The problem is the combination of hardware typically used to run these LED strings. They’re quite bright and draw significant amounts of power, each pixel drawing up to 60 mA at full-white. In a string of just 10 pixels, the strip is already drawing 600 mA. For this reason, it’s common for people to choose quite hefty power supplies that can readily deliver several amps to run these installations.
It’s here that the problem starts. Typically, wires used to hook up the LED strips are quite thin and the flex strips themselves have a significant resistance, too. This means it’s possible to short circuit an LED strip without actually tripping the overcurrent protection on something like an ATX power supply, which may be fused at well over 10 amps. With the resistance of the wires and strip acting as a current limiter, the strip can overheat to the point of catching fire while the power supply happily continues to pump in the juice. In a home workshop under careful supervision, this may be a manageable risk. In an unattended installation, things could be far worse.
Thankfully, the solution is simple. By installing an appropriately rated fuse for the number of LEDs in the circuit, the installation becomes safer, as the fuse will burn out under a short circuit condition even if the power supply is happy to supply the current. With the example of 10 LEDs drawing 600 mA, a 1 amp fuse would do just fine to protect the circuit in the event of an accidental short.
It’s a great explanation of a common yet dangerous problem, and [Thomas] backs it up by using a thermal camera to illustrate just how hot things can get in mere seconds. Armed with this knowledge, you can now safely play with LEDs instead of fire. But now that you’re feeling confident, why not check out these eyeball-searing 3 watt addressable LEDs?
Lately, [Ken Shirriff] has been on some of the most incredible hardware adventures. In his most recent undertaking we find [Ken] elbow-deep in the core memory of a 50-year-old machine, the IBM 1401. The computer wasn’t shut down before mains power was cut, and it has refused to boot ever since. The culprit is in the core memory support circuitry, and thanks to [Ken’s] wonderful storytelling we can travel along with him to repair an IBM 1401.
From a hardware standpoint core memory makes us giddy. It’s a grid of wires with ferrite toroids at every intersection. Bits can be set or cleared based on how electricity is applied to the intersecting wires. [Al Williams] walked through some of the core memory history last year and we enjoyed hearing [Pamela Liou] recount the story of how textile workers consulted on the fabrication of core memory for the Apollo missions during her OHWS Talk in October. But giddiness aside, core memory has pretty much gone the way of the dodo having been displaced by technologies that take up exponentially less space.
We chuckle at [Ken’s] mention of the core memory capacity for the IBM 1401. It has 4000 characters of memory built-in (with another 12,000 in an expansion box) and he goes on to detail that these are 6-bit characters on a machine that operates in decimal and not binary (hence 4k instead of the base-2 friendly 4096).
You may remember his work a few years back to repair core memory on the same model. The Museum has two 1401’s, which turned out to be a huge help in trouble-shooting this. After tracing out the control lines, the repair team began swapping cards between the working and non-working machines. They were able to bring it back online — establishing one of the green inductors was bad — only to be struck with a second fault in the power supply.
Get this, [Ken] comments that “the whole computer is pre-silicon”. When working through the PSU, some suspect transistors were replaced with germanium power transistors. Those may have been a red-herring, as a penciled-in fuse on the original schematics turned out to be the linchpin of the PSU repair. Buried deep in the assembly, replacing the designed-to-fail part let the ancient beast awake once more.
Machines of this quality were heavily documented, and the schematics make this type of trouble-shooting a lot more manageable. But it’s still as much an art as it is skill. Make sure to give [Ken’s] article a read, and look around at the other repair jobs he’s documented — keeping these machines in service is becoming wizard-level work and we love being able to follow along.
A lot of the items on [Medzik]’s BOM for this build are straight from the scrap bin. The aforementioned ATX supply case is one, as is the power transformer donated by a friend. Modules such as the 30V/2A regulator, the digital volt/ammeter, and a thermostat module to control the fan at higher power settings were all sourced via the usual suspects. The PSU boasts two outputs — an adjustable 0-22 volt supply, and a fixed 12-volt output. An unusual design feature is a secondary input which uses the 22-VAC supply from a Weller soldering station to give the PSU a little more oomph. This boosts the maximum output to 30 volts; one wonders why [Medzik] didn’t just source a bigger transformer, but you work with what you have sometimes. There are some nice touches, too, like custom-printed vinyl overlays for the case.
If you have an electronics bench, it follows that you will need some form of bench power supply. While many make do with fixed-voltage supplies it’s safe to say that the most useful bench power supplies have variable voltage and a variable current limiter. These are available in a range of sizes and qualities, and can be had from the usual online suppliers starting with a surprisingly small outlay.
There is however a problem with inexpensive bench power supplies. They are invariably switch-mode designs, and their output will often be noisy. Expensive linear supplies provide a much more noise-free output, but do so at the expense of excessive heat loss when regulating a high voltage drop.
One solution is a mixed-mode design, in which a switch-mode supply does the hard work of reducing the voltage most of the way, and a linear regulator drops the last couple of volts to provide a noise-free output. [Andrei] shows us his design for just such a mixed-mode supply, and it’s one you can have a go at building yourself.
His primary supply is an off-the-shelf switcher that turns mains AC into 24 V DC. This then feeds an LTC1624 buck converter that brings the voltage down to about 1.2 V above the final output voltage, this is in turn fed to a parallel pair of LT3081 linear regulators that deliver the final noise-free output. There is an INA260 for voltage and current measurement, and an Arduino with LCD display as a user interface. His prototype has been nicely constructed using a four-layer PCB, though he suggests it could be made on stripboard with the appropriate SMD adaptors. The cardboard chassis he’s used looks slightly alarming though.
But before [Andy Brown] could build this power supply, he had to reverse-engineer the modules. Based on what he learned, and armed with a data sheet for the modules, he designed a controller to take advantage of all the capabilities of them and ended up with a full-featured power supply. The modules are rated for 66 watts total dissipation at 3.3 volts and have a secondary 5-volt output. Using an ATmega328, [Andy] was able to control the module, provide a display for voltage and current, temperature sensing and fan control, and even a UART to allow data logging to a serial port. His design features mainly through-hole components to make the build accessible to everyone. A suitable case is yet to come, and we’re looking forward to seeing the finished product.
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.
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.
“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.
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. Continue reading “Get To Know Voltage Regulators with a 723”→