Instruction Manual

245364-V May 2002

Model 755A

4-3 ELECTRONIC CIRCUITRY

Electronic circuitry and internal interconnection wiring is shown in the schematic diagrams and wiring diagram in the rear of this manual. For detailed circuit analysis, refer to Section 5.

a.Detector/Magnet Assembly

A cross-sectional view of the optical bench and detector assemblies is shown in Figure 5-3B. Source lamp DS1, powered by a supply circuit on the Case Board (Section 4-3c, page 4-7), directs a light beam onto the mirror attached to the test body. The mirror reflects the beam onto dual photocell BT1, BT2.

The difference between the signals developed by the two halves of the photocell constitutes the error signal supplied to the input of amplifier U1 on the Control Board. Amplifier U1 drives U2 which, in turn, supplies the restoring current to the titanium wire loop on the test body (see Section 4-1, page 4-1).

Elements of Detector Temperature Control Circuit

Detector temperature is sensed by thermistor RT1, an integral part of the detector assembly (see Figure 4-3B, page 4-4). The thermistor provides the input signal to the detector temperature control circuit on the Case Board:

The output from this circuit is applied to the two heaters within the detector/magnet assembly; HR1, mounted on the top of the magnet, and HR2, mounted permanently on the rear of the detector assembly.

b.Control Board and Associated Circuitry

The Control Board contains signal conditioning and control circuitry. The Control Board is mounted on the inside of

the analyzer door (see Figure 1-2 on page 1-3).

The Control Board contains the following:

Input Amplifier U1

This amplifier receives the error signal from the dual photocell of the detector assembly and drives amplifier U2.

Amplifier U2 and Associated Zero Adjustment

Amplifier U2 supplies the restoring current to the titanium wire loop of the test body within the detector assembly. Front panel ZERO Control R13 applies an adjustable zero biasing signal to the input of U2 to permit establishing a zero calibration point on the display or recorder. With downscale zero standard gas flowing through the analyzer, the ZERO control is adjusted for the appropriate reading.

Amplifier U4 and ASsociated Span Adjustment

Amplifier U4 and associated feedback resistors provide a signal amplification of X4, resulting in a signal level suitable for analog divider circuit U6. Front panel SPAN adjustment R16 modifies the value of the input resistance and hence the signal amplification factor. Adjustment range is approximately ±30%.

The SPAN adjustment permits establishing an upscale calibration point on the display or recorder. With upscale standard gas flowing through the analyzer, the SPAN control is adjusted for the appropriate reading.

Pressure Compensation Circuit

The pressure compensation circuit consists of divider U6 and associated components. This circuit provides a pressure-corrected output signal conditioned to the range of 0 to 10 VDC. The circuit solves the follow equation:

Vo = k (Vx/Vz)

4-6

Theory

Rosemount Analytical Inc. A Division of Emerson Process Management