Saturday, March 27, 2021

A Ham Radio Series 23 - Introduction to Power Supplies

One thing that every radio and virtually every accessory needs is power.  In some cases, especially handheld portables, the power is supplied by batteries; in many other cases the power is obtained from the AC power lines.  Typically 120V 60 Hz in the US and 220V 50 Hz in much of the rest of the world.  If you're operating a 100W output power transceiver that may run on 12V and take 20 Amps on peaks a few percent of the operating time, for example, that can be run off a deep cycle 12V battery, like the marine or RV batteries or it can be run on a line-operated power supply.

I want to take a short dive into line-operated power supplies.  The details of building one vary with the requirements, but they're well within the range of things amateur builders can design, build and get running.  For low power supplies, say the 1 to 2 amp supplies that you might be familiar with for little things around your house, there are literally less than 10 parts in the entire thing.  More parts are required for something like that 12V 20 to 30 amp supply, but aren't really much more complicated, conceptually. 

Broadly speaking, power supplies can be divided into two kinds: linear and switching.  Linear supplies use components called voltage regulators which are analog components; that is, the voltages and currents in them and through them are smoothly varying, continuous in time.  These regulators operate like the feedback circuits I've talked about before.  Somewhere inside the regulator is a reference voltage.  The output voltage is compared to this reference continuously, producing an error signal that causes the circuit to turn the voltage up if it's too low and turn it down if it's too high.

Switching regulators, as the name implies, use switching techniques to change the input voltage to the output voltage, so they're on-off circuits more like digital than analog.  They use components called regulators as well, but they work on different principles; a very common technique is Pulse Width Modulation.  Briefly, the switched on/off voltage they're producing gets filtered into clean DC, and if it the voltage runs too low, they keep the pulses on longer (so that they're wider) and if the voltage is too high, they turn the pulses off sooner (making them narrower) reducing the output voltage.   

The advantage of switching regulators is they tend to be more efficient, sometimes reaching efficiencies of 85 to 90% or higher.  The linear regulators are lower efficiency.  That might matter to you for running things when the grid is down and you're trying to stretch every watt you can produce.  The advantage moves over to the linear regulator when you want an electrically "quiet" regulator.  The fast switching voltages in the switching reply produce broad spectrum electromagnetic interference (EMI).  Nevertheless, I can assure you that every system I worked on in military, space and commercial aviation for the 40-ish years I worked in those fields were switching power supplies.  Their drawbacks can be designed around.  After almost 30 years using the same linear supply in my ham station, I switched to a switching supply 15 months ago and haven't heard it on the receiver at all.

The biggest advantage of the linear regulators for the hobbyist and experimenter is that they are incredibly easy to use.  How easy?  How about "just add two parts?"

This example is based on the series of parts called the "seventy eight hundred" series.  The last two digits are the voltage.  Over the years they've been produced (since the late 1970s) they've been produced in 5V (7805) 8V (7808) 12V (7812) 7815 and more.  Furthermore, they're available in different packages and can deliver different currents depending on the package they're in.  They're the 78L series (low power) 78M series (medium power) 78P (high power) series, for example.  There's also a 79XX series for negative voltages. 

The bigger the case, the more power dissipation the part can handle.  The TO-3 (TO is from Transistor Outline) is the highest power (78P05, for example) while the TO-92 would be the lowest power 78L05.

What makes these so easy to use is they're extremely rugged; they've been designed to handle anything that could happen to the regulator.  Too much current?  It'll shut down, and then turn on again periodically to see if the condition cleared.  You could short the output to ground and it will just stay shut down until the short is removed.  If you over heat the part by drawing too much current, it just shuts down.  If you see the power turning on and off, that's the first thing to suspect. 

Practically, the input voltage will be coming from a transformer that drops the wall voltage down to "a few" volts above the desired regulator output voltage, rectified with a diode bridge and filtered with a large capacitor (although not as large as if there were no 78XX regulator there). It never hurts to read the datasheet, and the sheets for these, along with other design notes and help, are extremely widespread on the 'net. The power dissipation in the regulator is the voltage drop across the regulator times the current it's delivering, so say you're putting 8V into a 7805 putting out 5V at 1 Amp; that dissipation is 3W (8V-5V)*1A. 

