Get To Know Voltage Regulators with a 723

“Chapter 5; Horowitz and Hill”. University students of all subjects will each have their standard texts of which everyone will own a copy. It will be so familiar to them as to be referred to by its author as a shorthand, and depending on the subject and the tome in question it will be either universally loathed or held onto and treasured as a lifetime work of reference.

For electronic engineers the work that most exemplifies this is [Paul Horowitz] and [Winfield Hill]’s The Art Of Electronics. It definitely falls into the latter category of course books, being both a mine of information and presented in an extremely accessible style. It’s now available in its third edition, but the copy in front of me is a first edition printed some time in the mid 1980s.

The Art of Electronics, on regulators.
The Art of Electronics, on regulators.

Chapter 5 probably made most of an impression on the late-teenage me, because it explains voltage regulation and power supplies both linear and switching. Though there is nothing spectacularly challenging about a power supply from the perspective of experience, having them explained as a nineteen-year-old by a book that made sense because it told you all the stuff you needed to know rather than just what a school exam syllabus demanded you should know was a revelation.

On the first page of my Art of Electronics chapter 5, they dive straight in to the μA723 linear voltage regulator. This is pretty old; a design from the legendary [Bob Widlar], master of analogue integrated circuits, which first made it to market in 1967. [Horowitz] and [Hill] say “Although you might not choose it for a new design nowadays, it is worth looking at in some detail, since more recent regulators work on the same principles“. It was 13 years old when they wrote that sentence and now it is nearly 50 years old, yet judging by the fact that Texas Instruments still lists it as an active product without any of those ominous warnings about end-of-life it seems plenty of designers have not heeded those words.

So why is a 50-year-old regulator chip still an active product? There is a huge range of better regulators, probably cheaper and more efficient regulators that make its 14-pin DIP seem very dated indeed. The answer is that it’s an incredibly useful part because it does not present you with a regulator as such, instead it’s a kit of all the parts required to make a regulator of almost any description. Thus it is both an astonishingly versatile device for a designer and the ideal platform for anyone wanting to learn about or experiment with a regulator.

Block diagram of a 723 regulator. From the TI data sheet.
Block diagram of a 723 regulator. From the TI data sheet.

Running through the package contents, there is a temperature compensated voltage reference, an error amplifier, an output transistor, and a current sense transistor, all presented almost as separate components on a blank slate for the designer. It can be configured as a negative or a positive voltage series regulator, it can use an external transistor to boost its 150mA rated current, it can incorporate a current limiter, it can be a shunt regulator, and there is even a circuit for its use as a switching regulator in the data sheet. To fully understand the 723 then is to fully understand low voltage linear regulators.

The datasheet's 723 suply with external pass transistor and current limit.
The datasheet’s 723 supply with external pass transistor and current limit.

On my bench there is a low voltage supply that is my go-to battery replacement when I am prototyping. It’s the supply I made using a 723 after reading the Art of Electronics power supply chapter all those years ago, and it is not unlike the circuit shown in figure 4 of the device data sheet.

A standard transformer, bridge rectifier and large capacitor produces an unregulated supply of about 14 volts. This is taken through a power transistor whose base is driven by the 723 output, and thence through a current sense resistor to the PSU output and the current sense line. A potentiometer lies across the output, whose wiper goes to the negative input of the error amplifier while the positive input comes from the reference. This feedback from the potentiometer sets the output voltage, which ranges from around 2 volts to just over 12 volts.

The 723 PSU, inside and out. Suddenly I'm a kid again.
The 723 PSU, inside and out. Suddenly I’m a kid again.

The magic part to me as a 19-year-old was the point at which I understood the current limiter. The sense resistor is a 1 ohm wire-wound component connected across the base and emitter of the current sense transistor, so when the current through the resistor reaches 600mA the voltage across it becomes enough to turn on the transistor. This in turn pulls down the base of the output transistor and limits the voltage to keep the current at the 600mA limit. Simple and straightforward as a grown-up, but as a rookie this was one of those lightbulb moments in which everything comes together and makes sense.

The supply itself is rather tatty, with my hand-written calibration on sticky luggage labels stuck to its front panel. Inside the box it’s a bit messy, with a mixture of second-hand components and the 723 on a piece of stripboard. Decades later I’d make a far better job of it, but it has served me well for all that time and will no doubt continue to do so.

I don’t know whether other engineers have a favorite integrated circuit, or whether I’m alone in coming to the realization that I’d nominate the 723 as mine. It might seem an odd choice, given that it’s not a component I’ve used many times in my professional career. But for me the elegance of a circuit that provides such versatile access to so many functions is attractive, and that it does so with such a simple but clever design is ample reason to like it even if I rarely need to design a linear regulator. Perhaps I should drop a few of them on my next order, and explore some of its other configurations.

Who knows, perhaps you should too.

40 thoughts on “Get To Know Voltage Regulators with a 723

  1. You are not really a hacker unless you have a copy of Horowitz and Hill (I have two, the first edition nearly falling apart). This is one of those books where the authors are taking you under their wing and doing their best to teach you all they know and love.

