TXP-C20_Instruction_Manual_(Rev1.0)
REV – 1.0
[34-2900-0207-5 ]
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with a bridge amplifier, balance potentiometer, and an adjustable switching regulator for setting the correct sensor
excitation voltage. A 3-position coarse gain jumper allows setting the gain of the bridge amplifier. Fault supervision
circuitry forces the TXP-C20 into a FAULT condition upon sensor failure or removal.
This option may also be configured to accept 4-20mA inputs for mixing a sensor and a current loop into the same
board. Placing either channel’s 2 position SENSOR/4-20mA jumper (JP3 or JP4) into the 4-20mA position, and
installing the associated precision 100 ohm socketed resistor, allows 4-20mA signals to be applied to the mA+ / mA-
terminals (see Optional 4-20mA notes in Figure 4-5). 100-ohm resistors are taped to the inside of the TXP-C20
enclosure.
Bridge sensors not mounted to the TXP-C20 enclosure at the factory will require the Initial Setup calibration
procedure described in section 3.1.4. After performing the one time only Initial Setup all subsequent calibrations
are by the TXP-C20’s electronic Cal Mode menus (see section 2.2.5). Sensors mounted locally to the TXP-C20
normally do not require Initial Setup since it is performed at the factory.
4.1.4
Bridge Sensor Initial Setup
Bridge sensors vary widely in power requirements and sensitivity. It is therefore important to configure each
channel to match the sensor with which it will operate. Sensors attached to the TXP-C20 enclosure at the factory
have already had Initial Setup.
1.
Prior to connecting remote sensors, apply power to the system. Measure the voltage between each
channel’s A and R terminals and set the Voltage Adjust potentiometers for the correct sensor excitation
voltage (remove TXP-C20 terminal cover). This may range from 1.5 volts to 7.5 volts depending upon sensor
specifications. Sensors may be damaged by accidental over voltage conditions. It is recommended the
Voltage Adjust potentiometer screws be covered by a dollop of RTV or similar material after completion of
this procedure.
2.
Remove system power and connect sensor wires to the A-C-R terminals. Reapply system power and confirm
correct voltage across each sensor’s A & R terminals. Note if sensor wires are long, it will be necessary to
measure the excitation voltage at the sensor end to compensate for I * R voltage losses in the wiring.
3.
With the minus voltmeter lead on TB2-2 (common), connect the plus lead to the channel’s red test point.
With zero air on that sensor, adjust its Balance potentiometer for .4 volts at the test point.
4.
Apply 50% span gas to the sensor and allow the test point voltage to stabilize. Two volts = 100% input to the
A – D Converter and .4 volts = 0%. Therefore, 1.2 volts = 50%. Place the 3 position Coarse Gain jumper into
the position which reads between 1.1 volts and 1.4 volts on the test point with 50% gas on the sensor. Gain
settings for each jumper position are as follows: no jumper = 1, LO = 7, MED = 12, HI = 24. Multiple jumpers
have an additive affect upon gain, so for example the LO and MED jumpers together provide a gain of 19.
Initial setup is now complete and normally only requires repeating if a sensor is replaced. Final calibration of this
channel may now be performed using the TXP-C20’s electronic Cal Mode feature described in section 2.2.5.
