Model 755R

Instruction Manual

748213-S April 2002

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 zero standard gas flowing through the ana- lyzer, the ZERO control is adjusted for the appropriate reading.

Amplifier U4 and Associated Span Adjustment

Amplifier U4 and associated feed back resistors provide a signal amplification of X4. Front panel SPAN adjustment R20 modifies the value of the input resistance and hence the signal amplification factor. Adjustment range is approximately ±30%.

Amplifier U8

This unity gain amplifier provides zeroing capability and a buffered output for the anticipation circuit feeding U10.

Amplifier U10

U10 is an inverting buffer amplifier that incorporates an anticipation arrangement in its input network, thus providing slightly faster response on the readout device(s).

Potentiometer R30 provides a continuously variable adjustment of 5 to 25 seconds for the electronic anticipation time and is factory-set for 20 seconds.

Since the anticipation network attenuates the signal, a gain of 10 is provided in U10 to restore the signal to the desired full- scale range of 0 to 10 VDC.

The output signal from U10 is routed to two output circuits: a digital and an ana- log.

In the Digital Output Circuit, the signal from U10 passes to an integrating analog -to-digital converter. The resulting digital signal drives the liquid crystal dis- play.

In the Analog Output Circuit, the output from U10 is provided as an input to the recorder output amplifier. This circuitry provides scale expansion, and amplification preparatory to use for potentiometric recorder, voltage-to-current conversion for current recorder, and/or alarm functions.

Potentiometric output is strap-selectable for 0 to 10 mV, 0 to 100 mV, 0 to 1 V, or 0 to 5VDC. Potentiometer R88 permits adjustment of recorder span on 0 to 1 V, 100 mV and 10 mV outputs.

c.Power Supply Board Assembly

The Power Supply Board assembly con- tains power supply and temperature con- trol circuitry. The assembly is mounted within the analyzer case.

As shown in DWG 617186, the various circuits operate on main power trans- former T1. During instrument assembly, the two primary windings of T1 are fac- tory-connected for operation on either 115 VAC or 230 VAC, as noted on the name rating plate.

The same circuit board contains the fol- lowing:

Source Lamp Power Supply Section

This circuit provides a regulated output of 2.20 VDC to operate incandescent source lamp DS1 within the optical bench as- sembly. One secondary of main power transformer T1 drives a fullwave rectifier consisting of CR7 and CR8. The output of DS1 is held constant by a voltage regu- lator circuit utilizing U7, Q4 and Q5.

Rosemount Analytical Inc. A Division of Emerson Process Management

Theory

4-5