    Get to know the 723 and the 555 and you will have done yourself a favor.

    1. I don’t have a copy, too expensive at the very least.

      It may have been around since 1980, but I don’t recall seeing reference to it until about 1995, suddenly people talking about it like “anyone in the know” would know about it. If there were earlier references in hobby circles, it was minimal, yet then at some point everyone jumped on the bandwagon. I suspect the internet had something to do with the propagation, suddenly all those former students talking about it.

      When I was a kid, we had other books, including databooks and collections of application notes from the manufacturers, which often were the source for other books. There was also (still is) the ARRL Handbook, vital for those interested in radio, till useful for the electronic hobbyist, and cheap enough that one could “waste” the radio sections if you weren’t interested And we had Don Lancaster cookbooks. “You can’t be a ‘hacker’ without these books”.


      1. I have edition 1 and 2. I have been tempted to buy 3, but probably won’t.

        I just went to Abebooks and typed in the ISBN 0-521-37095-7 (from my 2nd edition). There are plenty of used copies for $20 or so shipped. So you have no excuse, unless you just gotta have the 3rd edition.

      2. Oh I had other books as well. The Art of Electronics is not the most detailed in topics. It doesn’t give a complete picture of everything. It certainly isn’t the only thing you should have either.

        But it is by far the most readable and understandable electronics book I have seen bar none. The price is high, but the contents are priceless, and I’d recommend it at an even higher cost.

      3. Ah – H&H – “The Art of Electronics” .

        I was into electronics as a teenager and that led to enrolling in electrical engineering. Second year was a slog of mostly theoretical stuff and that goddamn second order differential equation. Important, I suppose, but nothing that I felt was readily applicable. A friend in physics pointed out that the physics dept had a one-semester electronics course that had you whipping up prototypes by the end of it. Their text? “The Art of Electronics”. Take transistors. This book runs you through the semiconductor basics and the various configurations and their parameters, then BANG they give you a set of quick recipes that will meet 90% of all transistor applications.

        I bought that book, it’s been my go-to throughout my electronic career.

        Re the 723 – I was seduced early by the three-terminal linear regulators, so I didn’t mess much with the 723. But it’s definitely worth studying.

      4. ARRL Handbook and Don Lancaster cookbooks: I’ll absolutely second these. Ditto the Forest Mimms notebooks. Great ideas in all of these places. Art of Electronics is a phenomenal text too.

        There was a series of “Electronic Circuits” books that were just random analog circuits one after the other. These aren’t worth anything until you build them, but if you do you can learn a ton.

        The nice thing about the Internet age is that you can be a hacker without any of these, but also that the state of the hacker art is getting so advanced in places that you’ll want a college textbook too.

        What’s missing from the mini-library above? Anything about DSP, robotics, or computers at all. Bruce Land’s microcontroller lecture videos. :)

    2. Damn myself and who knows how many others aren’t hackers, because we done quite well without this book while using other resources. Might see if I get the book via the inter-library network to give it a look over, as I’m not so set in my ways that I don’t try to learn about other resources.

  2. “For electronic engineers the work that most exemplifies this is [Paul Horowitz] and [Winfield Hill]’s The Art Of Electronics. It definitely falls into the latter category of course books, being both a mine of information and presented in an extremely accessible style”

    Good book. Price-gouging on the eTextbook edition.

  3. The first power supply I built, well after realizing tat ” a big electrolytic” wasn’t good enough for sold state, used a 723. I must have got that t Radio Shack, which maybe impacted on choosing the 723.

    But I do remember when three-terminal regulators arrived, which would have preceded the 723 power supply. So much simpler, but way less versatile. But they were also referred to as “on-card” regulators, cheap and simple to use, put them near the load rather than at the central power supply. Suddenly you could regulate more than in the past, it got better with the three terminal regulators in to-5 or to-92 packages. Suddenly zeners got a lot less prominence in hobby circuits.

    But that’s solid state. The devices became to cheap, and took up so little space, that adding a device complicated circuits, but made overall design simpler.


  4. The fact that this device is denoted ‘µA…’ is indicative of the fact that this part was designed by Bob Widlar while he was at Fairchild Semiconductor in Mountain View, CA before he moved to National Semi (a spin-off of Fairchild).

    My all-time favorite device is, coincidentally, the Fairchild F-8 microprocessor. Fairchild jumped through hoops to get all the functionality needed in a small amount of silicon (things like trashing the accumulator on a ‘jump’ instruction because the accumulator was used to hold one byte of the jump address; things like using a MLSR as a counter, rather than an INCREMENTING binary up-counter–which as everyone knows is the only type allowed by the Bible; things like being the FIRST microprocessor to have a DRAM controller and DRAM refresh on-board…).
    It really was a pleasure to design with it, in spite of having to store the ACC before performing a JMP.

    1. I do find it amusing that mine pictured above is a NatSemi part :)
      I ordered one in case I needed it for photos fo this piece, it’s a TI. Not sure if anyone else is still making it.

