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.
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Bench Testing a Switch Mode Drop In Replacement For the LM7805

Throwing a 5V regulator like the LM7805 at our projects can become habit forming, after all they’re dirt cheap and the circuit is about as basic as they come with only two external components, an input and output cap. As this is a good enough solution to most of our 5V circuits we can come into some issues if we aren’t paying attention. Linear regulators can only dissipate so much power in the form of heat before they need a heat sink and/or active cooling. Even if they can produce a cleaner output, in an embedded system, large power losses to heat are less than ideal to say the least.

[Daniel] needed an efficient solution to use in the place of an LM7805, after looking at the drop-in replacement switching solutions available on Adafruit’s website, he headed to DigiKey for a similar and less expensive part. [Daniel] collected some data and found the regulator to be 92% efficient with a 12V input, which is not quite the claimed 97% but a good solution nonetheless.

Switching voltage regulators are nothing new, so don’t even act like we just jumped on this switch-mode bandwagon! But it pays to give a little thought to your power supply. And while you’re in the mood, have an extremely thorough look inside the LM7805.

Retro TO-3 Switching Voltage Regulator

Restoring old gear often means replacing unavailable parts with modern equivalents. [Alex Eisenhut] needed to replace some old TO-3 voltage regulators and decided to make an authentic-looking switching power supply replacement. These three pin metal cans were very common, especially the LM340 5V regulator which was, of course, a linear regulator. Today, you are more likely to see a 7805 in a TO-220 case or something surface mount for a comparable linear regulator.

As you might expect, the board uses surface mount components. [Alex] used Mill Max machine pins to match the original regulator footprint and calls the regulator Ton3y. He plans to cover it up with a 3D printed lid, but it seems a shame to hide the fine PCB work.

In the pictures, you can see that the machine pins are a tight fit. [Alex] used a hammer to lightly tap them into place. Of course, the original TO-3 regulators were linear and would generate a lot of heat. The Ton3y, as you’d expect from a switching power supply, runs cool (according to the scientific measurement made with [Alex]’s pinky finger) and surely has a wider input voltage range and more output current capacity.

We’ve seen replacement switching regulators before, but this one is really a work of art.

Hackaday Prize Entry: A 7805 Replacement

The 7805 voltage regulator is a great device if you want a simple way of bringing a voltage down to 5V. It’s a three-pin, one-component solution that puts out five volts and a lot of heat. Simple, not efficient. For his Hackaday Prize entry, [K.C. Lee] is working on a much more efficient drop-in replacement for the 7805.

Linear regulators like the 7805 are great, but they’re not terribly efficient. Depending on the input voltage you might see 50% efficiency. Going to a switch mode supply, that efficiency shoot up to about 90%.

For his drop-in replacement, [K.C. Lee] is using the LM3485, a switch mode regulator that only needs a few extra parts to turn it into a replacement for the 7805. You will need a cap on the input, but you should already be putting those in your circuit anyway, right?


The 2015 Hackaday Prize is sponsored by:

A Detailed Look at the 7805 Voltage Regulator

We’re quite sure that all hobbyists have used the 7805 voltage regulator at least once in their lives. They are a simple way to regulate 7V+ voltages to the 5V that some of our low power projects need. [Ken Shirriff] wrote an amazingly detailed article about its theory of operation and implementation in the silicon world.

As you may see in the picture above such a regulator is composed of very different elements: transistors, resistors, capacitors and diodes, all of them integrated in the die. [Ken] provides the necessary clues for us to recognize them and then explains how the 7805 can have a stable output even when its temperature changes. This is done by using a bandgap reference in which the difference between transistor base-emitter voltages for high and low current is used to counter the effects of temperature. As some elements looked a bit odd during [Ken]’s reverse engineering process, he finally concluded that what he purchased on Ebay may be a counterfeit (read this Reddit comment for another opinion).

Another switch mode regulator swap for the Raspberry Pi

[Karl Lunt] is working to slim the Raspberry Pi current draw as much as possible. The first step in his journey was to replace the linear voltage regulator with this switch mode version. It’s a step-down voltage regulator circuit with a tiny footprint and a matching price tag (about $10) made by Pololu. It’s small enough to be mounted in the empty space between the LCD ribbon connector and the main processor.

The project was based on the hack we saw at the end of June. But we give much more credit to [Karl] for removing the old part in a safer way. He clipped the two small leads on the bottom of the old part, then used a beefy iron to sufficiently heat the large pad before removing the body of it. With the old part out of the way it’s just a matter of connecting the three wires in the right configuration.

This cut consumption by about 50 mA. He’s hoping to do more by removing the on-board LEDs. His goal is a draw of under 250 mA in order to make it last a reasonable amount of time when running from batteries.

Non-invasive AC power adapter for exercise equipment

We often look at battery-operated hardware and shake our heads at the wastefulness of throwing away disposable batteries. There are some devices that minimize the waste, like those TV remotes that seem to never need new cells. But the C cells that [Quinn Dunki] kept replacing in her elliptical trainer were only lasting about three months at a time. The manufacturer hadn’t cared enough to build a power jack into the machine, so she built her own AC adapter without modifying the stock hardware.

The first thing she did was to patch in a couple of wires between two of the batteries. This let her measure the current consumption, which topped out at around 200mA. This is good news because that’s easily sourced with a cheap linear regulator. Out of the junk box came a 12V/1A wall wart transformer, which just leaves the need for a fuse and some capacitors to finish out a voltage regulator circuit.

Since [Quinn] didn’t want to permanently alter the exerciser, she came up with a way that it could take the same physical space as the batteries. Two long stand-offs are used as prongs to interface the spring terminals in the battery compartment. They attach to a piece of protoboard which hosts the rest of the circuitry. Now she just needs to remember to unplug this from the wall after each session and she’ll be in business.