That's not all in the world of linear voltage regulators; there are adjustable regulators - and ways to make the 78XX series adjustable, too.  There are higher power regulators, and ways to make higher power with lower power regulators.  There are regulators optimized for lower voltage drops across the part, Low Drop Out or LDO regulators, so that instead of putting 8V into a 5V regulator you'd use 6V or 5.5V (depending on the part).  

It's literally an entire world of circuit design open to anyone interested in making something useful.


  1. I seem to recall that a linear regulator can be hooked up as an audio amplifier, but my books are not here.

    1. I've never seen that, but it would be interesting to see if someone figured out how to do it.

      Amplifying a varying signal versus keeping the output constant under all conditions seem to be opposite concepts.

    2. I've seen them used for similar things by floating the ground pin, or using the "reference" pin on the adjustable regulators. I'm not sure how well it works, but I do remember seeing it.

  2. A good intro SiG. I have looked at building a linear power supply but bought instead. I do know enough to trouble shoot them.

    As to switching power supplies, I have looked at the design of those. The design can get complicated for a mains to dc system. I was looking at building a universal supply for old tube rigs like the Heathkits and some others; one would want to try to make it "universal" with adjustments for all of the needed inputs (plate, screen, grid, filament,?). I was also looking at doing a DC-DC for operating the same off of 12 volt batteries. Other activities overtook those endeavors.

    1. In my case the "other activities" might take over because that's a pretty major design undertaking. Everything is adjustable? How? Pots? Digipots so its programmable? My mind kinda boggles at that.

      I have a vintage Collins KWM-2, and it's powered by a Heathkit supply, just rewired for the different connector Collins used.

      A 12VDC to 120VAC supply (usually called an inverter) is a pretty serious design, and also pretty difficult to do better than what you can just go buy. As the comment below this, from Clayton W at 1745 on March 29 says, it's likely there are going to be lethal levels of voltage in there.

    2. I was looking at separate switching supplies for each of the voltages and using a microcontroller like an Arduino to set each voltage. Yeah, it gets complex but it might be doable.

      I saw in QST or QEX where a gentleman developed a 3000+ volt switching supply for a tube power amplifier. He had a lot of problems with overshoot on some of the driving waveforms that switched the output transistors that drove the stepup transformer. It is where I let this project drop. Who knows I might take it up again.

  3. Lines powered supplies can be very dangerous and I caution any hobbyist to do their research before attempting to build one. It's not hard to get to 400 VDC in a design and transients can get much higher. There are a lot of safety factors that need to be considered.

    1. I would never suggest handling the 120 VAC out of the wall isn't dangerous, but you've got a mental circuit model there which doesn't apply to any linear regulator circuit like those 7800 series parts I was talking about.

      Imagine a transformer on the 120VAC that drops it to 8VAC before it's rectified and put into the three pin regulator to produce 5V DC. Exactly where does 400 VDC come from? This is like the power supply for the USB slots on your computer. Repeat for voltage regulators up to 24 VDC. Where does anything more dangerous than the 120 VAC input come from?

      I can see 400 VDC in a switching power supply that has to work on 120 and 240 VAC. It's pretty typical for them to step up the voltage to something higher than either input voltage before they step down to the final voltage and regulate it, but that's not what this article is talking about.

    2. For low power supplies that is certainly true. There are some regulatory concerns once you get past a few tens of watts, however. Much of the world, although notably not the US, has power factor requirements past very low supplies. Most PF corrected designs have an intermediate voltage on the order of 400 volts. That jumped out at me when I read the opening paragraph and it described a ~250W supply.

    3. Interesting. There are some regulatory concerns once you get past a few tens of watts, however. Much of the world, although notably not the US, has power factor requirements past very low supplies.

      Does that apply to people who are making things for their own use? I don't follow any of the European ham radio magazines, but the last time I read some British (RSGB) articles they were allowed to make their own radios and other accessories. Same for the German hams. Is that no longer true?

      PF corrected designs having a step up/step down architecture might make a little more sense with their inputs being 220V in most of the EU. Is this an EU regulation? Stepping up 220 to a 400V supply to step down to a regulated 3.3 or 5V delivering less than 20 Watts seems like overkill.

      That 12V 20A supply I was mentioning is a transformer, stepping 120 down to around 30V, a shunt regulator (another 45 year old part, the LM723) and four series pass transistors to bear the load. There is no voltage in it higher than the line.

      The same basic design is still being sold.