MDS 05-4055A01, Rev. C
MDS entraNET System Guide
21
Power Supply
Connections at 28
Vdc
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 impor-
tant 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 cur-
rents 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 pow-
ered 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 min-
imized during initial power-up if the supply cannot be turned
off.
• Add a 1 to 2
Ω
, 2 Watt resistor in series with the positive lead.
This 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.
• Minimize the length of the power cabling, within reason.
• When power is applied from a power source having a relatively
high (1 or 2
Ω
) source impedance, or from a power source with-
out a large amount of output capacitance, no overshoot occurs.
Therefore, use a power supply that is rated appropriately for the
radio if possible—avoid using power supplies that far exceed
the radio's current requirements.
Please direct any questions you may have about interfacing to MDS
radios to MDS Technical Services at TechSupport@Microwave-
Data.com, or telephone
+1-585-241-5510.
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