Eberline RO-2 - Functional Theory: Amplifier, Meter, and Range Switching

MODEL КО-2
hose connected to а drying box filled with silica gel `
desiccant. In this way, any air drawn into the chamber
(caused by atmospheric pressure changes, temperature
changes, transporting RO-2 by air, etc.) must first pass
over the drying desiccant. Dry air in the chamber is
necessary to help prevent leakage.
An idealized air chamber, the size of the one used on
the Model RO-2, produces approximately 1.93 x 1071 A
- per mR/h at standard temperature and pressure. (STP is
0 °С and sea level pressure of 760 mm of Hg.) At
5 mR/h it should produce 9.65 x 10“ A and at
5000 mR/h it should produce 9.65 x 107" A. It is seen
that at full scale on the most sensitive range (5 mR/h),
less than one-tenth of а micro-microamp is produced in
the chamber, which makes protection against leakage
current paramount. The silica gel desiccant should be
changed as soon as it shows any clear or pink crystals.
The positive voltage supplied to the inside chamber
wall is provided by a separate battery, BT3. This battery
does not power any other circuit but the chamber and,
therefore, its current drain is insignificant and its life is
indefinitely long (shelf life). For this reason, it is not
checked by the battery checking circuits.
2. Operational Amplifier (See Figure 6-1)
Operational amplifier A1 is contained in a single TO-5
size transistor package located in the top of the chamber
assembly. It has dual MOS FET inputs which result in a
very. high input impedance on the order of 10" Q.
Noninverting input pin 3 is connected to ground and the
other input, pin 2, is connected to the ion chamber
center electrode and the feedback elements. The output
of amplifier pin 6 feeds the meter circuit and the feed-
back circuit.
When ion chamber current flows toward the amplifier
input at pin 2, the input becomes slightly more positive
which causes the output to go more negative, This
negative output of the amplifier draws a current from
one of the feedback elements to exactly match the
amount of current the ion chamber is contributing. If a
higher ion chamber current occurs, the amplifier must
produce a higher voltage across the feedback element to
draw off the current. In this way, the amplifier voltage
output is proportional to the rate of radiation in the
chamber.
3. Meter Circuit
The meter is driven directly from the output. of
amplifier pin 6, through dropping resistor R8 and one
of four calibrating resistors R9 through R12. Nominal-
ly, the meter reaches full scale on all ranges at approx-
imately the same voltage output from the amplifier. The
four calibrating resistors allow for manufacturing
tolerances in various components and provide for
calibration at various elevations and temperatures.
Function switch 51, section C, determines which
calibration resistor is in the circuit. Capacitor C4 sets
the time constant for the meter circuit.
4. Range Switching
The range that the Model RO-2 is operating on
depends entirely on the feedback circuitry between the
output of the amplifier and the amplifier input, pin 2,
which is connected to the ion chamber. Table 3-1 shows
the nominal current and voltage condition of the circuit `
when at full scale on the four ranges (sea level, 0 °С).
Feedback resistor КІ is connected to the circuit at all
times. When using the 500 and 5000 mR/h ranges, S3
closes and R2 is put into the circuit. Since R2 has 100 `
times less resistance than КІ, the effect of КІ is negli-
gible. When on the ZERO, BAT 1, BAT 2, or OFF
positions of the range switch, S1 closes and shorts out
the feedback resistors so that the feedback current can-
not generate any voltage across the resistors, When this
occurs, any voltage remaining on the amplifier output
and showing on the meter is zero offset error. This off-
set can be removed with ZERO control R3 which re-
balances the amplifier with both inputs of Al at zero
potential.
Switches S2 and S3 are glass encapsulated magnetic
ION CHAMBER VOLTAGE ON
RANGE CURRENT, AMPS | FEEDBACK FEEDBACK VOLTAGE OUT
SETTING FULL SCALE ELEMENT ELEMENT OF AMPLIFIER
5 mR/h 9.65 х 10:45 КІ, 3 x 10 Q 2.9
50 mR/h 9.65 x 1075 R1, 3 x 10” Q 2.9 2.9 i
500 mR/h 9.65 x 107: R2, 3 x 10 Q 0.29 2.9
5000 mR/h 9.65 x 10! R2,3x 10" Q 2.9 2.9
Table 3-1. Full Scale Currents and Voltages
CHANGE 2