BEKA BA334E, Руководство пользователя, Страница: 10 / 64

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Alternatively the pulse source may be located in the
safe area. Fig 3 shows how an additional Zener
barrier is used to transfer the signal to the rate
totaliser in the hazardous area. When more than
one Zener barrier is used in a system all must have
the same polarity, i.e. all positive or all negative
barriers.
When designing a system it is important to
remember that terminals 2, 6 and RS2 are
interconnected within the BA334G See Fig 1.
Fig 3 BA334G used with Zener barriers pulse
source in the safe area.
4.1.1 Power supply
The BA334G Rate Totaliser requires a minimum of
10V between terminal 1 & 2 and consume:
10mA without optional backlight
plus 6mA when terminals 3 & 4 are linked
Any certified Zener barrier may be used to power a
BA334G Rate Totaliser providing the output safety
parameters of the barrier are equal to or less than
the input safety parameters of terminals 1 & 2 of the
Rate Totaliser.
Although this allows a wide variety of barriers to be
used, a positive polarity 28V; 93mA; 300
Ω Zener
barrier, which has an end-to-end resistance of
about 340
Ω , is an industry standard device which
is frequently used. With this barrier the supply
voltage in the safe area must be between 15.5V
and the maximum working voltage of the Zener
barrier which, depending upon manufacturer, will
be approximately 26V.
Note: The optional factory fitted display backlight
increases the instrument's current
consumption to 32mA and therefore
increases the minimum safe area
operating voltage, see section 10.2 for
details.
4.1.2 Pulse input
As shown in Figs 2 and 3 the BA334G can display
the rate and total flow from flowmeters with a wide
variety of pulse outputs located in safe and
hazardous areas.
No Zener barrier is required in series with the input
if the intrinsically safe flowmeter is located within
the same hazardous area as the Rate Totaliser.
The following table shows the Rate Totaliser's input
switching thresholds when conditioned for use with
flowmeters having different outputs, For reliable
totalisation the Rate Totaliser pulse input must fall
below the lower threshold and rise above the
upper
threshold.
Input transducer
Switching thresholds
Lower Upper
Open collector
2k
Ω
10k
Ω
Voltage pulse
low
1.0V 3.0V
Voltage pulse
high
3.0V 10.0V
Magnetic pick-off 0 40mV peak
Proximity
detector
1.2mA 2.1mA
Switch
100
Ω
1000
Ω
Flowmeters with a switch contact, proximity
detector or an open collector output require
energising which is achieved by linking Rate
Totaliser terminals 3 and 4.