BEKA BA314E, Manual

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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 Tachometer
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 BA314G. See Fig 1.
Fig 3 BA314G used with Zener barriers pulse source
in the safe area.
4.1.1 Power supply
The BA314G Tachometer requires a minimum of 10V
between terminal 1 & 2 and consumes:
10mA without optional backlight
plus 6mA when terminals 3 & 4 are linked
Any certified Zener barrier may be used to power a
BA314G Tachometer providing the output safety
parameters of the barrier are equal to or less than the
input safety parameters of terminals 1 & 2 of the
Tachometer.
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
increase the instrument's current
consumption to 32mA and therefore
increases the minimum safe area operating
voltage, see section 9.2 for details.
4.1.2 Pulse input
As shown in Figs 2 and 3 the BA314G can display
the rate and total flow from sensors 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 sensor is located within the
same hazardous area as the Tachometer. The
following table shows the Tachometer's input
switching thresholds when conditioned for use with
sensors having different outputs, For reliable
totalisation the Tachometer 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
Switch contact, proximity detector or open collector
sensors require energising which is achieved by
linking Tachometer terminals 3 and 4.
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