App note: High-frequency automotive power supplies


Switching power supply used in automotive electronics app note from Maxim Integrated. Link here (PDF)

The combination of high switching frequency and high-voltage capability is difficult to achieve in IC design. You can, however, design an automotive power supply that operates with high frequency if you protect it from temporary high-voltage conditions. High-frequency operation is becoming important as more and more electronic functions are integrated into the modern automobile. This article discusses several ways to protect low-voltage electronic circuits from the harsh effects of the automotive electrical environment. Also included are the results of laboratory tests for noise immunity.

Modifying a computer ATX power supply for higher output voltage


Kerry Wong wrote a post on his blog showing how he modified the 12V rail of a TL494 based ATX power supply to 14.6V for 4S LiFePO4 battery charging:

To charge the 110Ah battery bank I built, I need a power supply that can provide at least 10A at 14.6V. Since I have many old ATX power supplies lying around and the 12V rails of these power supplies are more than capable of providing 10A, I decided to modify one such power supply for using as a 4S LiFePO4 battery charger.

More details at his blog here.

Check out the video after the break.

0-30V/0-7A power supply unit with LM723 and Arduino volt and ampermeter


Nicu Florica has been working on a power supply project, inspired by Albasete’s power supply unit with LM723 and Arduino volt and ampermeter:

It use an Arduino nano board with i2c LCD1602 display, a active buzzer for indicate shortcircuit case. Also, I put DS18B20 temperature sensor and relay for power a cooler when tenmperature is bigger than a threshold level.
For albasete version, I write psu_reber_ver3ro.ino sketch. In this sketch I put value for albasete case (R1 = 1k put between GND and A1 port, R2 = 39k put between +OUT and A1) and value for threshold (temax) and hysteresis value

See the full post and more details on his blog, Arduinotehniq.

So Where’s My Low Voltage DC Wall Socket?

What are the evocative sounds and smells of your childhood? The sensations that you didn’t notice at the time but which take you back immediately? For me one of them is the slight smell of phenolic resin from an older piece of consumer electronics that has warmed up; it immediately has me sitting cross-legged on our living room carpet, circa 1975.

"Get ready for a life that smells of hot plastic, son!" John Atherton [CC BY-SA 2.0], via Wikimedia Commons.
“Get ready for a life that smells of hot plastic, son!” John Atherton [CC BY-SA 2.0], via Wikimedia Commons.
That phenolic smell has gone from our modern electronics, not only because modern enclosures are made from ABS and other more modern plastics, but because the electronics they contain no longer get so hot. Our LCD TV for instance nowadays uses only 50 watts, while its 1970s CRT predecessor would have used several hundred. Before the 1970s you would not find many household appliances that used less than 100 watts, but if you take stock of modern electrical appliances, few use more than that. Outside the white goods in your kitchen and any electric heaters or hair dryers you may own, your appliances today are low-powered. Even your lighting is rapidly being taken over by LEDs, which are at their heart low-voltage devices.

There are many small technological advancements that have contributed to this change over the decades. Switch-mode power supplies, LCD displays, large-scale integration, class D audio and of course the demise of the thermionic tube, to name but a few. The result is often that the appliance itself runs from a low voltage. Where once you would have had a pile of mains plugs competing for your sockets, now you will have an equivalent pile of wall-wart power supplies. Even those appliances with a mains cord will probably still contain a switch-mode power supply inside.

History Lesson

Mains electricity first appeared in the homes of the very rich at some point near the end of the nineteenth century. Over time it evolved from a multitude of different voltages supplied as AC or DC, to the AC standards we know today. Broadly, near to 120 V at 60 Hz AC in the Americas, near to 230 V at 50 Hz AC in most other places. There are several reasons why high-voltage AC has become the electrical distribution medium of choice, but chief among them are ease of generation and resistance to losses in transmission.

international-mains-plugs-featured_thumbnailThe original use for this high-voltage mains power was in providing bright electric lighting, which must have seemed magical to Victorians accustomed to oil lamps and gas light. Over the years as electrical appliances were invented there evolved the mains wiring and domestic connectors we’re all used to, and thereafter all mains-powered appliances followed those standards.

So here we are, over a hundred years later, with both 21st century power conversion technology, and low power, low voltage appliances, yet we’re still using what are essentially 19th-century power outlets. Excuse me for a minute, I need to hire a man to walk in front of my horseless carriage with a red flag — I’m off to secure some banging tunes for my phonograph.

Given that so many of the devices we use these days have a low power consumption and accept a low voltage, surely it’s time to evolve a standard for low voltage power distribution in the home? Not to supplant the need for mains sockets entirely, after all there are times when 3 KW on tap is quite handy, but to do away with the need for all those power cubes and maybe allow the use of other low voltage sources if you have them. Does it still makes sense to send the power to our houses the 19th century way?

