Power supplies
| Power supplies | |
|---|---|
| Standard output | 13.8 V DC |
| Why 13.8 V | It matches a car's electrical system with the engine running, which mobile and HF radios are designed around |
| Main types | Linear and switching (switch-mode) |
| Typical size for a 100 W HF radio | 25–30 A continuous |
| Connections | |
| Common connector | Anderson Powerpole |
| Powerpole convention | Red right, tongue top, viewed from the contact end |
| Protection | Fuses, current limiting, overvoltage protection |
A power supply in amateur radio usually means a DC supply that converts household AC power to the 13.8 volts that most transceivers need. HF transceivers, VHF and UHF mobile radios, and many accessories are built to run from a car's electrical system, so a base station runs them from a supply that imitates one. Choosing a supply comes down to three things: enough current, a clean output that doesn't add noise to the receiver, and safe wiring between the supply and the radio.
Why 13.8 volts
A 12-volt lead-acid car battery sits at about 12.6 volts at rest and around 13.8 to 14.4 volts while the engine is charging it. Radios meant for vehicles are designed for that range, with 13.8 volts as the nominal figure. Most are specified to work within about 15 percent of it, but transmitter output and audio quality can suffer as the voltage falls, and some radios shut down or reset when it sags under transmit current.
Linear and switching supplies
A linear supply uses a heavy transformer to step the AC voltage down, rectifiers and large capacitors to turn it into rough DC, and transistors that act as a variable resistance to hold the output steady. Linear supplies are simple, durable and electrically quiet, but they are inefficient, run warm, and a 30-amp model can weigh 20 pounds or more. A good linear supply includes overvoltage protection, often a "crowbar" circuit that shorts the output and blows a fuse if the regulator fails, since a failed regulator could otherwise send the full unregulated voltage into the radio.
A switching supply rectifies the AC first and then switches it on and off tens of thousands of times a second through a small, high-frequency transformer. The result is a supply a fraction of the size and weight of a linear one, and more efficient. The drawback is that the switching can produce RF noise, heard as hash or as steady tones called birdies at regular intervals across the HF bands. Supplies sold for amateur use are filtered to keep this down, and some have a control that shifts the switching frequency to move any remaining noise off the frequency in use. Generic supplies made for computers or LED lighting are often much noisier. Ferrite chokes on the DC and AC leads sometimes help; see Radio frequency interference.
Current ratings
A 100-watt HF transceiver draws about 20 to 25 amps at full output, and a 50-watt VHF/UHF mobile roughly 10 to 15 amps. The radio's manual gives the exact figure.
Read the rating carefully. Many supplies list both a continuous rating and a higher intermittent or surge rating, and the model name often reflects the higher one. A supply sold as "35 amps" may be rated for 25 amps continuous. The continuous figure matters most for FT8, RTTY and other digital modes, which transmit at full power for the whole of each transmission. Several radios can share one supply, provided the total current they might draw at once stays within its rating.
Connectors and distribution
The Anderson Powerpole has become the standard DC connector in amateur radio, adopted by many ARES and RACES groups so that any radio can plug into any supply or battery. The plastic housings are genderless, so any two mate, and the same housing accepts contacts rated at 15, 30 or 45 amps. Contacts are crimped onto the wire with a proper crimping tool. The common convention is "red right, tongue top": looking at the contact end with the small tongue on top, red is on the right. Following it prevents reversed polarity.
A DC distribution panel, a strip of Powerpole sockets each with its own fuse, lets one supply feed several radios and accessories.
Fusing and voltage drop
A fuse protects the wire as much as the radio, so its rating should suit both the load and the wire it feeds, and it should sit close to the source. Most radios have fuses in their DC cords. Replace a blown fuse only with one of the same rating, and find out why it blew.
Wire has resistance, and at 20 amps even a little matters. By Ohm's law, the voltage lost in a cable is the current times the resistance. A 10-foot run of 12-gauge wire means 20 feet of conductor out and back, about 0.03 ohm, which drops roughly 0.6 volts at 20 amps. Keep DC leads short, use heavy wire (10 or 12 gauge for an HF radio over more than a few feet), and place the supply near the radio. Some supplies offer remote sensing, which measures the voltage at the far end of the leads and compensates.
The supply should plug into a properly grounded outlet; see Grounding and lightning protection.