Teledyne 9700 instruction manual Detector

2 Theory of Operation Model 9700

An optional CAUTION alarm circuit utilizes potentiometer P4 to establish the setpoint. When the setting of P4 is exceeded. transistor Q2 is turned on and relay K1 is energized, resulting in an alarm.

The optional FAILURE alarm circuit is set up so that if the detector opens up, an alarm comparator will trip and turn off a relay, which is normally energized, i.e., during the non-alarm condition, the normally open contact will be closed and the normally closed contact will be open.

The power supply circuitry includes transformer T1 which is a 40-volt center-tapped unit. Two sets of rectifiers are used to supply two regulated power circuits and +/-24 VDC unregulated. The circuit that includes Zener diodes ZD2 and ZD3 supplies +/-15 VDC regulated. Regulator VR2 is the current regulator for the detector. This regulator is set up so that current through the detector is determined by resistors R4 and R5. Current output of 0.3A can be measured at a pair of test points. Capacitor C9, connected across the input, gives protection from line transients, high voltage spikes, etc.

2. 3 Detector

The basic elements of the combustible gas detector are shown in Figure 5. The two beads each consist of a very small coil of wire coated with an appropriate material. The active (or measuring) bead coating is a mixture of a catalytic material with an inert binder. This catalytic material is selected to enhance the oxidation of combustible gases. The reference bead coating is an inert material having similar thermal properties to the other bead.

Upon exposure of the detector to an atmosphere containing com- bustible gases and oxygen, these will combine at the surface of the measur- ing bead. Energy produced by this reaction will heat the active bead and cause the electrical resistance of its wire coil to change. The change in resistance of this coil is, then, for a particular gas, a measure of the reaction rate at the bead surface. The reaction rate and energy production depend strongly on the nature of the gas. By raising the temperature of the bead, the reaction rate can be increased, making the effects of different gases more nearly equal. Thus, the sensitivity of the detector is made greater and more nearly equal for different gases. This heating is accomplished by passing a constant electrical current through the wire coil supporting the bead.

2-4 TELEDYNE BROWN ENGINEERING

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