switch mode power supply

13 Articles

Making A Fixed Voltage Power Supply Adjustable

May 16, 2016 by 33 Comments

Switch-mode power supplies are ubiquitous. Standard off-the-shelf modules in a consistent range of form factors available from multiple manufacturers. Globalized manufacturing and trade has turned them from expensive devices into commodity parts, and they long ago replaced iron-cored transformers as the go-to choice when a high-current low-voltage mains supply is required.

[Lindsay Wilson] faced a power supply problem for a motor he was working with, it required 7.4V and no off-the-shelf power supplies were to be found with that voltage. His solution was to take a 12V supply and modify it to deliver a variable voltage so he could dial in his requirement. A Chinese-made 12v 33A switch-mode supply was purchased, and he set to work.

In the event he was able to design a replacement feedback divider incorporating a rotary potentiometer, and achieve a voltage range of 5 to 15V. A small LED voltmeter mounted next to it in the PSU case gave him a very neat result.

Modifying a switch-mode supply to deliver a different voltage is a well-worn path we’ve covered at least once before. What makes Lindsay’s article worth a read is his reverse-engineering and examination in detail of the PSU circuit. If you’d like to learn more about all the different facets of design that go into a switch-mode PSU, it’s a detailed yet readable primer. We’d suggest reading our recent series on mains and high voltage safety before cracking open a switch-mode PSU yourself, but even if you’re never going to do it there’s something to be gained from knowing in detail how they work.

We’ve featured [Lindsay]’s work here at Hackaday a few times over the years. Check out his ultrasonic transducer power supply, which might be of use were you were building the ultrasonic soldering iron we featured not long ago, his laser stripping of ribbon cables, and his tale of decapping a USB isolator chip.

Re-Capping An Ancient Apple PSU

April 30, 2016 by 35 Comments

It sometimes comes as a shock when you look at a piece of hardware that you maybe bought new and still consider to be rather high-tech, and realise that it was made before someone in their mid-twenties was born. It’s the moment from that Waylon Jennings lyric, about looking in the mirror in total surprise, hair on your shoulders and age in your eyes. Yes, those people in their mid-twenties have never even heard of Waylon Jennings.

[Steve] at Big Mess o’Wires has a Mac IIsi from the early 1990s that wouldn’t power up. He’d already had the life-expired electrolytic capacitors replaced on the mainboard, so the chief suspect was the power supply. That miracle of technology was now pushing past a quarter century, and showing its age. In case anyone is tempted to say they don’t make ’em like they used to, [Steve]’s PSU should dispel the myth.

It’s easy as an electronic engineer writing this piece to think: So? Just open the lid, pop out the old ones and drop in the new, job done! But it’s also easy to forget that not everyone has the same experiences and opening up a mains PSU is something to approach with some trepidation if you’re not used to working with line power. [Steve] was new to mains PSUs and considered sending it to someone else, but decided he *should* be able to do it so set to work.

The Apple PSU is a switch-mode design. Ubiquitous today but still a higher-cost item in those days as you’ll know if you owned an earlier Commodore Amiga whose great big PSU box looked the same as but weighed ten times as much as its later siblings. In simple terms, the mains voltage is rectified to a high-voltage DC, chopped at a high frequency and sent through a small and lightweight ferrite-cored transformer to create the lower voltages. This means it has quite a few electrolytic capacitors, and some of them are significantly stressed with heat and voltage.

Forum posts on the same PSU identified three candidates for replacement – the high voltage smoothing capacitor and a couple of SMD capacitors on the PWM control board. We’d be tempted to say replace the lot while you have it open, but [Steve] set to work on these three. The smoothing cap was taken out with a vacuum desoldering gun, but he had some problems with the SMD caps. Using a hot air gun to remove them he managed to dislodge some of the other SMD components, resulting in the need for a significant cleanup and rework. We’d suggest next time forgoing the air gun and using a fine tip iron to melt each terminal in turn, the cap only has two and should be capable of being tipped up with a pair of pliers to separate each one.

So at the end of it all, he had a working Mac with a PSU that should be good for another twenty years. And he gained the confidence to recap mains power supplies.

