The log count-rate meter (LCRM) is a pulse-counting component used to convert input pulses from the detector and preamplifier analog signal for use in control compo­nents The LCRM has five basic functions (1) pulse-height discrimination, (2) count-rate indication, (3) period indica­tion, (4) scaler output signal, and (5) adjustable alarm output Each of these functions is discussed below.

Figure 5.7 is a block diagram of an LCRM. The unit has all the items listed above along with a built-in calibrator,

B-12P

Fig. 5.4—Vacuum-tube preamplifier.

• •

power supply, and test source. The unit uses all solid-state elements for improved reliability and low maintenance

(a) Pulse-Height Discriminator. The solid-state pulse — height discriminator shown in Fig 5.8 performs three functions (1) provides for pulse-height discrimination, (2) reshapes pulses for counting, and (3) provides a scaler output signal The heart of the pulse discriminator is the dual n—p—n transistor and the potentiometer R3. Resistor R3 is a 10-turn potentiometer mounted on the front panel of the LCRM. Resistor R3 provides a d-c bias on one-half of the dual n—p—n transistor. This turns this haf of the transistor on while the input half is off. A pulse applied to the input with amplitude greater than the d-c bias turns the input transistor on, forces the biased half off, and generates an output pulse for the counting circuits A pulse of height less than the d-c bias has no effect on the output and is thus uncounted. The discriminator then allows only pulses of amplitude larger than a set threshold to be counted, thus providing a convenient means for eliminating low-amplitude noise pulses generated in the sensor and cable.

The pulses from the discriminator are reshaped in a trigger circuit so that each pulse has the same height and width, essentially a square wave. Transistor Q9 provides a pulse output for a scaler, and transistor Q8 provides a pulse output for the LCRM counting circuitry.

(b) Count-Rate Indication. Figure 5.9 shows a Cook— Yarborough log circuit. The function of this circuit is to convert the constant-width and constant-height pulses into an analog signal. Since five or six decades of counts are to be covered by the LCRM, the circuit provides a logarithmic signal.

The diode pump is composed of CR5 to CR25, C7 to C27, and resistor RIO to R20. The purpose of the diode pump is to convert from a count rate to a d-c voltage. This is accomplished in the following manner. Consider compo­nents RIO, C7, CR4, CR5, and CR6. Component CR6 is a pulse-coupling capacitor Component CR4 is a positive — voltage clipper which prevents a positive voltage from appearing at the cathode of diode CR5 Diode CR5 and capacitor C7 form a low-impedance charge circuit for negative pulses. After the negative pulse has passed through CR5, the diode prevents C7 from discharging back through CR1 or the input circuit Hence the capacitor C5 must discharge through resistor RIO. The time constant is then RIO times C5. Should a second pulse follow the first pulse rapidly, the capacitor will not have time to discharge, and, as a result, the input to the d-c amplifier through RIO is essentially a d-c or rectified a-c voltage.

There are 11 such circuits with various time constants, varying from 40 sec to 8 msec. Resistors RIO to R20, C7 to C27, and CR5 to CR25 serve the same purpose.

The pulses are thus converted to a d-c voltage output proportional to different count rates. The d-c outputs for high count rates are provided for by the shorter time — constant circuits (C27 and R20), whereas the circuits with long time constants provide a voltage output for both low and high count rates.

The output of the diode pump is a d-c voltage amplified in the scaling amplifier. The output of the scaling is properly sized for meter operation and for a potentiometric recorder output The analog signal from the scaling ampli­fier is also fed to a differentiator circuit for period and to the level alarm circuits.

(c) Period Indication. The reactor period, T, is the reciprocal of the fractional change in the neutron popula­tion per unit time (see Chap 1, Sec 1-3.1)

_ dn/n _ dn/dt _ d(ln n)

T dt n dt *

where n is the neutron density, In is the natural logarithm, and t is time

Figures 5.10 and 5.11 show two circuits used to obtain a period signal from the rate of change of the log-count-rate (LCR) signal The circuit shown in Fig. 5.10 uses opera­tional amplifier A2 with feedback to achieve the period signal Operational amplifier A1 serves only as a circuit calibrator and to provide for a test ramp signal to A2. The output signal from the diode pump is supplied to A2. Amplifier A2 differentiates any input-signal changes in level and provides an output signal for the time rate of change of the input signal

The circuit shown in Fig. 5.11 is similar in function to that shown in Fig. 5.10. The circuit is essentially an operational amplifier with a high-impedance input, FET A8, a resistive feedback element, R29, and an input capacitor, Cl2. The output voltage is then of the form

e0=Rk = RC^1 (5 2)

dt

where dej/dt is a measure of the time rate of change of the neutron flux When dn/dt is constant, dT/dt is unity (assuming T and t to be measured in the same units). The output voltage produced by the period amplifier will be some base level to keep the meter reading up scale A change in dn/dt produces a change in period-amplifier output

The output from the period differentiator drives an operational amplifier for proper signal scaling for the meter, recorder, and period-trip (alarm) board

(d) Scaler Output. Associated with the discriminator in Fig. 5.8 is a pulse output stage used to drive a pulse scaler or counter. (The scaler itself is discussed in Sec. 5-2.6.) The pulse-generating equipment consists of the circuit described in Sec (a) above and also transistor Q9. Transis­tor Q9 is used to isolate the scaler output from the LCR circuit

(e) Alarm Unit. The alarm unit provides a signal at selected and adjustable values, such as low count rate, high

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Fig. 5.8—Pulse-height discriminator

count rate, period, or loss of chamber high voltage A solid-state alarm (trip) unit is shown in Fig 5.12 Relays R1 and R2 are deenergized when the set point is exceeded. Transistors Q3 and Q7 must change from the saturated to the unsaturated (off) condition. The output of Q1 or Q2 will change any time the input-signal voltage levels exceed the set-point voltage as determined by potentiometers R28 and R29.

Each alarm function has two identical trip amplifiers similar to the one shown. The redundancy is necessary to decrease the probability of a failure in the operation of the alarm unit This is very important for reactor protection since the output transistor Q3 or Q7 could short between collector and emitter, in which case the relay would not deenergize (fail to trip) when the set point was exceeded. The contacts of the trip relays are connected in series so that either relay deenergizing will cause an alarm