 6 mH

Saw-tooth voltages: V

= 100 V

pp

f = 3.5..650 Hz

4. Operation

4.1 To start operation

In order to supply current to the demonstration oscil­loscope, power supplies are required that can output
the following voltages:

+250 V DC,
0-50 V DC, regulated,
6-8 V DC, regulated.
Power supplies U8521371 and U33000, which can

supply all of these voltages, are particularly well
suited to this end.

• Turn off the power supply.

• Connect the inputs of the demonstration oscillo-

scope to the outputs of the power supply, that
supply the specified voltages.

• Adjust the voltage so as to not to exceed the lim-

its.

• Turn on the power supply.

After 10-30 sec, a green spot appears on the fluores­cent screen, which denotes where the electron beam
is hitting the screen. In order to keep the tube as
simple and clear as possible for educational purposes,
it was decided to do without additional apparatus for
secondary acceleration and focusing of the beam. For
this reason, it is not usually possible to focus the
beam to the sharpness seen in conventional oscillo­scopes.

• Vary the Wehnelt voltage until the spot reaches its

minimum dimensions.

The electron beam is visible as a reddish thread inside
the tube, but only in a darkened room because of its
low intensity.

4.2 Deflecting apparatus

4.2.1 Electrical deflection
Using the deflecting plates located inside the tube,

the electron beam can be deflected horizontally by
applying a voltage of up to 100 V. For most applica-

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