Page 24 of 27 Filename: 7970H8
8. Sensor Information
8.1. Flammable Sensors
The flammable gas sensors are pellistor based flammable gas sensors
operating in the range 0-100% LEL (Lower Explosive Limit).
There are a number of components which inhibit flammable gas sensors.
For this reason regular checking will help maintain any pellistor based
system in good operating condition.
AVOID exposure to silicones, chlorinated hydrocarbons, lead and sulphur
containing compounds, halogenated compounds.
The TQ122 flammable sensors are wire colour coded and should be
connected to the control panel in accordance with Drawing 7970V.
8.2. Toxic Sensors
Most toxic sensors are 4-20 mA based and the majority of these are two
wire. The TQ4000 can interface to almost any 2 wire sensor. The TQ4000
source voltage is 24V and the 4-20mA is developed into 220 R. In general
the supply +ve to the sensor is taken from the input pin marked BHI and
the 4-20mA returned to the middle pin which is SIG. It is very important to
ensure that the correct input card is used. If a flammable input card is used
it may cause severe damage to the sensor, similar comments apply if a 4-
20mA is used on a flammable sensor. Connection detail for the TQ122/123
series of sensor is shown in drawing 7970V.
8.3. Infra-red and other 3-wire 4-20mA Sensors
3 wire systems indicate that the sensor at the end of the TQ4000 requires
power. The TQ4000 can adequately drive up to 3x24Volt sensors each
requiring less than 0.25W per sensor. The voltage available falls towards
the lower limit of 18Volts when loaded with 4 sensors of this power and
may cause intermittent operation of the sensors. When using higher rated
sensors it is advisable to order a larger capacity power supply to cope with
the additional demand. The power supply in the TQ4000 is designed to
deliver 18-24V at 1.5A. The electronics consume approx 0.5A which leaves
approx 1A available to drive sensors and alarms. The power supply is an
easily interchanged unit.
Connection is made to the BHI for the 24V, the 0V is obtained at the BLO
terminal and the 4-20mA is returned to the SIG terminal for the selected
channel. It is very important to ensure that the correct input card is used.
The 4-20mA input card must be used for this application.
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