The Old Plugs

XKCD 927, on the nail as always. (CC BY-NC 2.5)
XKCD 927, on the nail as always. (CC BY-NC 2.5)

There will be some among you who will rush to point out that the last thing we need is Yet Another Connector System. And you’d be right, with decades of development time behind them you’d think that the connector industry would already have something for low voltage and medium power in their catalogue. The IEC60309 standard, so-called CEE-form connectors, for example has a variant for 24 V or below. Or there are connectors based on the familiar XLR series that could be pressed into service in this application. You might even be positioning USB C for the role.

The Mennekes 662A, a low-voltage CEE-form plug.
The Mennekes 662A, a low-voltage CEE-form plug.

Sadly there are no candidates that fit the bill perfectly. Those low-voltage CEE-forms for example are bulky, expensive, and difficult to source. And the many variants of XLR already have plenty of uses which shouldn’t be confused with one delivering power, so that’s a non-starter. USB C meanwhile requires active cables, sockets, and devices, sacrificing any pretence of simplicity. Clearly something else is required.

Oddly enough, we do have a couple of established standards for low voltage power sockets. Or maybe I should say de facto standards, because neither of them is laid down for the purpose or indeed is even a good fit for it. They have just evolved through need and availability of outlets, rather than necessarily being a good solution.

The USB A socket is our first existing standard. It’s a data port rather than a power supply, but it can supply 10W of power as 5V at 2A, so it has evolved a separate existence as a power connector. You can find it on wall warts, in wall sockets, on aeroplane seats, trains, cars, and a whole host of other places. And in turn there are a ton of USB-powered accessories for when you aren’t just using it to charge your phone, some of which aren’t useless novelties. It’s even spawned a portable power revolution, with lithium-ion battery packs sporting USB connectors as power distribution points. But 10 W is a paltry amount for more than the kind of portable devices you are used to using it with, and though the connector is fairly reliable, it’s hardly the most convenient.

Is this really the best low voltage connector we can create?
Is this really the best low voltage connector we can create?

Our other existing standard is the car accessory socket. 120 W of power from 12 V at 10 A is a more useful prospect, but the connector itself is something of a disaster. It evolved from the cigarette lighter that used to be standard equipment in cars, so it was never designed as a general purpose connector. The popularity of this connector only stems from there being no alternative way to access in-car power, and with its huge barrel it is hardly convenient. About all that can be said for it is that a car accessory plug has enough space inside it to house some electronics, making some of its appliances almost self-contained.

The New Plug?

So having torn to pieces the inadequate state of existing low voltage connectors, what would I suggest as my solution? Given a blank sheet, how would I distribute low voltage?

First, it’s important to specify voltages and currents, and thus look at the power level. Where this is being written the wiring regulations do not apply to voltages under 50 V, so that sets our upper voltage limit. And while it’s tempting to pick a voltage near the top of that limit it also makes sense to stay with one more likely to be useful without further conversion. So while part of me would go straight for 48 V I’d instead remain with the familiar 12 V.

Looking at our existing 12 V standard, the car accessory socket delivers 120 W, that should be enough for the majority of low voltage lighting and appliances. So 12 V at 10 A per socket is a reasonable compromise even if the car socket isn’t. We thus need to find a more acceptable socket, and given that a maximum of 10 A isn’t a huge current, it shouldn’t be too difficult to imagine one. If I had a plug and socket manufacturer at my disposal and I was prepared to disregard the XKCD cartoon above I’d start with a fairly conventional set of pins not unlike the CEE-form connector above but without the industrial ruggedisation, and incorporate a fuse to protect the appliance cable from fire. Those British BS1363, fused-mains-plug habits die hard. In the socket surround I’d probably also incorporate a 5 V regulator for a USB outlet.

These are best kept out of sight. Smial (Own work) [FAL], via Wikimedia Commons.
These are best kept out of sight. By Smial (Own work) [FAL], via Wikimedia Commons.
A socket is all very well, but no use without a supply and cabling. Off-grid self-sufficiency enthusiasts will no doubt have a lead-acid battery or two and a bank of solar panels, but the majority of us would be more likely to hook our low-voltage socket to a switch-mode PSU running from our AC mains. An ATX PSU might struggle to supply more than one socket taking the full 12 V at 10 A, but would be a good place to start.

