GE MDS entraNET 900 Technical Manual page 30

Power Supply
Connections at
28 Vdc
20
Lead
Binding
Screws (2)
The transceiver must be used only with nega-
CAUTION
tive-ground systems. Make sure the polarity of the
POSSIBLE POSSIBLE
power source is correct. The radio is protected from
EQUIPMENT EQUIPMENT
DAMAGE DAMAGE
reverse polarity by an internal diode and an on-board
fuse.
Common 28 Vdc supplies are often high-current power supplies
designed primarily to charge battery banks. The radio can be operated
from these supplies, providing there are no transients on the leads as
power is applied to the radio. Transients can be created that rise above
30 Vdc to a voltage that exceeds the primary voltage rating of the radio
and can destroy its voltage regulators and other components. It is
important to keep this potential hazard in mind when designing 28 Vdc
power supply connections for the radio.
• Use a two-conductor cable to power to the radio. Then the
currents in the positive and negative wires are equal and
opposite, causing their magnetic fields to cancel. The result is
no net inductance in the connection to cause voltage overshoot.
• Do not connect a radio to a power supply that is already
powered up, unless necessary (that is, when connecting a radio
to a battery bank and charger). When power is applied by
switching on a power supply, the rise time of the supply is too
slow to cause overshoot.
• Typically, there are multiple return paths for the negative side
of the power supply, through the coaxial cable shield and the
chassis, for example. Any imbalance in the currents in the
power cable results in voltage overshoot, so this should be
minimized during initial power-up if the supply cannot be
turned off.
• Add a 1 to 2
greatly limits voltage overshoot. Since these radios draw very
little current in receive mode, and transmit only briefly, there is
little loss in power efficiency. In transmit, the voltage drop is
minimal and has no effect.
MDS entraNET Technical Manual
Invisible place holder
Figure 2-4. Power Connector
(polarity: left +, right –)

, 2 W resistor in series with the positive lead. This
Wire Ports
05-4055A01, Rev. E