Sorting Out Linear And Switching Power Supplies.

Two of a kind?

Some radio amateurs, especially those who are new to the hobby, may not even know there are two main types of power supplies. They could be forgiven for that. When you need a power supply, you just jump online, pick out that one suits your needs and budget, and click. No one really thinks about what goes on in the guts of a power supply. Juice is juice, right? Well, sort of. There are huge differences between linear and switching power supplies even if the final product is the same. linear vs. switching power supply

Old School. linear vs. switching power supply

Linear power supplies are one of the grand elders of ham radio. They’ve been around as long as radio itself. The principle of operation is fairly simple. Household AC voltage is fed to a transformer and dropped down to 12-15 volts. The low voltage AC goes through a rectifier which changes it to DC. That raw DC still has some residual AC in it, so it must be cleaned up. To solve that issue, the power goes through a capacitor to filter out any leftover AC “ripple”. After that, it’s ready for your radio.

In the early days of radio, rectifiers were a series of vacuum tubes that themselves gulped a lot of power. Later models had solid state diodes, which were much smaller and more efficient. Today, diodes are still used but manufacturers use components with several diodes mounted into one integrated rectifier package (these devices are sometimes referred to as “bridges”). The diagram below shows the basic parts of a linear power supply. Some better quality linear power supplies have additional regulation & control circuitry, as well as chokes for filtering. You’ll also see linear power supplies with variable outputs, multiple outputs, built-in meters, and other handy add ons. Regardless of the “bells and whistles,” all linear power supplies perform the same basic function.

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Used linear power supplies are very easy to find because they’ve been around so long. New models are still available but they will typically cost more than equivalent switching power supplies, and they’re also large and heavy. A linear power supply may be a good choice if you have a fixed station where space/weight is not a big concern or you want to reduce RF interference. linear vs. switching power supply

A new breed. linear vs. switching power supply

switching power supply block diagram

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Switching power supplies are a relative newcomer to ham radio. In a switching power supply, AC input is immediately converted to DC, but it is not stepped down to a lower voltage right away. The high voltage DC is fed into a switching transistor. The switch pulses or “switches” the DC on and off at a very high frequency, effectively making it AC again. While the AC in your house is 60 Hz, the switch cycles much faster, usually between 30kHz and 150 kHz. After that, it’s basically a traditional linear power supply: The high frequency AC goes through a transformer to become low voltage AC, then it’s rectified, filtered, and sent to your radio.

The obvious question is, why bother to go through the AC-DC conversion process twice? There’s a one-word answer: Physics. High frequency voltage coming out of the switch requires a much smaller transformer. This is why switching power supplies are less expensive, more efficient, smaller, and lighter than their elders. And like old school linear power supplies, they can also include extra features. On the down side, switching power supplies have the potential to generate RF interference, especially on the HF bands. Switchers may be a good pick if you are on a budget or have space/weight limitations.

So which one should you have? linear vs switching

Most hams today use switching power supplies and are very satisfied, but don’t let popularity alone decide for you. Many old timers swear by tried-and-true linear power supplies, and the belief is not without merit. After all, linear power supplies have been around this long for a reason. I personally use a switching power supply. I’ve used linear versions in the past and they were fine too. For my situation, it’s a moot point anyway because my entire radio station is powered directly from storage batteries charged by solar panels. I rarely pull power from the grid through a power supply to run my gear.

The concerns about RF interference from switching power supplies is probably more hype than reality. Unless you’re using a junky low-end unit or have incorrect grounding, it’s not likely to be a problem.

Linear power supply.

Pros:

  • Time-tested design and principle of operation.
  • Very clean output-no RF noise.
  • Widely available on the used market.
  • Generally better build quality…decades-old units are still in service going strong.

Cons:

  • Large & heavy compared to switching equivalents.
  • More expensive.
  • Less efficient.

Switching power supply.

Pros:

  • Inexpensive.
  • Small & light weight compared to linear equivalents.
  • Most popular type used today.
  • More efficient.

Cons.

  • Do your research! There is a lot of cheap junk on the market.
  • Potential for RF interference.

What we learned today.

There is no clear winner in the linear vs switching power supply debate. Like everything, what’s “best” will vary with each individual. Chances are good that you’ll be happy no matter what you ultimately get. Understanding the difference between the two and their respective principles of operation gives you the information you’ll need to make a decision as well as expand your skills as a radio amateur.

14 thoughts on “Sorting Out Linear And Switching Power Supplies.

  1. Randall Krippner

    well, let’s see if this works. I’ve been having problems with commenting and “likes” the last 2 days. I still can’t do a “like” for this. Hmph…

    Nice article Chris. A lot of newcomers to radio and electronics don’t know exactly what the difference is and the advantages and disadvantages.

