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Instruction Manual
748213-S
April 2002
Rosemount Analytical Inc. A Division of Emerson Process Management Circuit Analysis 5-7
Model 755R
C35
.01uF
-
+
AR7
Q5
DS1
R66
1.0
2
3
+15V
120 V
RMS
6.1 VAC
CR7
CR8
C31
2000uF
R65
4530
R64
14K
R63
7.5K
VR3
9.0V
C34
.01uF
T1
α
2.2V
Q4
α
+8.5V BUS
+
6.1 VAC
5-5 DETECTOR LIGHT SOURCE CONTROL
CIRCUIT
Refer to Figure 5-5 below. The detector light
source control circuit maintains the light out-
put 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 cen-
ter-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 con-
figuration of Q4.
Q4 controls the basic amount of current
through DS1.
Amplifier AR7 has a fixed value, approxi-
mately +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 con-
duct more current, causing a further increase
in current flow through DS1. The net result is
that the voltage across DS1 will remain uni-
form and the operation of Q4 and Q5 will ad-
just 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.
Figure 5-5. Detector Light Source Control Circuit