Ham Radio Power Supplies

Astron 13.8 Volt Power Supply. Very stable and very reliable.

There are some Amateur radios today that are sold with the power supply built into them so all you have to do to obtain power is just plug the power cord into a wall socket. With the advent of solid state radios this has become less true then it was in the vacuum tube days. The reason this is true is because the modern radio can be built to run directly off the 12 volt power available in modern motor vehicles so to use them as a base station you can use a 120 VAC to 12 VDC power supply. One power supply can be used to run more then one radio.

Vacuum Tube Power Supply

In the days of vacuum tube radios we usually had to have a variety of voltages in a radio for it to operate. There was a 6 or 12 volt AC or DC supply to run the tube filaments. If it was a transceiver or transmitter there was a lower plus voltage needed to run the plate voltage in the receiver and lower amplifier states of the transmitter, this voltage would usually be around 250 volts to 300 volts DC. Transmitters and transceivers transmitter sections sometimes used a negative 100 to 200 volt bias supply and a 750 to 1,000 volt positive voltage for the final amplifier. These voltages varied from unit to unit so each piece of equipment had to have its own power supply to convert the 120 VAC home current to the necessary voltages. If they were used mobile then a power supply was necessary to convert the vehicle’s DC power to the voltages necessary to run the unit.

Now most modern ham transceivers are designed to run directly off the vehicle’s 12 volt system. Building an AC power supply inside the unit would not only increase its size but also its weight. All that is really needed is to add a 120 AC to 12 DC volts power supply to make a base radio out of it.

We call them 12 volt power supplies but they are really 13.8 volt power supplies. They will usually operate just fine with a voltage variation of 10 to 15 volts but I do not recommend pushing the limits.

We usually get real fussy about the voltage remaining the same and not changing at all between receive and transmit. But once you get use to how much a vehicle’s voltage can vary between when the engine is running and when it is off you will start loosing some of that paranoia. While I was working in commercial two way radio there were Motorola power supplies that would run at 15 volts when in receive and drop to 13.8 volts during transmit. The first few times I saw this I thought there was something wrong with the power supplies but finally I learned that it was just the characteristics of that power supply.

When choosing a power supply determine how much current the radio uses during transmit and be sure the power supply can deliver that current for an extended amount of time. If your power drain on an FM transmitter is 3 amperes (that is about a 30 watt transmitter [13.8 X 3 = 41.4 X .7 = 28.9 watts]) then a 5 ampere or higher power supply should be used. The power supply for a CW 30 watt output transmitter would be about the same as that of an FM transmitter.

AM and SSB transmitters with 30 watt output power need to be higher. An AM transmitter running 30 watts with no modulation can run as high as 45 watts on peak modulation. Power is needed not only for the lower RF stages but also for the audio portion which is far less efficient so peak current would be about 8 amperes so a 10 ampere power supply would be the minimum necessary for a 30 watt AM transmitter.

A 30 watt PEP SSB transmitter will run about 15 watts average power so it would draw about 3 amperes while transmitting with a peak of about 6 amperes. The power supply to run that transmitter would need to be about 8 amperes peak and 5 amperes continues duty.

These figures are based on a class C amplifier running at about 70 to 80 percent efficiency, class B amplifier running at about 50 percent efficiency and a class A amplifier running at about 25 percent efficiency then increasing the current by about 20 percent so the power supply is not running on the ragged edge. Just remember you can always use a power supply with a higher current ratting then necessary. These are minimum standards.

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