    I’ve had good luck with both. I can’t remember ever having problems with RFI with either type so I agree that the RFI issue is really overblown. As long as they’re made from a decent design with good components, they should both provide years of service without a problem. Switching PS are generally a lot cheaper. And lighter. I have one transformer supply sitting around here that must weigh 40 lbs for heaven’s sake. A switching PS with the same rating would weigh less than 5 I would think.

    Only issue I’ve ever had with switching PSs is with the rectifying and filtering on the really cheap power supplies, but even that has become rare these days. The last one I bought was a variable DC 5 amp bench supply that was almost ridiculously cheap and even that had a voltage fluctuation of only about 0.005V.

    Reply
    1. Chris Warren Post author

      I agree the RFI problem in switching power supplies is probably overrated. I have noticed though that a disproportionate number of them on the market are cheap crap. Buyer beware!

      Reply
  2. -Jed

    I have wondered what the difference was but never looked it up. Thank you, you made it simple to understand.

    Reply
  3. Mike Hohmann

    Thanks, Chris… always good information. Our radio club held this month’s meeting ‘on-the-air’ and all went well, w/no disease transmission. “Go Boxes’ were a hot topic… imagine that! My best to all, stay healthy! KE0GZT

    Reply
    1. Chris Warren Post author

      Yes, these are interesting times, aren’t they? Recent events have caused an increase of traffic to this website, probably by people who should have prepared themselves long ago and are now playing catch-up.

      Reply
  4. ethrane

    Another big difference between the two is behavior under short circuit conditions. A switching supply will generally limit the current through a short and some will actually turn off. PC power supplies are commonly built that way. With a linear, a short will turn your power supply into a 12v welding rig. There are generally no short protections other than the line fuse which are slow to blow.

    The other consideration are the filtering capacitors that if not equipped with bleeder resistors can store a fair amount of charge for a while. A couple farads at 15 volts won’t kill, but it can burn. On the topic of filtering electrolytics, if the power supply has not been used in a long time (measured in years, like you picked it up at a hamfest or such) you may want to first power it up on a variac. With time electrolytic capacitors will lose their dielectric layer. They can be reconstituted by slowly applying increasing levels of voltage in a current limiting fashion. If you just flip the switch, the cap can explode. Not a life threatening event, but it has to be replaced.

    Great article! My first real home brew ham project was building a linear back in the day. Good times.

    Reply
    1. Chris Warren Post author

      In high school electronics shop class we built a “wall wart” power supply which was basically four diodes in a bridge configuration with a small transformer and a filter cap. Not very high tech even by 1980’s standards, but it did teach a lot of fundamentals that I still use today.

      Reply
      1. randall krippner

        Something similar to that simple power supply, although with a bit better filtering and a fuse, was what Atari and Commodore used back in the early 80s for some of their computers, only with the case filled with epoxy so it a) over heated and failed, and b) couldn’t be repaired. I was in a computer club back then and we had huge problems with those failing. I think at least a third of them failed. And they were very expensive. I sawed one open on a bandsaw (that was a mess) to see what was in it and we reverse-engineered it. Was a very simple item with all parts available off the shelf at the local Radio Shack. I think it cost something like $20 to make our own and the companies were charging at least twice that. I made a few bucks building ’em for people who were uncomfortable with rolling their own.

        Reply
        1. Chris Warren Post author

          Power supplies at their heart are fairly simple devices. My first computer was a VIC-20 and its power supply wasn’t any different than any other. All we had to do was find the proper connector, or recycle the old one. The manufacturers got even more greedy…today many devices require “smart” power supplies. There is a chipset on the supply that talks to the device, and if they don’t agree, you get no juice! Oh it was so much simpler back in the day…

          Reply
          1. randall krippner

            A lot of people are trying to fight back over the efforts of manufacturers to lock consumers into using their repair services, parts and supplies. The EU is trying to mandate a single, universal charger for all cell phones to eliminate the need to constantly buy new chargers whenever you get a new phone, for example. And there is a pretty active “right to repair” movement going on in the US right now that might have a chance to succeed.

            But when it comes down to it we’re becoming more and more dependent on the original manufacturers because modern equipment is loaded with custom made ICs, custom software, etc. all of it locked down with patent and copyright law. When the equipment goes out of production and the supply of repair parts runs out, well, that’s pretty much it for that piece of equipment. I have radios on the shelf that are older than I am and work just fine because I can still get parts for them. But I doubt if that’s going to be true for the stuff being produced today.

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