Instruction Manual

748213-S April 2002

5-4 DETECTOR HEATER CONTROL CIRCUIT

Figure 5-4 below is a simplified heater control circuit drawing for the detector. Heaters 1 and 2 are actually connected in parallel and have a combined resistance of about 17 ohms.

The thermistor resistance (RT1) in the resistance bridge varies inversely with tempera- ture. The bridge is designed to maintain the temperature of the detector at 150°F (65.5°C).

The junction point between R55 and R56 is maintained at a specific voltage since these resistances maintain a definite ratio. The thermistor resistance is 149 K at 150•F

(65.5°C) and increases rapidly as the temperature decreases. R59 in this bridge circuit represents the setpoint value for temperature. Suppose that, at temperature, resistance of the bridge (R55, R56, R59 and RT1) equals 149 K.

Model 755R

If the temperature goes down, RT1 increases in resistance and causes the junction of RT1 and R59 to go positive in voltage value. Since R55 and R56 are of equal resistance, their junction is at zero volts. Therefore, terminal 3 of AR6 is more positive than terminal 2 and the base of Q2 is positive. Q2 conducts, allowing alternating current to flow through heaters 1 and 2. The voltage drop across the heaters, when completely cold, would be about 20 VAC and, when controlling, would be AC of very low amplitude.

As the temperature increases, the resistance of RT1 decreases and the junction point between RT1 and R59 becomes less positive.

Terminal 3 of AR6 becomes less positive with respect to terminal 2. The output of AR causes Q2 and Q3 to conduct less. When terminal 3 equals terminal 2, or is less than terminal 2, the output of AR6 is zero or less. Q2 and Q3 do not conduct and the heater would not be supplying heat energy to the detector.

HR1 +2

F1

CR6

WO4

120 V

25 VAC

RMS

 

 

 

 

 

 

 

R60

 

 

 

R58

 

 

100

 

 

 

 

 

 

 

 

 

5M

 

 

 

 

+15V

2

 

 

 

Q3

 

 

-

 

R62

R55

R59

 

6

1K

 

 

U6

 

700K

700K

 

 

 

Q2

 

 

3 +

 

 

 

R56

 

 

 

 

 

CR12

RT1 C31

 

 

 

 

 

149K

R88

 

 

 

 

.01uF

5M

 

 

R61

 

 

 

 

 

 

-15V

2.0

 

Figure 5-4. Detector Heater Control Circuit

5-6 Circuit Analysis

Rosemount Analytical Inc. A Division of Emerson Process Management