DC and AC Power

The basic requirement for providing amateur radio communications is a radio and a means to power that radio. The power can come from a power supply or from batteries or both. The AC power for the power supply can come from the power grid or from a generator or both.

Power systems may have backup solutions and these solutions can be manually or automatically activated. An example of a manual backup system is a power supply and a battery. The back up solution may be to unplug the radio from the power supply and plug into the battery if the AC power fails. The manual solution can be automated if the power supply is set up to keep the battery charged and to automatically power the radio from the battery if the AC fails.

The size of battery backup varies by application. In a building with a built in generator, the battery backup may be small (1 hour). If the AC power fails, the battery powers the radio during the 30 to 60 seconds it takes the generator to sense the AC failure and come on line. The minimum 1 hour back up capacity keeps the radio running if the generator does not start. The outage period is longer as maintenance staff works to start the generator, but there is time for additional battery power to be delivered to keep the radio running if required. Other locations may not have any electricity or the potential outage period may be much longer as portable generators are brought in, so a larger battery capacity is required.

Computers add another challenge, since they typically run off AC power and they can be damaged by spikes on their power. Laptop computers have batteries, but the operating time on battery is only a few hours on good batteries and most amateurs are using older laptops which have less battery capacity due to age. If the AC power fails, there are devices for generating 110VAC from a battery source. The simplest solution is an inverter, which connects to the battery and provides an AC outlet. These are available from many sources. The more elaborate solution is a UPS (Uninterrupted Power Supply). The UPS provides power conditioning on the normal AC power, internal batteries and automatic switching if the AC power fails.

One more important component of any solution with backup power, is light. If the power fails, the radio operator will be in darkness or near darkness. Even for 30 to 60 seconds, this may interrupt or degrade the operation. Adding a small 50 watt inverter and lamp with a screw in 15 watt fluorescent bulb to the system, ensures that there will be lighting, even if the power fails.

Modular Power System

This project defines a modular power system that can be deployed as individual components, or as a complete system. The full system would be deployed for example at a shelter where there is voice radio operations as well as computer based communications.


  1. Power Supply
    The power supply module is intended to operate a radio at a site that has AC power, but provides a short term (approximately 1 hour) battery backup. The battery is charged by the power supply, using a modification to the power supply. This module also includes a 50 watt inverter to power a 15 watt fluorescent lamp. The wiring is included, to be able to add more battery power if required. The power supply module is for use in sites that have a built in generator backup, where interruptions would be least likely and short duration. IF the power fails, the radio continues to operate and there is light for the radio operator.
  2. Battery
    Long term operation requires a deep cycle battery, such as used in marine applications. The basic deep cycle battery is missing some components for use as a backup battery. Installing the battery in a container provides some protection and makes it easier to carry. The battery will require an automatic charger, to charge it when it needed and to float charge it for long term maintenance. Standard connectors with fuses are also required and a voltmeter is a useful addition. The battery module is fully self contained, with its own attached charger, so it can be used to power a radio at sites that do not have AC power. For locations that are using the Power Supply module, but that require additional battery power, the Battery module plugs in to add the extra capacity.
  3. AC Module

    There are two issues for AC power; getting power and providing power. Getting power involves plugging into the wall. For sites with an external generator, two plugs are required, with a means to switch between the two sources. Incoming power may not be clean, due to storms or the condition of the generator, so some filtering on incoming power is a good idea.

    Providing power means ensuring enough outlets with filtered power, switching between sources of power, so the end user equipment does not know and providing power AC power from a battery and UPS or Inverter, to cover periods when there is no other source of AC power.

DC Load Tester

Questions are often asked about DC power. For example, how long will battery A last? Will the power supply really handle 10A? The tool for answering these questions is a DC load tester. The load tester applies a load to the battery or power supply, simulating a radio or other device connected, drawing a specific amp rating.

There are suggestions for load testers, such as car headlights and large resistors, but these solutions have some drawbacks such as heat, how to protect them, the current changes over time and how to select the amp value you want to test. In an e-mail list, Jeff Steinkamp (N7YG) recently posted a design that he prepared, for a solid state load tester: "http://home.earthlink.net/~n7yg/el-1200.htm". (That web site still exists but the document is no longer available. Ed.)

The load tester functionality could be expanded with a few additional components.

  1. Variable Duty Cycle Controller
    Using a timer to turn the load tester on and off, a battery could be tested to see how long it will last with a 10 amp load and a 40% duty cycle (talk 20 seconds, listen 40 seconds).
  2. Low Voltage Cut Off Sensor
    Using a reference voltage (you need power to run fans anyway), a low voltage cutout could be set up, so when testing a battery, the battery cannot be drained below the reference voltage.
  3. Discharge Duration Timer
    With the variable rate duty cycle to turn the load tester on and off, plus the voltage sensor to turn the system off once the battery reached its low voltage limit, adding a simple clock would tell how long the battery was tested. The clock would start at 0 when the test started and would stop counting (but not clear the display) when the low voltage cut off stopped the test. Now it is possible to know how long the battery would last with the 10 A load and 40% duty cycle.
  4. Charger Control
    Most batteries have a charger and it would be nice to know how long it takes for the battery to charge from its discharged state. Adding a switch for AC power, the battery charger can be turned on and off by the load tester. When the test starts, the AC power to the charger is cut off. The battery drains during the test, but when the low voltage cut off is reached, the AC is turned back on and the load test is terminated, so the battery will be recharged.

With another voltage reference, set to the value for a fully charged battery, then it is possible to know when the battery is fully charged. Add another duration timer, that starts at 0 when the AC power is turned on for the charger and which stops (but not clear the display) when the battery full charge reference is reached. Now it is possible to automate the test of the battery, including draining the battery under load and then charging it back up.