      Sadly the F-8 never came my way.

    1. It’s pretty typical of the era. Perfboard construction, re-used parts, roughly constructed metal chasis with a sticker and pen front panel.

      It’s awesome! Way to go Jenny!

      At least it has a sturdy case. Look how many people today consider a project done when it is still just a PCB with no holes to even allow one to mount it to a case, or… don’t even get past the solderless breadboard!

      Re-used parts? Sweet! Today we can get most anything cheap off the net but working with what you have was an art to be admired!

    2. Sounds like a call for a photo gallery. Stuff we built as kids that we still use.

      Know my bench has a few things. Besides the power supply many of us built, probably most notable on my bench is a TTL signal generator made up with the usual inverter loop as an oscillator feeding some oddball CMOS counters to divide down to oddball waveforms. It’s in a wonderful grey hammertone steel slope front case I got from RadioShack that I wish I could get a dozen more of…

      1. Know how that is. Got good at taking stuff apart for parts and identifying useful bits to keep. Although I tend to update the still usefull stuff I built and still use, and replace worn out/bad components. Still have two PSUs with 723s, accually included one of them in a new Dual PSU i built last year with a switched side and analog side. Replaced and beefed up the old caps, new multiturn potentiometers and digital displays.

        1. Actually, kinda sad for modern larval stage electronics hackers, the most frequently discarded 5-15 year old electronic equipment has very little discrete stuff to scavenge, or even general purpose ICs, these days.

  5. I remember seeing a circuit that had a 723 acting as a self-ovening reference where the device would heat up and maintain a constant temperature to stabilise the reference source. Can’t remember where though. My first (1986) home built supply used two 723s. I still use it daily.

    1. That sounds familiar. Back when “Electronics” magazine was good, they had monthly columns, “Engineer’s notebook” and “Designer’s casebook” where readers sent in short buts, getting fifty dollars if published.

      The 723 circuit you describe sounds familiar, and certainly the sort of thing they published there. They once did publish a softcover book that collected the bits, but that was only a partial collection. And I’m not going to dig out the file box where I collected the pages of those columns which I pulled out if the magazine.


  6. Dozens of years ago I skipped over the uA723 for my bench supply and went with an LM317K. Still my primary benchtop supply. As far as favorite IC’s (you did wonder about that), I thought the LM3909 was a small wonder- it could flash a 1.7V forward drop LED once a second for 3 years on a “D” cell running it down to 1V or less. Clever switched capacitor all bipolar circuit. I also love the utility and hacking potential of the 555 timer.

    1. The LM317 was the next stage of evolution after the 2N3055 plus zener diode guts (*) of my bench supply. Unfortunately the Specs of the LM317 – up to 30V and 1,5A – seemed not to be true. Or was the reason for this the fact that I did not yet understand the concepts of power dissipation and thermal resistance as a 14(?) year old? :-)

      (*) with an incandescent lamp as current source for the Z-diode and a big potentiometer MLSR cause of the low current gain of the 2N3055 and no short circuit protection a t all. What lead to immediate destruction of the transistor by the stored energy of the 4700µF cap.

  7. I mean the following as a complement to you and your work, so please take it in the best sense possible.

    At 19, when I was a young junior engineer looking for a wife, I couldn’t find any Horowitz-and-Hill-reading, LM723-power-supply-designing, geek gals in my area. Don’t get me wrong–I’m very happy now after 30 years with my wife, but I gotta say: some jewel you must be, Jenny.

    Never stop designing, Jenny. Or writing about it.

  8. I only ever used a ‘723 once in a commercial design. Its role was to produce an extremely low noise supply voltage for a low phase noise oscillator.
    The ‘723 was good for low noise supplies for two reasons:
    – The reference is available on a pin, allowing it to be filtered. I used a single pole RC LPF.
    – The reference voltage is quite high, so the noise gain from the reference to the output voltages is low.

    C.F. the three terminal regulators that were available at the time – noisy 1.2V bandgap references that couldn’t be filtered, with a big gain from the reference voltage to the output voltage.

    Many recent LDOs address these issues though.

  9. Wow – Flashback to when I was a teen (40 yrs ago) and was building power supplies with LM723 and 2n2094’s.
    Built on for my fathers CB radio that worked for years (actually still does but CB is long gone.)

  10. I’m looking for a smart charger circuit for 18V NiMH batteries that monitors charge state using only the + and – connections. Whyfor? Charging cheap cordless tools where the original charger is nothing but a wall wart and a thing to plug the battery into.

  11. IIRC, the first/early versions of these chips had a very fragile voltage reference section. If the input voltage rose over 37V or so, the reference died a silent and quite certain death. Some typical transformer/bridge rectifier/capacitor arrangements aimed at achieving the full voltage range of the regulator were almost certain to deliver peak voltages that killed these chips stone dead. As a beginner and unaware of such issues, that was a mystery that drove me to discrete component regulators and later the LM317. That problem is still hinted at in the current generation spec sheets (50mS pulse limit of 50V).

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