Making A Fixed Voltage Power Supply Adjustable

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.

Dual-channel, variable voltage test box is a busy console modder’s dream


It seems like [Chris Downing] is always up to something new. If he’s not keeping busy by creating slick portable iterations of previous-gen gaming consoles, he is dreaming up ways to make his modding life a bit easier.

Recently while working on a Nintendo controller designed to control three different consoles, [Downing] found his desk buried in a pile of power supply and A/V cabling. Annoyed with his growing rat’s nest, he decided to build a universal power supply that would allow him to quickly switch between consoles with little effort.

He dug up an old PC power supply, and fed it into a LED control box built for cars. [Downing] then mounted an array of nine rocker switches on the box, adding A/V inputs and outputs along the way. A set of voltage regulators hidden inside allow [Downing] to dial in whatever custom voltages he might need at the moment.

The test box should come in pretty handy as [Downing] pursues even more modding projects in the months to come. In the meantime, be sure to check out the video below where he covers the finer points of the device’s design.

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