One of the least confusing purchases you can make in amateur radio has lately become one of the most complicated, namely the one of buying a power supply. Years ago, it was a simple decision, because choices were limited. You could only buy a linear power supply as that was all that was available. With the advent of computers, a second type of power supply was born, namely the switching type. So now, there are two basic types of power supplies to choose from. Which one should you choose, and which one is best? Let’s try to answer these questions one step at a time…………..
Why do you need a power supply in your shack at all?
Doesn’t most ham equipment come with its own power supply? The answer is both yes and no. Many manufacturers provide a either a built-in or external power supply for their equipment. This fact doesn’t completely solve your power problem, however. There’s plenty of ham radio accessories that require 12 volts to operate, and unless you want to have a multiple outlet strip cluttered with a dozen “wall-warts”, a common 12 volt D.C. power supply is the way to go. Then too, some radios do not come with a 12 volt DC supply, because they’re designed to be operated primarily in a vehicle. So now you have it. You need to buy a 12 volt D.C. power supply. What kind should you buy, switching or linear? Read on!
Basically, a 12 volt DC power supply has one chore, namely that of converting household current from 120 volts AC to 12 volts DC. The only thing you’ll need to be concerned about, other than whether to buy a linear or switching supply, is how much current you’ll need. The current question (pun intended) is easy to answer. Add up the amount of current (in amperes) drawn by each piece of equipment that you want to power. Then, I suggest you add a safety and “growth” factor of an additional 10 amperes or so. The result is the “continuous” rating of the power supply you want to buy. Most VHF/UHF radios will operate quite nicely on a 20 or 25 amp power supply, while others require greater current. A few require less.
Remember to allow a “safety margin”. Don’t buy a 20 ampere rated power supply to power equipment that draws 20 amperes! Watch for manufacturer’s ratings that contain the words “surge”. It’s the “continuous” rating that’s the most significant factor here. A power supply with a 30 ampere surge rating may only be capable of providing 25 amperes continuously. Overloading a power supply is a
major cause of allowing smoke to escape! Remember Murphy’s Law. In case of an overload, a component will blow out first in order to protect the fuse!
So what’s the difference between a switching and linear power supply? Without going into high level mathematics and detailed lists of advantages and disadvantages of each, let’s provide an oversimplified explanation, as follows. A linear power supply uses a power transformer, which takes line voltage (generally 120 VAC) and “transforms” it to a lower voltage (generally but not always 12 volts AC). The lower voltage is rectified (changed to pulsating DC) and then filtered with relatively large capacitors. More complex analog power supplies also contain regulators, chokes and additional devices to stabilize and control the output voltage so that it will remain constant regardless of load.
A switching power supply takes incoming line voltage (120 VAC), rectifies it, then filters it in order to produce a high voltage DC. From there, a power transistor is generally connected in series with a transformer to switch power on and off at a pre-set frequency, usually between 30 KHz and 150 KHz. From there, the voltage is rectified again and regulated down to 12 VDC. Remember that this is an oversimplified explanation, but basically accurate.
So, what’s the difference between the two types of supplies? There are numerous advantages and disadvantages of both, but here are just a few……………
Analog power supplies are generally more expensive because the transformer involved works at a line frequency of 60 Hz, so they’re larger and heavier than switching power supply transformers that work at higher frequencies.
Did I say they’re more expensive too? Capacitors that are required for linear power supplies are larger and more expensive than filter capacitors in switching supplies, so they’re more expensive too. Linear power supplies are, however, quite reliable, since they have fewer components. Switching power supplies can produce noise, related to the switching frequency. Considerable care must be taken in the design and manufacture of switching power supplies to make certain that “hash” and harmonics of the switching frequency are not radiated to nearby ham radio receivers. Switching power supplies are more complex than linear supplies, because they contain many more components.
The biggest advantage of a switching power supply is the weight factor. They are very powerful for their weight, which is small when compared to a linear supply with comparable specifications. Switching power supplies can be physically smaller than linear ones, because there is no need for a massive transformer that operates at 60 Hz inside the enclosure.
Choose your power supply carefully, according to your needs. There are, of course, other comparisons that can be made between linear and switching power supplies, but space limitations do not allow for any more discussion here. Perhaps we’ll meet at the next club meeting and go in to greater detail then?
