Badger Meter 106-255 - Manual (Page 3)

Model 106-255 Signal Conditioner, Modulated Carrier, Meter Mounted
3
As in the two wire interface, the flowmeter pick-
off is connected directly to terminals “A” and “B”.
The power supply required can be at any
voltage level between 15 and 28 volts and
elements (gates, flip-flops, etc.) that level would be
equal to the logic supply voltage. The maximum
voltage level to be used for this purpose is +40 volts.
The resistor value is not critical and is determined in a
manner which minimizes power consumption while
insuring sufficient rise and fall times given the shunt
capacitance. For example, if the total expected shunt
capacitance due to all contributors is 1000 picofarads,
the maximum pull-up should not exceed 20,000
ohms. For a total shunt capacitance of 10,000
picofarads, the maximum pull-up would be reduced to
2,000 ohms. INTRINSIC SAFETY BARRIERS
A variation of the Two Wire Interface is illustrated in
Figure 6 which uses Intrinsic Safety Barriers to satisfy
the requirements typical of operation in hazardous or
potentially explosive environments. By appropriate
selection of the barrier, the series resistor common to
the Two Wire Interface is replaced by the barrier
device. Barriers are available which provide series
resistance from a few ohms up to several hundred
ohms. By returning the modulation signal to the rate
indicator via a second safety barrier, the
instrumentation is effectively isolated from the
hazardous area and satisfies the requirements for an
intrinsically safe design.
Figure 4. Output peak to peak voltage vs. R
series
and
V
supply
. The shaded area indicates R
series
greater than
maximum allowed from Figure 3.
Figure 5. Minimum level of output signal. DC component on
which the signal rides. Max supply current is (V
supply
– V
min
)
/R
series
.
Figure 6. Interconnect using Intrinsic Safety Barriers for operation in hazardous potentially explosive
environments. Instrumentation located in “safe” area; flowmeter and signal conditioner are located in
the hazardous area. Safety Barrier isolates the two.
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