If you are tempted to look inside a mains power supply you should not necessarily be put off by the fact it handles mains voltage as long as you treat it with respect. Don’t power it up while you have it open unless it is through an isolation transformer, and remember at all times that it can generate lethal voltages so be very careful and don’t touch it in any way while it is powered up. If in doubt, just don’t power it up at all while open. If you are concerned about high voltages remaining in capacitors when it is turned off, simply measure those voltages with your multimeter. If any remain, discharge them through a suitable resistor until you can no longer measure them. There is a lot for the curious hacker to learn within a switch mode PSU, why should the electronic engineers have all the fun!

This isn’t the first recapping story we’ve covered, and it will no doubt not be the last. Browse our recapping tag for more.

How A Hacker Jump Starts A Car

January 31, 2016 by 73 Comments

Here’s the Scenario: you need to get somewhere in a hurry. The problem is that your car has a dead battery and won’t turn over. The Obvious solution would be to call a friend for a jump. But is the friendless hacker out of luck in such a situation? Not if you can whip up a quick parts bin jump starter.

Clearly, [Kedar Nimbalkar]’s solution would be practical only under somewhat bizarre circumstances, so we’ll concentrate on what we can learn from it. A spare PC power supply provides the electrons – [Kedar]’s 250W supply pushes 15A at 12 volts, which is a pretty respectable amount of current. The voltage is a little anemic, though, so he pops it up to 14.2 volts with a 150W boost converter cooled with a PC fan. A dual panel meter reads out the voltage and current, but a VOM could substitute in a pinch. About the only thing you might not have on hand is a pair of  honking 10A diodes to keep current from creeping back into the boost converter. [Kedar] claims he got enough of a charge back in the battery in five minutes to start his car.

As jump-starting goes, this hack is a bit of a stretch. It’s not the first time we’ve seen a MacGyver’d jump starter, though, and you never know when the principles and hardware behind these hacks will come in handy.

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Hackaday Prize Entry: A 7805 Replacement

May 13, 2015 by 75 Comments

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:

Emergency Power Based On Cordless Drill Batteries

January 31, 2015 by 56 Comments

[Don Eduardo] took matters into his own hands after experiencing a days-long power outage at his house. And like most of us have done at least one, he managed to burn his fingers on a regulator in the process. That’s because he prototyped a way to use power tool batteries as an emergency source — basing his circuit on a 7812 linear regulator which got piping hot in no time flat.

His next autodidactic undertaking carried him into the realm of switch-mode buck converters (learn a bit about these if unfamiliar). The device steps down the ~18V output to 12V regulated for devices meant for automotive or marine. We really like see the different solutions he came up with for interfacing with the batteries which have a U-shaped prong with contacts on opposite sides.

The final iteration, which is pictured above, builds a house of cards on top of the buck converter. After regulating down to 12V he feeds the output into a “cigarette-lighter” style inverter to boost back to 110V AC. The hardware is housed inside of a scrapped charger for the batteries, with the appropriate 3-prong socket hanging out the back. We think it’s a nice touch to include LED feedback for the battery level.

We would like to hear your thoughts on this technique. Is there a better way that’s as easy and adaptive (you don’t have to alter the devices you’re powering) as this one?

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A Primer On Buck (and Boost) Converters

January 24, 2015 by 13 Comments

We all know that the reason the electrical system uses alternating current is because it’s easy to step the voltage up and down using a transformer, a feature which just isn’t possible with a DC system… or is it? Perhaps you’ve heard of mysterious DC-DC transformers before but never really wanted to look at the wizardry that makes them possible. Now, SparkFun Director of Engineering [Pete Dokter] has a tutorial which explains how these mysterious devices work.

Known as buck converters if they step the input voltage down and boost converters if they step the voltage up, [Pete] explains how these circuits exploit the properties of an inductor to resist changes in current flow. He goes into exquisite detail to explain how components like transistors or MOSFETs are used to switch the current flow to the inductor very rapidly, and just exactly what happens to the magnetic field which makes these devices possible.

The video gives a good amount of background knowledge if you’ve always wanted to understand these devices a little bit better. There are also a few projects floating around that exploit these devices, such as one that uses an AVR microcontroller to perform the switching for a small circuit, or another that uses the interesting properties of these circuits to follow the I-V curve of a solar panel to help charge a bank of batteries. The possibilities are endless!

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