Instruction and operating guideline CO-detector
Type:
VCD2 CO-Detector
5
Power Supply and Total Loop Resistance
Transmitters can be operated with any single-sided power supply within the
range 10 - 35 VDC. However the power supply used will impose constraints on
the total loop resistance in the external circuit, and this must be taken into
account when choosing the supply voltage. This includes the measuring resistor
at the remote receiver and any meters for calibration etc. The example below
illustrates the relationship between the two considerations.
So that the sensor only requires a few seconds to settle after start-up, the
transmitter circuit has an FET powered shorting link between the sensing and
reference electrodes of the CiTiceL when left unpowered. However, due to the
different operating nature of biased sensors (which include nitric oxide and
hydrogen chloride sensors), this link is omitted with these transmitters. They
therefore require a longer start-up time when first powered, and must remain
powered continuously in the actual application.
IMPORTANT :
The transmitter is designed for power supplies within the range
10 - 35 VDC only. Connection to mains electricity will result in transmitter failure.
Relationship Between Power Supply Voltage and Total Loop Resistance
Example : Using a transmitter with a standard 24 VDC power supply, the total
allowable resistance is as follows:
The minimum voltage required at
the transmitter for operation is 10
Volts. Therefore, the maximum
drop allowed due to loop resistance
is 14 Volts.
Full scale deflection current
requirement : 20 mA
Therefore, the maximum loop
resistance allowable
(using Ohm's law):
R = V / I
R = (14 x 1000) / 20
R = 700 Ohms
In this case, a measuring resistor of 250 Ohms is appropriate, giving a scale of 1
V to 5 V.
