NOTE: (1) Zener Diode 36V: 1N4753A or Motorola
P6KE39A. Use lower voltage zener diodes with loop
power supply voltages less than 30V for increased
protection. See “Over-Voltage Surge Protection.”
less than minimum
voltage rating of zener
The diode bridge causes
a 1.4V loss in loop supply
FIGURE 4. Reverse Voltage Operation and Over-Voltage Surge Protection.
The XTR115/6 low compliance voltage rating (7.5V) per-
mits the use of various voltage protection methods without
compromising operating range. Figure 4 shows a diode
bridge circuit which allows normal operation even when the
voltage connection lines are reversed. The bridge causes a
two diode drop (approximately 1.4V) loss in loop supply
voltage. This results in a compliance voltage of approxi-
mately 9V—satisfactory for most applications. A diode can
be inserted in series with the loop supply voltage and the V+
pin to protect against reverse output connection lines with
only a 0.7V loss in loop supply voltage.
OVER-VOLTAGE SURGE PROTECTION
Remote connections to current transmitters can sometimes be
subjected to voltage surges. It is prudent to limit the maximum
surge voltage applied to the XTR115/6 to as low as practical.
Various zener diode and surge clamping diodes are specially
designed for this purpose. Select a clamp diode with as low a
voltage rating as possible for best protection. For example, a
36V protection diode will assure proper transmitter operation
at normal loop voltages, yet will provide an appropriate level
of protection against voltage surges. Characterization tests on
several production lots showed no damage with loop supply
voltages up to 65V.
Most surge protection zener diodes have a diode character-
istic in the forward direction that will conduct excessive
current, possibly damaging receiving-side circuitry if the
loop connections are reversed. If a surge protection diode is
used, a series diode or diode bridge should be used for
protection against reversed connections.
RADIO FREQUENCY INTERFERENCE
The long wire lengths of current loops invite radio frequency
interference. RF can be rectified by the input circuitry of the
XTR115/6 or preceding circuitry. This generally appears as
an unstable output current that varies with the position of
loop supply or input wiring.
Interference may also enter at the input terminals. For
integrated transmitter assemblies with short connection to
the sensor, the interference more likely comes from the
current loop connections.