We do not however want to replace a load of wall warts with a load of switch mode PSU boxes. The point of the sockets described here is to take away other paraphernalia, not to just deliver power. Clearly some form of cabling is required. At this point there has to be a consideration of topology, will there be one cable per socket, and if not how many sockets will go on one cable? If the former then 16 A insulated flex from our distribution point with a single 10 A fuse per cable would suffice with enough margin to allow for sensible cable runs, while if the latter then something more substantial would be required. There are flexible insulated copper busbar products from the domestic low voltage lighting industry that would suffice, for example.


This has been an epic rant, a personal manifesto if you will, for a future with slightly safer and more convenient power outlets. Somehow I don’t expect to see a low-voltage home distribution system like this emerge any time soon, though I can keep hoping.

Your vision for low-voltage power distribution may differ from mine, for example it may include some smart element which I have eschewed. You might even be a mains-voltage enthusiast. Please let us know in the comments.

Filed under: rants

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.

Filed under: Featured, parts

Not Quite 101 Uses For An ATX Power Supply

The PC power supply has been a standard of the junk box for the last couple of decades, and will probably continue to be for the foreseeable future. A product that is often built to a very high standard and which will give years of faithful service, yet which has a life of only a few years as the PC of which it is a part becomes obsolete. Over the decades it has evolved from the original PC and AT into ATX, supplying an ever-expanding range of voltage rails at increasing power levels. There have been multiple different revisions of the ATX power supply standard over the years, but they all share the same basic form factor.

So a pile of ATX supplies will probably feature in the lives of quite a few readers. Most of them will probably be old and obsolete versions of little use with today’s motherboards, so there they sit. Not small enough to ignore, yet Too Good To Throw Away. We’re going to take a look at them, try to work out what useful parts they contain, and see a few projects using them. Maybe this will provide some inspiration if you’re one of those readers with a pile of them seeking a purpose.

What’s Inside The Box?

A typical ATX PSU schematic using a TL494. forum [Public domain], via Wikimedia Commons.
A typical ATX PSU schematic using a TL494. forum [Public domain], via Wikimedia Commons.
ATX power supplies follow a closely defined standard, so it should come as little surprise that many of them share very similar circuitry inside even if they come from different manufacturers. There are a variety of integrated circuits you’ll find running the show whose data sheets will often give you an entire ATX power supply schematic, but since their circuitries will often be very similar we’re showing you one of the most common.

The TL494 is a switched-mode power supply controller designed to work in a variety of configurations and manufactured by multiple semiconductor companies.

The basic operation of a switch-mode power supply is fairly straightforward, and ATX supplies have very few deviations from the norm. There is a mains rectifier and filter, a pair of high-voltage power transistors that switch the resulting DC at a few tens of kHz into a ferrite-cored transformer whose output is rectified to low voltage DC. The TL494 samples the output voltage and produces the PWM switching signal which is fed to the bases or gates of the power transistors through a drive transformer. There will also be a standby 5V supply using another small transformer, and a “power good” circuit to tell the motherboard that the PSU is ready and to activate the supply on an external input.

A typical ATX PSU interior. Alan Liefting (Own work) [Public domain], via Wikimedia Commons.

A typical ATX PSU interior



A: bridge rectifier

B: input filter capacitors, between B and C – Heatsink for HV transistors

C: transformer, between C and D – Heatsink for low-voltage rectifiers

D: output filter coil

E: output filter capacitors


Alan Liefting [PD], via Wikimedia Commons.

These supplies are slightly unusual in the age of surface-mount components, in that the majority of them you’ll find in a junk box still have through-hole construction. This makes them suitable targets for the electronic scavenger, as parts can more easily be retrieved intact. It’s worth taking a moment to look at the components you’ll find, and suggest some uses for them.

Parts, Parts, Parts

Most obvious when dismantling one of these boxes are the metal case, IEC connector, power switch, and fan. You shouldn’t need an explanation of how these could be reused, if you don’t mind a bit of steel drilling and your project case being obviously that of a PC power supply then they are very robust enclosures. The same goes for the wiring loom of motherboard and disk power connectors, a handy source of medium-sized hook-up wire.

If you look at the components on the PCB, many of them are standard discretes. Yes, we’ve all scavenged a 10K resistor at some point, but outside a few high-voltage capacitors in general they are not much to get excited about. So what is there on that board that’s worth lifting?

Just a selection of ATX power supply magnetics and cores.
Just a selection of ATX power supply magnetics and cores.

One thing that’s in abundance on an ATX PSU board are magnetics. Toroidal chokes and ferrite bobbins used in filters as well as the various ferrite cored transformers. The transformers are wound for the specific purpose so unless you have the patience to rewind them they may be of little use, but the chokes have more application. These are not exotic RF ferrites but more utilitarian iron-dust cores, though they still can find plenty of use wherever a choke is required. I’ve even used them as cores for co-ax baluns, when their purpose is simply to stop RF leaking down the feeder their poor RF performance is an asset. It’s also worth noting from an RF perspective that these chokes are also a handy source of plenty of heavy-gauge enamelled copper wire for your other inductors.

