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GFK-2489 Chapter 2 Tabletop Setup and Evaluation 2-5
2
Power Supply Connections at 28VDC
Common 28VDC 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 30VDC 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 28VDC 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 Ohm, 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 Ohm) source
impedance, or from a power source without 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.