CRI insures that only
the correct polarity current will flow in the set or
reset coils. CR2 clamps the back-spike voltage which
is present from either coil when power is suddenly removed.
2. The sensitivity of
the indicator is specified as the voltage required at
a minimum pulse duration to produce an indicator transfer.
In general, longer pulse
applications will not reduce the peak pulse voltage
The pulse shape must be
such that the required voltage is present over the minimum
pulse length time to effect transfer.
Rise time of the applied
pulse does not affect operation of the Ball or Drum
Indicators. Response is obtained to fast and slow pulses,
D.C., and rectified A.C. as long as the pulse has the
prescribed amplitude and duration. The Flag Indicators
will operate upon the application of a 40-millisecond
minimum, DC voltage square wave pulse with a rise time
of 5-millisecond minimum. The fault indicator does not
operate satisfactorily from a slowly increasing ramp
voltage due to the anti-vibration feature designed into
the fault indicator.
3. The minimum operate
voltage may be confused with minimum non-operate requirements.
The minimum voltage is designed to operate well below
the minimum operate voltage in order to insure that
the unit will always operate at the minimum voltage
at the maximum operating temperature specified. This
is necessary because the coil resistance increases with
an increase in ambient temperature.
4. Since the typical status
indicator is a magnetic latching non volatile device,
the resistance to vibration may often be increased over
specified values by reducing clearances and magnet spacing
within the device.
In practically all
instances, the amplitude of vibration a given type of
indicator can withstand is proportional to the magnetic
latching properties and therefore to the amount of voltage
required for transfer. Therefore, higher vibration level
resistance requires a higher voltage for transfer and
an increase in the minimum operate voltage level.
The fundamental driving
indicator elements are magnetic. To insure that the
magnetic environment is free from an interfering field,
the following should be considered;
1) Locate the indicators at least 0.5"
apart, center to center, except where otherwise noted
(some units can be mounted side by side).
2) Do not locate the device adjacent to magnetic sources
such as power relays, transformers, etc.
3) Mount indicators on
4) If it is impossible
to follow the 3 rules above, then magnetic shielding
may be necessary.
The nominal operating
voltage is marked on the indicator case. This voltage
applied @ 25°C will cause transfer at the rated
ampere turns (NI). The minimum NI product must be maintained
over the temperature range. An increase or decrease
in temperature will create the need for a proportional
change in voltage. Typically these indicators will operate
30% below and 25% above nominal voltage at 25°C.
It is recommended that nominal voltage be applied at
the ambient temperature since the indicator will still
be operable if the coil resistance increases due to
an unforeseen increase in ambient temperature. The graph
below shows the variation of minimum operate voltage
Since these devices
depend upon a constant ampereturns for proper operation,
care should be exercised when planning for temperature
extremes. See Figure 8 below.