The semiconductors in an ATX PSU include some specialist components, but there are still alternative applications for them. On the high voltage side are a selection of high-voltage diodes and those switching transistors, all of which are a fertile source of parts if you are hacking together high voltage inverters. On the low voltage side apart from the TL494 or other controller chip you’ll find some high current rectifiers and more than one three-terminal 78XX series regulator if you are lucky, as well as in many cases a TL431 adjustable voltage reference. You may also find the various heatsinks to be useful in other projects.

Use It, Don’t Break It!

As you can see, an ATX PSU can yield some useful components. But since there is an almost limitless supply of them it’s not worth breaking one unless you need the parts, so what can you do with an intact one?

A bench PSU project we covered back in 2010.
A rather nicely built bench PSU project we covered back in 2010.

The answer is pretty simple: how about using it as a bench power supply? These supplies are not electrically the quietest or best regulated in the world, but they do have the advantage of providing several useful voltage rails at significant current levels. There is a small modification required to use one in this way, one of the lines is an enable line that is held high. Pull pin 16 low (usually a green wire) and the supply will start up. There are numerous projects on showing how others have done this, and a quick search of OSH Park will yield a range of breakout PCBs like this one.

If fixed voltages aren’t enough, there have been many ATX bench PSU projects like the one pictured fitted with LM317 variable regulators on their 12V lines to provide an adjustable output. That is not the only way that this can be achieved though, the TL494 can be readily made into a variable regulator with a simple modification. The standard warnings and disclaimers apply with respect to the dangers of working on mains and high voltage equipment if you follow that route.

Of course, using a power supply as a power supply is all very useful, but it’s hardly groundbreaking even if it does sometimes involve a bit of hardware hacking. How about other uses for one? One area for which a supply capable of producing large currents might be suited for example is welding. It’s important to point out that by welding we do not mean the kind of welding you’d make ships or even cars out of, but that’s not the only place you’ll find a welder (This spot welder using just the case of an ATX supply is a fine project, but doesn’t quite count in this context). Last year for example we covered an ATX supply being used with a graphite electrode to weld thermocouples, providing a significant savings over commercial alternatives. And the metalworking potential of an ATX supply does not end there, you’ll find people using them for resistance soldering in the model making community.

So, you’ve still got that pile of metal bricks under the bench from all the old PCs that have come your way, but with luck after reading this you’ll have a bit of inspiration that may enable you to do something with them. Whatever you make be sure to share it with us on, and don’t forget to send us the link!

Filed under: Hackaday Columns, hardware, parts, Skills

App note: Method to reduce the output ripple & noise of power supplies


Another app note from Aimtec on power supplies and how to minimize their output noises. Link here

The switching power supplies have the fundamental advantage of high efficiency i.e. low power dissipation when compared to linear voltage regulation. However, there exists an important consideration concerning the presence of ripple and noise at their outputs. If the ripple and noise are left unfiltered their levels may be sufficiently high to adversely affect other devices connected to the same power supply. Fortunately there exists methods to cost effectively reduce the impact of ripple and noise.

Low voltage power supply unit


Dilshan Jayakody writes:

In this post we introduce simple and flexible, regulated low voltage power supply unit. This power supply has provision for 4 outputs such as 1.5V, 1.8V, 2.5V and 3.3V. We mainly build this low voltage power supply unit to test (and power-up) low voltage MCUs, CPLDs and radio receivers. For this power supply we choose 1.8V, 2.5V and 3.3V to get it compatible with most of the LVTTL/LVCMOS devices. Other than that, we include 1.5V because there are several analog ICs are available for that voltage level.
This power supply unit is based on LM1117/AMS1117 voltage regulator series and for this design we use AMS1117-1.5, AMS111-1.8, AMS1117-2.5 and AMS1117-3.3 fixed voltage regulators. Except to above regulators this board can be use with AMS1117-2.85 and AMS1117-5.0 regulators.

Project info at Dilshan Jayakody’s blog.

Mullard 3-3 Amplifier project (part 2)


Dilshan Jayakody writes:

This is a second article related to Mullard 3-3 Amplifier Project and in this article we introduce HT transformer and HT power supply related to this amplifier. As mentioned in previous post, power supply unit of this tube amplifier is constructed using 400V 5A bridge rectifier, 220µF (400V) and 82µF (400V) electrolytic capacitors.
The most vital component of this power supply is HT transformer and due to limited availability we construct this transformer by ourselves.

More details at Dilshan Jayakody’s blog.