Model 755R

Instruction Manual

748213-S April 2002

5-5 DETECTOR LIGHT SOURCE CONTROL CIRCUIT

Refer to Figure 5-5 below. The detector light source control circuit maintains the light output from the bulb (DS1) as uniform as possi- ble, regardless of voltage fluctuations or aging of the bulb.

The power source for the light bulb is a center -tapped secondary of transformer T1. This AC voltage is rectified by CR7 and CR8 and filtered (C32), presenting an approximate +8.5 V bus to the current-limiting Darlington configuration of Q4.

Q4 controls the basic amount of current through DS1.

Amplifier AR7 has a fixed value, approximately +2.2 VDC on terminal 3. The output of AR7 is positive, causing Q4 to conduct. As Q4 conducts, electrons flow from the center-tap of T1 to ground and from ground through DS1 for an input voltage to terminal 2 of AR7, through R66 to develop a bias on the base of Q5, through Q4 to the +8.5 V bus, and back to the secondary. As Q5 conducts, some of the

current going through DS1 is shunted from the main current path, and goes through Q5, which acts as a variable feedback resistance, goes to the positive output potential of AR7.

As DS1 ages, its light emission decreases and its resistance increases. The current through DS1 tends to decrease, causing a decrease in the voltage drop across DS1 and the input potential to terminal 2 of AR7. Now the output AR7 will increase, causing Q4 to conduct more current through R66. As the potential across R66 increases, Q5 will conduct more current, causing a further increase in current flow through DS1. The net result is that the voltage across DS1 will remain uniform and the operation of Q4 and Q5 will adjust the gain of AR7 to maintain the light emission from DS1 uniform for a long period of time.

Voltage fluctuations in the 115 VAC supply could cause some variation in the amount of current flowing through the bulb DS1. How- ever, the voltage drop across DS1 would cause AR7 to adjust Q4 and the voltage drop across R66 to adjust Q5. The net result would still be uniform current flow through DS1 and uniform light emission.

T1

CR7

α+8.5V BUS

 

Q4

 

 

 

 

 

 

6.1 VAC

+15V

 

 

 

120 V

R63

 

 

 

 

 

C34

 

RMS

 

7.5K

 

 

 

 

.01uF

 

 

6.1 VAC CR8

2

-

 

 

 

R64

AR7

 

 

 

VR3

14K

 

 

 

+

 

 

 

9.0V

3

C35

 

 

R65α2.2V

 

 

 

C31 +

 

 

 

 

.01uF

R66

 

2000uF

4530

 

Q5

1.0

 

 

 

DS1

Figure 5-5. Detector Light Source Control Circuit

Rosemount Analytical Inc. A Division of Emerson Process Management

Circuit Analysis

5-7